Python: Fix function_approval_response extraction in AG-UI workflow path (#4550)

* Extract function_approval_response from workflow messages (#4546)

_extract_responses_from_messages now handles function_approval_response
content in addition to function_result content. Previously, approval
responses sent via the messages field were silently dropped because the
function only checked for content.type == "function_result".

The approval response is keyed by content.id and includes the approved
status, id, and serialized function_call — consistent with how
_coerce_content identifies approval response payloads.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Apply pre-commit auto-fixes

* Fix #4546: Update docstring and add integration tests for message-based approvals

- Update _extract_responses_from_messages docstring to reflect that it
  now handles function_approval_response content in addition to
  function_result content.
- Add integration tests for run_workflow_stream across two turns with
  approval responses provided via messages (function_approvals) rather
  than resume.interrupts, covering both approved and denied scenarios.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review feedback for #4546

- Use safer 'not .get("interrupt")' assertion instead of 'not in'
  to handle Pydantic v2 model_dump() including keys with None values
- Add unit test for mixed function_result and function_approval_response
  in the same message to TestExtractResponsesFromMessages

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/actions/python-setup/action.yml

@@ -8,6 +8,10 @@ inputs:
     os:
       description: The operating system to set up
       required: true
+    exclude-packages:
+      description: Space-separated list of packages to exclude from uv sync
+      required: false
+      default: ''
 
 runs:
     using: "composite"
@@ -19,6 +23,20 @@ runs:
             enable-cache: true
             cache-suffix: ${{ inputs.os }}-${{ inputs.python-version }}
             cache-dependency-glob: "**/uv.lock"
+      - name: Exclude incompatible workspace packages
+        if: ${{ inputs.exclude-packages != '' }}
+        shell: bash
+        run: |
+          for pkg in ${{ inputs.exclude-packages }}; do
+            for f in python/packages/*/pyproject.toml; do
+              if grep -q "name = \"$pkg\"" "$f"; then
+                pkg_dir=$(dirname "$f" | sed 's|python/||')
+                echo "Excluding workspace package: $pkg ($pkg_dir)"
+                sed -i.bak '/\[tool\.uv\.workspace\]/a\exclude = ["'"$pkg_dir"'"]' python/pyproject.toml
+                sed -i.bak '/'"$pkg"' = { workspace = true }/d' python/pyproject.toml
+              fi
+            done
+          done
       - name: Install the project
         shell: bash
         run: |

.github/workflows/dotnet-format.yml

@@ -86,11 +86,10 @@ jobs:
         run: docker pull mcr.microsoft.com/dotnet/sdk:${{ matrix.dotnet }}
 
       # This step will run dotnet format on each of the unique csproj files and fail if any changes are made
-      # exclude-diagnostics should be removed after fixes for IL2026 and IL3050 are out: https://github.com/dotnet/sdk/issues/51136
       - name: Run dotnet format
         if: steps.find-csproj.outputs.csproj_files != ''
         run: |
           for csproj in ${{ steps.find-csproj.outputs.csproj_files }}; do
             echo "Running dotnet format on $csproj"
-            docker run --rm -v $(pwd):/app -w /app mcr.microsoft.com/dotnet/sdk:${{ matrix.dotnet }} /bin/sh -c "dotnet format $csproj --verify-no-changes --verbosity diagnostic --exclude-diagnostics IL2026 IL3050"
+            docker run --rm -v $(pwd):/app -w /app mcr.microsoft.com/dotnet/sdk:${{ matrix.dotnet }} /bin/sh -c "dotnet format $csproj --verify-no-changes --verbosity diagnostic"
           done

.github/workflows/python-code-quality.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.11"]
     runs-on: ubuntu-latest
     continue-on-error: true
     defaults:
@@ -55,7 +55,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.11"]
     runs-on: ubuntu-latest
     continue-on-error: true
     defaults:
@@ -84,7 +84,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.11"]
     runs-on: ubuntu-latest
     continue-on-error: true
     defaults:
@@ -117,7 +117,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.11"]
     runs-on: ubuntu-latest
     continue-on-error: true
     defaults:

.github/workflows/python-integration-tests.yml

@@ -170,7 +170,7 @@ jobs:
     environment: integration
     timeout-minutes: 60
     env:
-      UV_PYTHON: "3.10"
+      UV_PYTHON: "3.11"
       OPENAI_CHAT_MODEL_ID: ${{ vars.OPENAI__CHATMODELID }}
       OPENAI_RESPONSES_MODEL_ID: ${{ vars.OPENAI__RESPONSESMODELID }}
       OPENAI_API_KEY: ${{ secrets.OPENAI__APIKEY }}

.github/workflows/python-lab-tests.yml

@@ -67,6 +67,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           os: ${{ runner.os }}
+          exclude-packages: ${{ matrix.python-version == '3.10' && 'agent-framework-github-copilot' || '' }}
         env:
           # Configure a constant location for the uv cache
           UV_CACHE_DIR: /tmp/.uv-cache

.github/workflows/python-merge-tests.yml

@@ -288,7 +288,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: integration
     env:
-      UV_PYTHON: "3.10"
+      UV_PYTHON: "3.11"
       OPENAI_CHAT_MODEL_ID: ${{ vars.OPENAI__CHATMODELID }}
       OPENAI_RESPONSES_MODEL_ID: ${{ vars.OPENAI__RESPONSESMODELID }}
       OPENAI_API_KEY: ${{ secrets.OPENAI__APIKEY }}

.github/workflows/python-test-coverage.yml

@@ -20,7 +20,7 @@ jobs:
       run:
         working-directory: python
     env:
-      UV_PYTHON: "3.10"
+      UV_PYTHON: "3.11"
     steps:
       - uses: actions/checkout@v6
       # Save the PR number to a file since the workflow_run event

.github/workflows/python-tests.yml

@@ -34,12 +34,13 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           os: ${{ runner.os }}
+          exclude-packages: ${{ matrix.python-version == '3.10' && 'agent-framework-github-copilot' || '' }}
         env:
           # Configure a constant location for the uv cache
           UV_CACHE_DIR: /tmp/.uv-cache
       # Unit tests
       - name: Run all tests
-        run: uv run poe all-tests
+        run: uv run poe all-tests ${{ matrix.python-version == '3.10' && '--ignore-glob=packages/github_copilot/**' || '' }}
         working-directory: ./python
 
       # Surface failing tests

docs/decisions/0001-agent-run-response.md

@@ -64,7 +64,7 @@ Approaches observed from the compared SDKs:
 | AutoGen | **Approach 1** Separates messages into Agent-Agent (maps to Primary) and Internal (maps to Secondary) and these are returned as separate properties on the agent response object.  See [types of messages](https://microsoft.github.io/autogen/stable/user-guide/agentchat-user-guide/tutorial/messages.html#types-of-messages) and [Response](https://microsoft.github.io/autogen/stable/reference/python/autogen_agentchat.base.html#autogen_agentchat.base.Response) | **Approach 2** Returns a stream of internal events and the last item is a Response object. See [ChatAgent.on_messages_stream](https://microsoft.github.io/autogen/stable/reference/python/autogen_agentchat.base.html#autogen_agentchat.base.ChatAgent.on_messages_stream) |
 | OpenAI Agent SDK | **Approach 1** Separates new_items (Primary+Secondary) from final output (Primary) as separate properties on the [RunResult](https://github.com/openai/openai-agents-python/blob/main/src/agents/result.py#L39) | **Approach 1** Similar to non-streaming, has a way of streaming updates via a method on the response object which includes all data, and then a separate final output property on the response object which is populated only when the run is complete. See [RunResultStreaming](https://github.com/openai/openai-agents-python/blob/main/src/agents/result.py#L136) |
 | Google ADK | **Approach 2** [Emits events](https://google.github.io/adk-docs/runtime/#step-by-step-breakdown) with [FinalResponse](https://github.com/google/adk-java/blob/main/core/src/main/java/com/google/adk/events/Event.java#L232) true (Primary) / false (Secondary) and callers have to filter out those with false to get just the final response message | **Approach 2** Similar to non-streaming except [events](https://google.github.io/adk-docs/runtime/#streaming-vs-non-streaming-output-partialtrue) are emitted with [Partial](https://github.com/google/adk-java/blob/main/core/src/main/java/com/google/adk/events/Event.java#L133) true to indicate that they are streaming messages. A final non partial event is also emitted. |
-| AWS (Strands) | **Approach 3** Returns an [AgentResult](https://strandsagents.com/latest/documentation/docs/api-reference/python/agent/agent_result/) (Primary) with messages and a reason for the run's completion. | **Approach 2** [Streams events](https://strandsagents.com/latest/documentation/docs/api-reference/python/agent/agent/#strands.agent.agent.Agent.stream_async) (Primary+Secondary) including, response text, current_tool_use, even data from "callbacks" (strands plugins) |
+| AWS (Strands) | **Approach 3** Returns an [AgentResult](https://strandsagents.com/docs/api/python/strands.agent.agent_result/#agentresult) (Primary) with messages and a reason for the run's completion. | **Approach 2** [Streams events](https://strandsagents.com/docs/user-guide/concepts/streaming/) (Primary+Secondary) including, response text, current_tool_use, even data from "callbacks" (strands plugins) |
 | LangGraph | **Approach 2** A mixed list of all [messages](https://langchain-ai.github.io/langgraph/agents/run_agents/#output-format) | **Approach 2** A mixed list of all [messages](https://langchain-ai.github.io/langgraph/agents/run_agents/#output-format) |
 | Agno | **Combination of various approaches** Returns a [RunResponse](https://docs.agno.com/reference/agents/run-response) object with text content, messages (essentially chat history including inputs and instructions), reasoning and thinking text properties. Secondary events could potentially be extracted from messages. | **Approach 2** Returns [RunResponseEvent](https://docs.agno.com/reference/agents/run-response#runresponseevent-types-and-attributes) objects including tool call, memory update, etc, information, where the [RunResponseCompletedEvent](https://docs.agno.com/reference/agents/run-response#runresponsecompletedevent) has similar properties to RunResponse|
 | A2A | **Approach 3** Returns a [Task or Message](https://a2aproject.github.io/A2A/latest/specification/#71-messagesend) where the message is the final result (Primary) and task is a reference to a long running process. | **Approach 2** Returns a [stream](https://a2aproject.github.io/A2A/latest/specification/#72-messagestream) that contains task updates (Secondary) and a final message (Primary) |
@@ -496,7 +496,7 @@ We need to decide what AIContent types, each agent response type will be mapped
 |-|-|
 | AutoGen | **Approach 1** Supports [configuring an agent](https://microsoft.github.io/autogen/stable/user-guide/agentchat-user-guide/tutorial/agents.html#structured-output) at agent creation. |
 | Google ADK | **Approach 1** Both [input and output schemas can be specified for LLM Agents](https://google.github.io/adk-docs/agents/llm-agents/#structuring-data-input_schema-output_schema-output_key) at construction time. This option is specific to this agent type and other agent types do not necessarily support |
-| AWS (Strands) | **Approach 2** Supports a special invocation method called [structured_output](https://strandsagents.com/latest/documentation/docs/api-reference/python/agent/agent/#strands.agent.agent.Agent.structured_output) |
+| AWS (Strands) | **Approach 2** Supports a special invocation method called [structured_output](https://strandsagents.com/docs/user-guide/concepts/agents/structured-output/) |
 | LangGraph | **Approach 1** Supports [configuring an agent](https://langchain-ai.github.io/langgraph/agents/agents/?h=structured#6-configure-structured-output) at agent construction time, and a [structured response](https://langchain-ai.github.io/langgraph/agents/run_agents/#output-format) can be retrieved as a special property on the agent response |
 | Agno | **Approach 1** Supports [configuring an agent](https://docs.agno.com/input-output/structured-output/agent) at agent construction time |
 | A2A | **Informal Approach 2** Doesn't formally support schema negotiation, but [hints can be provided via metadata](https://a2a-protocol.org/latest/specification/#97-structured-data-exchange-requesting-and-providing-json) at invocation time |
@@ -508,7 +508,7 @@ We need to decide what AIContent types, each agent response type will be mapped
 |-|-|
 | AutoGen | Supports a [stop reason](https://microsoft.github.io/autogen/stable/reference/python/autogen_agentchat.base.html#autogen_agentchat.base.TaskResult.stop_reason) which is a freeform text string |
 | Google ADK | [No equivalent present](https://github.com/google/adk-python/blob/main/src/google/adk/events/event.py) |
-| AWS (Strands) | Exposes a [stop_reason](https://strandsagents.com/latest/documentation/docs/api-reference/python/types/event_loop/#strands.types.event_loop.StopReason) property on the [AgentResult](https://strandsagents.com/latest/documentation/docs/api-reference/python/agent/agent_result/) class with options that are tied closely to LLM operations. |
+| AWS (Strands) | Exposes a `stop_reason` property on the [AgentResult](https://strandsagents.com/docs/api/python/strands.agent.agent_result/#agentresult) class with options that are tied closely to LLM operations. |
 | LangGraph | No equivalent present, output contains only [messages](https://langchain-ai.github.io/langgraph/agents/run_agents/#output-format) |
 | Agno | [No equivalent present](https://docs.agno.com/reference/agents/run-response) |
 | A2A | No equivalent present, response only contains a [message](https://a2a-protocol.org/latest/specification/#64-message-object) or [task](https://a2a-protocol.org/latest/specification/#61-task-object). |

docs/decisions/0019-python-context-compaction-strategy.md

@@ -1240,3 +1240,10 @@ class AttributionAwareStrategy(CompactionStrategy):
 
 - [ADR-0016: Unifying Context Management with ContextPlugin](0016-python-context-middleware.md) — Parent ADR that established `ContextProvider`, `HistoryProvider`, and `AgentSession` architecture.
 - [Context Compaction Limitations Analysis](https://gist.github.com/victordibia/ec3f3baf97345f7e47da025cf55b999f) — Detailed analysis of why current architecture cannot support in-run compaction, with attempted solutions and their failure modes. Option 4 in this ADR corresponds to "Option A: Middleware Access to Mutable Message Source" from that analysis; Options 1-3 correspond to "Option B: Tool Loop Hook", adapted here to a `BaseChatClient` hook instead of `FunctionInvocationConfiguration`.
+
+### Implementation Rollout Note
+
+Implementation is split into two phases:
+
+1. **Phase 1 (PR 1):** runtime compaction foundation in `agent_framework/_compaction.py`, in-run integration, and extensive core tests, plus in-run compaction samples (`basics`, `advanced`, `custom`).
+2. **Phase 2 (PR 2):** history/storage compaction (`upsert`-based full replacement), provider support, storage tests, and storage-focused sample (`storage`).

dotnet/Directory.Packages.props

@@ -11,8 +11,8 @@
   </PropertyGroup>
   <ItemGroup>
     <!-- Aspire.* -->
-    <PackageVersion Include="Anthropic" Version="12.3.0" />
-    <PackageVersion Include="Anthropic.Foundry" Version="0.4.1" />
+    <PackageVersion Include="Anthropic" Version="12.8.0" />
+    <PackageVersion Include="Anthropic.Foundry" Version="0.4.2" />
     <PackageVersion Include="Aspire.Azure.AI.OpenAI" Version="13.0.0-preview.1.25560.3" />
     <PackageVersion Include="Aspire.Hosting.AppHost" Version="$(AspireAppHostSdkVersion)" />
     <PackageVersion Include="Aspire.Hosting.Azure.CognitiveServices" Version="$(AspireAppHostSdkVersion)" />
@@ -108,6 +108,7 @@
     <!-- Inference SDKs -->
     <PackageVersion Include="AWSSDK.Extensions.Bedrock.MEAI" Version="4.0.5.1" />
     <PackageVersion Include="Microsoft.ML.OnnxRuntimeGenAI" Version="0.10.0" />
+    <PackageVersion Include="Microsoft.ML.Tokenizers" Version="2.0.0" />
     <PackageVersion Include="OllamaSharp" Version="5.4.8" />
     <PackageVersion Include="OpenAI" Version="2.8.0" />
     <!-- Identity -->

dotnet/agent-framework-dotnet.slnx

@@ -56,6 +56,7 @@
     <Project Path="samples/02-agents/Agents/Agent_Step15_DeepResearch/Agent_Step15_DeepResearch.csproj" />
     <Project Path="samples/02-agents/Agents/Agent_Step16_Declarative/Agent_Step16_Declarative.csproj" />
     <Project Path="samples/02-agents/Agents/Agent_Step17_AdditionalAIContext/Agent_Step17_AdditionalAIContext.csproj" />
+    <Project Path="samples/02-agents/Agents/Agent_Step18_CompactionPipeline/Agent_Step18_CompactionPipeline.csproj" />
   </Folder>
   <Folder Name="/Samples/02-agents/DeclarativeAgents/">
     <Project Path="samples/02-agents/DeclarativeAgents/ChatClient/DeclarativeChatClientAgents.csproj" />
@@ -103,6 +104,7 @@
     <Project Path="samples/02-agents/AgentWithMemory/AgentWithMemory_Step01_ChatHistoryMemory/AgentWithMemory_Step01_ChatHistoryMemory.csproj" />
     <Project Path="samples/02-agents/AgentWithMemory/AgentWithMemory_Step02_MemoryUsingMem0/AgentWithMemory_Step02_MemoryUsingMem0.csproj" />
     <Project Path="samples/02-agents/AgentWithMemory/AgentWithMemory_Step04_MemoryUsingFoundry/AgentWithMemory_Step04_MemoryUsingFoundry.csproj" />
+    <Project Path="samples/02-agents/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/AgentWithMemory_Step05_BoundedChatHistory.csproj" />
   </Folder>
   <Folder Name="/Samples/02-agents/AgentWithOpenAI/">
     <File Path="samples/02-agents/AgentWithOpenAI/README.md" />

dotnet/agent-framework-release.slnf

@@ -14,6 +14,7 @@
       "src\\Microsoft.Agents.AI.Declarative\\Microsoft.Agents.AI.Declarative.csproj",
       "src\\Microsoft.Agents.AI.DevUI\\Microsoft.Agents.AI.DevUI.csproj",
       "src\\Microsoft.Agents.AI.DurableTask\\Microsoft.Agents.AI.DurableTask.csproj",
+      "src\\Microsoft.Agents.AI.FoundryMemory\\Microsoft.Agents.AI.FoundryMemory.csproj",
       "src\\Microsoft.Agents.AI.Hosting.A2A.AspNetCore\\Microsoft.Agents.AI.Hosting.A2A.AspNetCore.csproj",
       "src\\Microsoft.Agents.AI.Hosting.A2A\\Microsoft.Agents.AI.Hosting.A2A.csproj",
       "src\\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore\\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.csproj",

dotnet/global.json

@@ -1,6 +1,6 @@
 {
     "sdk": {
-        "version": "10.0.100",
+        "version": "10.0.200",
         "rollForward": "minor",
         "allowPrerelease": false
     },

dotnet/nuget/nuget-package.props

@@ -2,11 +2,11 @@
   <PropertyGroup>
     <!-- Central version prefix - applies to all nuget packages. -->
     <VersionPrefix>1.0.0</VersionPrefix>
-    <RCNumber>3</RCNumber>
+    <RCNumber>4</RCNumber>
     <PackageVersion Condition="'$(IsReleaseCandidate)' == 'true'">$(VersionPrefix)-rc$(RCNumber)</PackageVersion>
-    <PackageVersion Condition="'$(IsReleaseCandidate)' != 'true' AND '$(VersionSuffix)' != ''">$(VersionPrefix)-$(VersionSuffix).260304.1</PackageVersion>
-    <PackageVersion Condition="'$(IsReleaseCandidate)' != 'true' AND '$(VersionSuffix)' == ''">$(VersionPrefix)-preview.260304.1</PackageVersion>
-    <GitTag>1.0.0-rc3</GitTag>
+    <PackageVersion Condition="'$(IsReleaseCandidate)' != 'true' AND '$(VersionSuffix)' != ''">$(VersionPrefix)-$(VersionSuffix).260311.1</PackageVersion>
+    <PackageVersion Condition="'$(IsReleaseCandidate)' != 'true' AND '$(VersionSuffix)' == ''">$(VersionPrefix)-preview.260311.1</PackageVersion>
+    <GitTag>1.0.0-rc4</GitTag>
 
     <Configurations>Debug;Release;Publish</Configurations>
     <IsPackable>true</IsPackable>

dotnet/samples/02-agents/AGUI/Step01_GettingStarted/Server/Program.cs

@@ -4,7 +4,6 @@ using Azure.AI.OpenAI;
 using Azure.Identity;
 using Microsoft.Agents.AI;
 using Microsoft.Agents.AI.Hosting.AGUI.AspNetCore;
-using Microsoft.Extensions.AI;
 using OpenAI.Chat;
 
 WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
@@ -27,7 +26,7 @@ ChatClient chatClient = new AzureOpenAIClient(
         new DefaultAzureCredential())
     .GetChatClient(deploymentName);
 
-AIAgent agent = chatClient.AsIChatClient().AsAIAgent(
+AIAgent agent = chatClient.AsAIAgent(
     name: "AGUIAssistant",
     instructions: "You are a helpful assistant.");
 

dotnet/samples/02-agents/AGUI/Step01_GettingStarted/Server/Server.csproj

@@ -10,7 +10,6 @@
   <ItemGroup>
     <PackageReference Include="Azure.AI.OpenAI" />
     <PackageReference Include="Azure.Identity" />
-    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
   </ItemGroup>
 
   <ItemGroup>

dotnet/samples/02-agents/AGUI/Step02_BackendTools/Server/Program.cs

@@ -82,7 +82,7 @@ ChatClient chatClient = new AzureOpenAIClient(
         new DefaultAzureCredential())
     .GetChatClient(deploymentName);
 
-ChatClientAgent agent = chatClient.AsIChatClient().AsAIAgent(
+ChatClientAgent agent = chatClient.AsAIAgent(
     name: "AGUIAssistant",
     instructions: "You are a helpful assistant with access to restaurant information.",
     tools: tools);

dotnet/samples/02-agents/AGUI/Step02_BackendTools/Server/Server.csproj

@@ -10,7 +10,6 @@
   <ItemGroup>
     <PackageReference Include="Azure.AI.OpenAI" />
     <PackageReference Include="Azure.Identity" />
-    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
   </ItemGroup>
 
   <ItemGroup>

dotnet/samples/02-agents/AGUI/Step03_FrontendTools/Server/Program.cs

@@ -4,7 +4,6 @@ using Azure.AI.OpenAI;
 using Azure.Identity;
 using Microsoft.Agents.AI;
 using Microsoft.Agents.AI.Hosting.AGUI.AspNetCore;
-using Microsoft.Extensions.AI;
 using OpenAI.Chat;
 
 WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
@@ -27,7 +26,7 @@ ChatClient chatClient = new AzureOpenAIClient(
         new DefaultAzureCredential())
     .GetChatClient(deploymentName);
 
-AIAgent agent = chatClient.AsIChatClient().AsAIAgent(
+AIAgent agent = chatClient.AsAIAgent(
     name: "AGUIAssistant",
     instructions: "You are a helpful assistant.");
 

dotnet/samples/02-agents/AGUI/Step03_FrontendTools/Server/Server.csproj

@@ -10,7 +10,6 @@
   <ItemGroup>
     <PackageReference Include="Azure.AI.OpenAI" />
     <PackageReference Include="Azure.Identity" />
-    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
   </ItemGroup>
 
   <ItemGroup>

dotnet/samples/02-agents/AGUI/Step04_HumanInLoop/Server/Program.cs

@@ -60,7 +60,7 @@ ChatClient openAIChatClient = new AzureOpenAIClient(
         new DefaultAzureCredential())
     .GetChatClient(deploymentName);
 
-ChatClientAgent baseAgent = openAIChatClient.AsIChatClient().AsAIAgent(
+ChatClientAgent baseAgent = openAIChatClient.AsAIAgent(
     name: "AGUIAssistant",
     instructions: "You are a helpful assistant in charge of approving expenses",
     tools: tools);

dotnet/samples/02-agents/AGUI/Step04_HumanInLoop/Server/Server.csproj

@@ -10,7 +10,6 @@
   <ItemGroup>
     <PackageReference Include="Azure.AI.OpenAI" />
     <PackageReference Include="Azure.Identity" />
-    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
   </ItemGroup>
 
   <ItemGroup>

dotnet/samples/02-agents/AGUI/Step05_StateManagement/Server/Program.cs

@@ -4,7 +4,6 @@ using Azure.AI.OpenAI;
 using Azure.Identity;
 using Microsoft.Agents.AI;
 using Microsoft.Agents.AI.Hosting.AGUI.AspNetCore;
-using Microsoft.Extensions.AI;
 using Microsoft.Extensions.Options;
 using OpenAI.Chat;
 using RecipeAssistant;
@@ -37,7 +36,7 @@ ChatClient chatClient = new AzureOpenAIClient(
         new DefaultAzureCredential())
     .GetChatClient(deploymentName);
 
-AIAgent baseAgent = chatClient.AsIChatClient().AsAIAgent(
+AIAgent baseAgent = chatClient.AsAIAgent(
     name: "RecipeAgent",
     instructions: """
         You are a helpful recipe assistant. When users ask you to create or suggest a recipe,

dotnet/samples/02-agents/AGUI/Step05_StateManagement/Server/Server.csproj

@@ -10,7 +10,6 @@
   <ItemGroup>
     <PackageReference Include="Azure.AI.OpenAI" />
     <PackageReference Include="Azure.Identity" />
-    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
   </ItemGroup>
 
   <ItemGroup>

dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/AgentWithMemory_Step05_BoundedChatHistory.csproj

@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFrameworks>net10.0</TargetFrameworks>
+
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Azure.AI.OpenAI" />
+    <PackageReference Include="Azure.Identity" />
+    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
+    <PackageReference Include="Microsoft.SemanticKernel.Connectors.InMemory" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\..\src\Microsoft.Agents.AI.OpenAI\Microsoft.Agents.AI.OpenAI.csproj" />
+  </ItemGroup>
+
+</Project>

dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/BoundedChatHistoryProvider.cs

@@ -0,0 +1,133 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.Agents.AI;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.VectorData;
+
+namespace SampleApp;
+
+/// <summary>
+/// A <see cref="ChatHistoryProvider"/> that keeps a bounded window of recent messages in session state
+/// (via <see cref="InMemoryChatHistoryProvider"/>) and overflows older messages to a vector store
+/// (via <see cref="ChatHistoryMemoryProvider"/>). When providing chat history, it searches the vector
+/// store for relevant older messages and prepends them as a memory context message.
+/// </summary>
+/// <remarks>
+/// Only non-system messages are counted towards the session state limit and overflow mechanism. System messages are always retained in session state and are not included in the vector store.
+/// Function calls and function results are also dropped when truncation happens, both from in-memory state, and they are also not persisted to the vector store.
+/// </remarks>
+internal sealed class BoundedChatHistoryProvider : ChatHistoryProvider, IDisposable
+{
+    private readonly InMemoryChatHistoryProvider _chatHistoryProvider;
+    private readonly ChatHistoryMemoryProvider _memoryProvider;
+    private readonly TruncatingChatReducer _reducer;
+    private readonly string _contextPrompt;
+    private IReadOnlyList<string>? _stateKeys;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="BoundedChatHistoryProvider"/> class.
+    /// </summary>
+    /// <param name="maxSessionMessages">The maximum number of non-system messages to keep in session state before overflowing to the vector store.</param>
+    /// <param name="vectorStore">The vector store to use for storing and retrieving overflow chat history.</param>
+    /// <param name="collectionName">The name of the collection for storing overflow chat history in the vector store.</param>
+    /// <param name="vectorDimensions">The number of dimensions to use for the chat history vector store embeddings.</param>
+    /// <param name="stateInitializer">A delegate that initializes the memory provider state, providing the storage and search scopes.</param>
+    /// <param name="contextPrompt">Optional prompt to prefix memory search results. Defaults to a standard memory context prompt.</param>
+    public BoundedChatHistoryProvider(
+        int maxSessionMessages,
+        VectorStore vectorStore,
+        string collectionName,
+        int vectorDimensions,
+        Func<AgentSession?, ChatHistoryMemoryProvider.State> stateInitializer,
+        string? contextPrompt = null)
+    {
+        if (maxSessionMessages < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(maxSessionMessages), "maxSessionMessages must be non-negative.");
+        }
+
+        this._reducer = new TruncatingChatReducer(maxSessionMessages);
+        this._chatHistoryProvider = new InMemoryChatHistoryProvider(new InMemoryChatHistoryProviderOptions
+        {
+            ChatReducer = this._reducer,
+            ReducerTriggerEvent = InMemoryChatHistoryProviderOptions.ChatReducerTriggerEvent.AfterMessageAdded,
+            StorageInputRequestMessageFilter = msgs => msgs,
+        });
+        this._memoryProvider = new ChatHistoryMemoryProvider(
+            vectorStore,
+            collectionName,
+            vectorDimensions,
+            stateInitializer,
+            options: new ChatHistoryMemoryProviderOptions
+            {
+                SearchInputMessageFilter = msgs => msgs,
+                StorageInputRequestMessageFilter = msgs => msgs,
+            });
+        this._contextPrompt = contextPrompt
+            ?? "The following are memories from earlier in this conversation. Use them to inform your responses:";
+    }
+
+    /// <inheritdoc />
+    public override IReadOnlyList<string> StateKeys => this._stateKeys ??= this._chatHistoryProvider.StateKeys.Concat(this._memoryProvider.StateKeys).ToArray();
+
+    /// <inheritdoc />
+    protected override async ValueTask<IEnumerable<ChatMessage>> ProvideChatHistoryAsync(
+        InvokingContext context,
+        CancellationToken cancellationToken = default)
+    {
+        // Delegate to the inner provider's full lifecycle (retrieve, filter, stamp, merge with request messages).
+        var chatHistoryProviderInputContext = new InvokingContext(context.Agent, context.Session, []);
+        var allMessages = await this._chatHistoryProvider.InvokingAsync(chatHistoryProviderInputContext, cancellationToken).ConfigureAwait(false);
+
+        // Search the vector store for relevant older messages.
+        var aiContext = new AIContext { Messages = context.RequestMessages.ToList() };
+        var invokingContext = new AIContextProvider.InvokingContext(
+            context.Agent, context.Session, aiContext);
+
+        var result = await this._memoryProvider.InvokingAsync(invokingContext, cancellationToken).ConfigureAwait(false);
+
+        // Extract only the messages added by the memory provider (stamped with AIContextProvider source type).
+        var memoryMessages = result.Messages?
+            .Where(m => m.GetAgentRequestMessageSourceType() == AgentRequestMessageSourceType.AIContextProvider)
+            .ToList();
+
+        if (memoryMessages is { Count: > 0 })
+        {
+            var memoryText = string.Join("\n", memoryMessages.Select(m => m.Text).Where(t => !string.IsNullOrWhiteSpace(t)));
+
+            if (!string.IsNullOrWhiteSpace(memoryText))
+            {
+                var contextMessage = new ChatMessage(ChatRole.User, $"{this._contextPrompt}\n{memoryText}");
+                return new[] { contextMessage }.Concat(allMessages);
+            }
+        }
+
+        return allMessages;
+    }
+
+    /// <inheritdoc />
+    protected override async ValueTask StoreChatHistoryAsync(
+        InvokedContext context,
+        CancellationToken cancellationToken = default)
+    {
+        // Delegate storage to the in-memory provider. Its TruncatingChatReducer (AfterMessageAdded trigger)
+        // will automatically truncate to the configured maximum and expose any removed messages.
+        var innerContext = new InvokedContext(
+            context.Agent, context.Session, context.RequestMessages, context.ResponseMessages!);
+        await this._chatHistoryProvider.InvokedAsync(innerContext, cancellationToken).ConfigureAwait(false);
+
+        // Archive any messages that the reducer removed to the vector store.
+        if (this._reducer.RemovedMessages is { Count: > 0 })
+        {
+            var overflowContext = new AIContextProvider.InvokedContext(
+                context.Agent, context.Session, this._reducer.RemovedMessages, []);
+            await this._memoryProvider.InvokedAsync(overflowContext, cancellationToken).ConfigureAwait(false);
+        }
+    }
+
+    /// <inheritdoc/>
+    public void Dispose()
+    {
+        this._memoryProvider.Dispose();
+    }
+}

dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/Program.cs

@@ -0,0 +1,79 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample shows how to create a bounded chat history provider that keeps a configurable number of
+// recent messages in session state and automatically overflows older messages to a vector store.
+// When the agent is invoked, it searches the vector store for relevant older messages and
+// prepends them as a "memory" context message before the recent session history.
+
+using Azure.AI.OpenAI;
+using Azure.Identity;
+using Microsoft.Agents.AI;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.VectorData;
+using Microsoft.SemanticKernel.Connectors.InMemory;
+using OpenAI.Chat;
+using SampleApp;
+
+var endpoint = Environment.GetEnvironmentVariable("AZURE_OPENAI_ENDPOINT") ?? throw new InvalidOperationException("AZURE_OPENAI_ENDPOINT is not set.");
+var deploymentName = Environment.GetEnvironmentVariable("AZURE_OPENAI_DEPLOYMENT_NAME") ?? "gpt-4o-mini";
+var embeddingDeploymentName = Environment.GetEnvironmentVariable("AZURE_OPENAI_EMBEDDING_DEPLOYMENT_NAME") ?? "text-embedding-3-large";
+
+// WARNING: DefaultAzureCredential is convenient for development but requires careful consideration in production.
+// In production, consider using a specific credential (e.g., ManagedIdentityCredential) to avoid
+// latency issues, unintended credential probing, and potential security risks from fallback mechanisms.
+var credential = new DefaultAzureCredential();
+
+// Create a vector store to store overflow chat messages.
+// For demonstration purposes, we are using an in-memory vector store.
+// Replace this with a persistent vector store implementation for production scenarios.
+VectorStore vectorStore = new InMemoryVectorStore(new InMemoryVectorStoreOptions()
+{
+    EmbeddingGenerator = new AzureOpenAIClient(new Uri(endpoint), credential)
+        .GetEmbeddingClient(embeddingDeploymentName)
+        .AsIEmbeddingGenerator()
+});
+
+var sessionId = Guid.NewGuid().ToString();
+
+// Create the BoundedChatHistoryProvider with a maximum of 4 non-system messages in session state.
+// It internally creates an InMemoryChatHistoryProvider with a TruncatingChatReducer and a
+// ChatHistoryMemoryProvider with the correct configuration to ensure overflow messages are
+// automatically archived to the vector store and recalled via semantic search.
+var boundedProvider = new BoundedChatHistoryProvider(
+    maxSessionMessages: 4,
+    vectorStore,
+    collectionName: "chathistory-overflow",
+    vectorDimensions: 3072,
+    session => new ChatHistoryMemoryProvider.State(
+        storageScope: new() { UserId = "UID1", SessionId = sessionId },
+        searchScope: new() { UserId = "UID1" }));
+
+// Create the agent with the bounded chat history provider.
+AIAgent agent = new AzureOpenAIClient(new Uri(endpoint), credential)
+    .GetChatClient(deploymentName)
+    .AsAIAgent(new ChatClientAgentOptions
+    {
+        ChatOptions = new() { Instructions = "You are a helpful assistant. Answer questions concisely." },
+        Name = "Assistant",
+        ChatHistoryProvider = boundedProvider,
+    });
+
+// Start a conversation. The first several exchanges will fill up the session state window.
+AgentSession session = await agent.CreateSessionAsync();
+
+Console.WriteLine("--- Filling the session window (4 messages max) ---\n");
+
+Console.WriteLine(await agent.RunAsync("My favorite color is blue.", session));
+Console.WriteLine(await agent.RunAsync("I have a dog named Max.", session));
+
+// At this point the session state holds 4 messages (2 user + 2 assistant).
+// The next exchange will push the oldest messages into the vector store.
+Console.WriteLine("\n--- Next exchange will trigger overflow to vector store ---\n");
+
+Console.WriteLine(await agent.RunAsync("What is the capital of France?", session));
+
+// The oldest messages about favorite color have now been archived to the vector store.
+// Ask the agent something that requires recalling the overflowed information.
+Console.WriteLine("\n--- Asking about overflowed information (should recall from vector store) ---\n");
+
+Console.WriteLine(await agent.RunAsync("What is my favorite color?", session));

dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/README.md

@@ -0,0 +1,40 @@
+# Bounded Chat History with Vector Store Overflow
+
+This sample demonstrates how to create a custom `ChatHistoryProvider` that keeps a bounded window of recent messages in session state and automatically overflows older messages to a vector store. When the agent is invoked, it searches the vector store for relevant older messages and prepends them as memory context.
+
+## Concepts
+
+- **`TruncatingChatReducer`**: A custom `IChatReducer` that keeps the most recent N messages and exposes removed messages via a `RemovedMessages` property.
+- **`BoundedChatHistoryProvider`**: A custom `ChatHistoryProvider` that composes:
+  - `InMemoryChatHistoryProvider` for fast session-state storage (bounded by the reducer)
+  - `ChatHistoryMemoryProvider` for vector-store overflow and semantic search of older messages
+
+## Prerequisites
+
+- [.NET 10 SDK](https://dotnet.microsoft.com/download/dotnet/10.0)
+- An Azure OpenAI resource with:
+  - A chat deployment (e.g., `gpt-4o-mini`)
+  - An embedding deployment (e.g., `text-embedding-3-large`)
+
+## Configuration
+
+Set the following environment variables:
+
+| Variable | Description | Default |
+|---|---|---|
+| `AZURE_OPENAI_ENDPOINT` | Your Azure OpenAI endpoint URL | *(required)* |
+| `AZURE_OPENAI_DEPLOYMENT_NAME` | Chat model deployment name | `gpt-4o-mini` |
+| `AZURE_OPENAI_EMBEDDING_DEPLOYMENT_NAME` | Embedding model deployment name | `text-embedding-3-large` |
+
+## Running the Sample
+
+```bash
+dotnet run
+```
+
+## How it Works
+
+1. The agent starts a conversation with a bounded session window of 4 non-system, non-function messages (i.e., user/assistant turns). System messages are always preserved, and function call/result messages are truncated and not preserved.
+2. As messages accumulate beyond the limit, the `TruncatingChatReducer` removes the oldest messages.
+3. The `BoundedChatHistoryProvider` detects the removed messages and stores them in a vector store via `ChatHistoryMemoryProvider`.
+4. On subsequent invocations, the provider searches the vector store for relevant older messages and prepends them as memory context, allowing the agent to recall information from earlier in the conversation.

dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/TruncatingChatReducer.cs

@@ -0,0 +1,65 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.Extensions.AI;
+
+namespace SampleApp;
+
+/// <summary>
+/// A truncating chat reducer that keeps the most recent messages up to a configured maximum,
+/// preserving any leading system message. Removed messages are exposed via <see cref="RemovedMessages"/>
+/// so that a caller can archive them (e.g. to a vector store).
+/// </summary>
+internal sealed class TruncatingChatReducer : IChatReducer
+{
+    private readonly int _maxMessages;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TruncatingChatReducer"/> class.
+    /// </summary>
+    /// <param name="maxMessages">The maximum number of non-system messages to retain.</param>
+    public TruncatingChatReducer(int maxMessages)
+    {
+        this._maxMessages = maxMessages > 0 ? maxMessages : throw new ArgumentOutOfRangeException(nameof(maxMessages));
+    }
+
+    /// <summary>
+    /// Gets the messages that were removed during the most recent call to <see cref="ReduceAsync"/>.
+    /// </summary>
+    public IReadOnlyList<ChatMessage> RemovedMessages { get; private set; } = [];
+
+    /// <inheritdoc />
+    public Task<IEnumerable<ChatMessage>> ReduceAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken)
+    {
+        _ = messages ?? throw new ArgumentNullException(nameof(messages));
+
+        ChatMessage? systemMessage = null;
+        Queue<ChatMessage> retained = new(capacity: this._maxMessages);
+        List<ChatMessage> removed = [];
+
+        foreach (var message in messages)
+        {
+            if (message.Role == ChatRole.System)
+            {
+                // Preserve the first system message outside the counting window.
+                systemMessage ??= message;
+            }
+            else if (!message.Contents.Any(c => c is FunctionCallContent or FunctionResultContent))
+            {
+                if (retained.Count >= this._maxMessages)
+                {
+                    removed.Add(retained.Dequeue());
+                }
+
+                retained.Enqueue(message);
+            }
+        }
+
+        this.RemovedMessages = removed;
+
+        IEnumerable<ChatMessage> result = systemMessage is not null
+            ? new[] { systemMessage }.Concat(retained)
+            : retained;
+
+        return Task.FromResult(result);
+    }
+}

dotnet/samples/02-agents/AgentWithMemory/README.md

@@ -8,5 +8,6 @@ These samples show how to create an agent with the Agent Framework that uses Mem
 |[Memory with MemoryStore](./AgentWithMemory_Step02_MemoryUsingMem0/)|This sample demonstrates how to create and run an agent that uses the Mem0 service to extract and retrieve individual memories.|
 |[Custom Memory Implementation](../../01-get-started/04_memory/)|This sample demonstrates how to create a custom memory component and attach it to an agent.|
 |[Memory with Azure AI Foundry](./AgentWithMemory_Step04_MemoryUsingFoundry/)|This sample demonstrates how to create and run an agent that uses Azure AI Foundry's managed memory service to extract and retrieve individual memories.|
+|[Bounded Chat History with Overflow](./AgentWithMemory_Step05_BoundedChatHistory/)|This sample demonstrates how to create a bounded chat history provider that overflows older messages to a vector store and recalls them as memories.|
 
 > **See also**: [Memory Search with Foundry Agents](../FoundryAgents/FoundryAgents_Step22_MemorySearch/) - demonstrates using the built-in Memory Search tool with Azure Foundry Agents.

dotnet/samples/02-agents/Agents/Agent_Step18_CompactionPipeline/Agent_Step18_CompactionPipeline.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFrameworks>net10.0</TargetFrameworks>
+
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Azure.AI.OpenAI" />
+    <PackageReference Include="Azure.Identity" />
+    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\..\src\Microsoft.Agents.AI.OpenAI\Microsoft.Agents.AI.OpenAI.csproj" />
+  </ItemGroup>
+
+</Project>

dotnet/samples/02-agents/Agents/Agent_Step18_CompactionPipeline/Program.cs

@@ -0,0 +1,120 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample demonstrates how to use a CompactionProvider with a compaction pipeline
+// as an AIContextProvider for an agent's in-run context management. The pipeline chains multiple
+// compaction strategies from gentle to aggressive:
+//   1. ToolResultCompactionStrategy - Collapses old tool-call groups into concise summaries
+//   2. SummarizationCompactionStrategy - LLM-compresses older conversation spans
+//   3. SlidingWindowCompactionStrategy - Keeps only the most recent N user turns
+//   4. TruncationCompactionStrategy - Emergency token-budget backstop
+
+using System.ComponentModel;
+using Azure.AI.OpenAI;
+using Azure.Identity;
+using Microsoft.Agents.AI;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+var endpoint = Environment.GetEnvironmentVariable("AZURE_OPENAI_ENDPOINT") ?? throw new InvalidOperationException("AZURE_OPENAI_ENDPOINT is not set.");
+var deploymentName = Environment.GetEnvironmentVariable("AZURE_OPENAI_DEPLOYMENT_NAME") ?? "gpt-4o-mini";
+
+// WARNING: DefaultAzureCredential is convenient for development but requires careful consideration in production.
+// In production, consider using a specific credential (e.g., ManagedIdentityCredential) to avoid
+// latency issues, unintended credential probing, and potential security risks from fallback mechanisms.
+AzureOpenAIClient openAIClient = new(new Uri(endpoint), new DefaultAzureCredential());
+
+// Create a chat client for the agent and a separate one for the summarization strategy.
+// Using the same model for simplicity; in production, use a smaller/cheaper model for summarization.
+IChatClient agentChatClient = openAIClient.GetChatClient(deploymentName).AsIChatClient();
+IChatClient summarizerChatClient = openAIClient.GetChatClient(deploymentName).AsIChatClient();
+
+// Define a tool the agent can use, so we can see tool-result compaction in action.
+[Description("Look up the current price of a product by name.")]
+static string LookupPrice([Description("The product name to look up.")] string productName) =>
+    productName.ToUpperInvariant() switch
+    {
+        "LAPTOP" => "The laptop costs $999.99.",
+        "KEYBOARD" => "The keyboard costs $79.99.",
+        "MOUSE" => "The mouse costs $29.99.",
+        _ => $"Sorry, I don't have pricing for '{productName}'."
+    };
+
+// Configure the compaction pipeline with one of each strategy, ordered least to most aggressive.
+PipelineCompactionStrategy compactionPipeline =
+    new(// 1. Gentle: collapse old tool-call groups into short summaries
+        new ToolResultCompactionStrategy(CompactionTriggers.MessagesExceed(7)),
+
+        // 2. Moderate: use an LLM to summarize older conversation spans into a concise message
+        new SummarizationCompactionStrategy(summarizerChatClient, CompactionTriggers.TokensExceed(0x500)),
+
+        // 3. Aggressive: keep only the last N user turns and their responses
+        new SlidingWindowCompactionStrategy(CompactionTriggers.TurnsExceed(4)),
+
+        // 4. Emergency: drop oldest groups until under the token budget
+        new TruncationCompactionStrategy(CompactionTriggers.TokensExceed(0x8000)));
+
+// Create the agent with a CompactionProvider that uses the compaction pipeline.
+AIAgent agent =
+    agentChatClient
+        .AsBuilder()
+        // Note: Adding the CompactionProvider at the builder level means it will be applied to all agents
+        // built from this builder and will manage context for both agent messages and tool calls.
+        .UseAIContextProviders(new CompactionProvider(compactionPipeline))
+        .BuildAIAgent(
+            new ChatClientAgentOptions
+            {
+                Name = "ShoppingAssistant",
+                ChatOptions = new()
+                {
+                    Instructions =
+                        """
+                        You are a helpful, but long winded, shopping assistant.
+                        Help the user look up prices and compare products.
+                        When responding, Be sure to be extra descriptive and use as
+                        many words as possible without sounding ridiculous.
+                        """,
+                    Tools = [AIFunctionFactory.Create(LookupPrice)]
+                },
+                // Note: AIContextProviders may be specified here instead of ChatClientBuilder.UseAIContextProviders.
+                // Specifying compaction at the agent level skips compaction in the function calling loop.
+                //AIContextProviders = [new CompactionProvider(compactionPipeline)]
+            });
+
+AgentSession session = await agent.CreateSessionAsync();
+
+// Helper to print chat history size
+void PrintChatHistory()
+{
+    if (session.TryGetInMemoryChatHistory(out var history))
+    {
+        Console.ForegroundColor = ConsoleColor.Cyan;
+        Console.WriteLine($"\n[Messages: #{history.Count}]\n");
+        Console.ResetColor();
+    }
+}
+
+// Run a multi-turn conversation with tool calls to exercise the pipeline.
+string[] prompts =
+[
+    "What's the price of a laptop?",
+    "How about a keyboard?",
+    "And a mouse?",
+    "Which product is the cheapest?",
+    "Can you compare the laptop and the keyboard for me?",
+    "What was the first product I asked about?",
+    "Thank you!",
+];
+
+foreach (string prompt in prompts)
+{
+    Console.ForegroundColor = ConsoleColor.Cyan;
+    Console.Write("\n[User] ");
+    Console.ResetColor();
+    Console.WriteLine(prompt);
+    Console.ForegroundColor = ConsoleColor.Cyan;
+    Console.Write("\n[Agent] ");
+    Console.ResetColor();
+    Console.WriteLine(await agent.RunAsync(prompt, session));
+
+    PrintChatHistory();
+}

dotnet/samples/02-agents/Agents/Agent_Step18_CompactionPipeline/README.md

@@ -0,0 +1,132 @@
+# Compaction Pipeline
+
+This sample demonstrates how to use a `CompactionProvider` with a `PipelineCompactionStrategy` to manage long conversation histories in a token-efficient way. The pipeline chains four compaction strategies, ordered from gentle to aggressive, so that the least disruptive strategy runs first and more aggressive strategies only activate when necessary.
+
+## What This Sample Shows
+
+- **`CompactionProvider`** — an `AIContextProvider` that applies a compaction strategy before each agent invocation, keeping only the most relevant messages within the model's context window
+- **`PipelineCompactionStrategy`** — chains multiple compaction strategies into an ordered pipeline; each strategy evaluates its own trigger independently and operates on the output of the previous one
+- **`ToolResultCompactionStrategy`** — collapses older tool-call groups into concise inline summaries, activated by a message-count trigger
+- **`SummarizationCompactionStrategy`** — uses an LLM to compress older conversation spans into a single summary message, activated by a token-count trigger
+- **`SlidingWindowCompactionStrategy`** — retains only the most recent N user turns and their responses, activated by a turn-count trigger
+- **`TruncationCompactionStrategy`** — emergency backstop that drops the oldest groups until the conversation fits within a hard token budget
+- **`CompactionTriggers`** — factory methods (`MessagesExceed`, `TokensExceed`, `TurnsExceed`, `GroupsExceed`, `HasToolCalls`, `All`, `Any`) that control when each strategy activates
+
+## Concepts
+
+### Message groups
+
+The compaction engine organizes messages into atomic *groups* that are treated as indivisible units during compaction. A group is either:
+
+| Group kind | Contents |
+|---|---|
+| `System` | System prompt message(s) |
+| `User` | A single user message |
+| `ToolCall` | One assistant message with tool calls + the matching tool result messages |
+| `AssistantText` | A single assistant text-only message |
+| `Summary` | One or more messages summarizing earlier conversation spans, produced by compaction strategies |
+
+`Summary` groups (`CompactionGroupKind.Summary`) are created by compaction strategies (for example, `SummarizationCompactionStrategy`) and do not originate directly from user or assistant messages.
+Strategies exclude entire groups rather than individual messages, preserving the tool-call/result pairing required by most model APIs.
+
+### Compaction triggers
+
+A `CompactionTrigger` is a predicate evaluated against the current `MessageIndex`. When the trigger fires, the strategy performs compaction; when it does not fire, the strategy is skipped. Available triggers are:
+
+| Trigger | Activates when… |
+|---|---|
+| `CompactionTriggers.Always` | Always (unconditional) |
+| `CompactionTriggers.Never` | Never (disabled) |
+| `CompactionTriggers.MessagesExceed(n)` | Included message count > n |
+| `CompactionTriggers.TokensExceed(n)` | Included token count > n |
+| `CompactionTriggers.TurnsExceed(n)` | Included user-turn count > n |
+| `CompactionTriggers.GroupsExceed(n)` | Included group count > n |
+| `CompactionTriggers.HasToolCalls()` | At least one included tool-call group exists |
+| `CompactionTriggers.All(...)` | All supplied triggers fire (logical AND) |
+| `CompactionTriggers.Any(...)` | Any supplied trigger fires (logical OR) |
+
+### Pipeline ordering
+
+Order strategies from **least aggressive** to **most aggressive**. The pipeline runs every strategy whose trigger is met. Earlier strategies reduce the conversation gently so that later, more destructive strategies may not need to activate at all.
+
+```
+1. ToolResultCompactionStrategy  – gentle:    replaces verbose tool results with a short label
+2. SummarizationCompactionStrategy – moderate: LLM-summarizes older turns
+3. SlidingWindowCompactionStrategy – aggressive: drops turns beyond the window
+4. TruncationCompactionStrategy   – emergency:  hard token-budget enforcement
+```
+
+## Prerequisites
+
+- .NET 10 SDK or later
+- Azure OpenAI service endpoint and model deployment
+- Azure CLI installed and authenticated
+
+**Note**: This sample uses `DefaultAzureCredential`. Sign in with `az login` before running. For production, prefer a specific credential such as `ManagedIdentityCredential`. For more information, see the [Azure CLI authentication documentation](https://learn.microsoft.com/cli/azure/authenticate-azure-cli-interactively).
+
+## Environment Variables
+
+```powershell
+$env:AZURE_OPENAI_ENDPOINT="https://your-resource.openai.azure.com/"  # Required
+$env:AZURE_OPENAI_DEPLOYMENT_NAME="gpt-4o-mini"                       # Optional, defaults to gpt-4o-mini
+```
+
+## Running the Sample
+
+```powershell
+cd dotnet/samples/02-agents/Agents/Agent_Step18_CompactionPipeline
+dotnet run
+```
+
+## Expected Behavior
+
+The sample runs a seven-turn shopping-assistant conversation with tool calls. After each turn it prints the full message count so you can observe the pipeline compaction doesn't alter the source conversation.
+
+Each of the four compaction strategies has a deliberately low threshold so that it activates during the short demonstration conversation. In a production scenario you would raise the thresholds to match your model's context window and cost requirements.
+
+## Customizing the Pipeline
+
+### Using a single strategy
+
+If you only need one compaction strategy, pass it directly to `CompactionProvider` without wrapping it in a pipeline:
+
+```csharp
+CompactionProvider provider =
+    new(new SlidingWindowCompactionStrategy(CompactionTriggers.TurnsExceed(20)));
+```
+
+### Ad-hoc compaction outside the provider pipeline
+
+`CompactionProvider.CompactAsync` applies a strategy to an arbitrary list of messages without an active agent session:
+
+```csharp
+IEnumerable<ChatMessage> compacted = await CompactionProvider.CompactAsync(
+    new TruncationCompactionStrategy(CompactionTriggers.TokensExceed(8000)),
+    existingMessages);
+```
+
+### Using a different model for summarization
+
+The `SummarizationCompactionStrategy` accepts any `IChatClient`. Use a smaller, cheaper model to reduce summarization cost:
+
+```csharp
+IChatClient summarizerChatClient = openAIClient.GetChatClient("gpt-4o-mini").AsIChatClient();
+new SummarizationCompactionStrategy(summarizerChatClient, CompactionTriggers.TokensExceed(4000))
+```
+
+### Registering through `ChatClientAgentOptions`
+
+`CompactionProvider` can also be specified directly on `ChatClientAgentOptions` instead of calling `UseAIContextProviders` on the `ChatClientBuilder`:
+
+```csharp
+AIAgent agent = agentChatClient
+    .AsBuilder()
+    .BuildAIAgent(new ChatClientAgentOptions
+    {
+        AIContextProviders = [new CompactionProvider(compactionPipeline)]
+    });
+```
+
+This places the compaction provider at the agent level instead of the chat client level, which allows you to use different compaction strategies for different agents that share the same chat client.
+
+> Note: In this mode the `CompactionProvider` is not engaged during the tool calling loop. Agent-level `AIContextProviders` run before chat history is stored, so any synthetic summary messages produced by `CompactionProvider` can become part of the persisted history when using `ChatHistoryProvider`. If you want to compact only the request context while preserving the original stored history, register `CompactionProvider` on the `ChatClientBuilder` via `UseAIContextProviders(...)` instead of on `ChatClientAgentOptions`.

dotnet/samples/02-agents/Agents/README.md

@@ -44,6 +44,7 @@ Before you begin, ensure you have the following prerequisites:
 |[Deep research with an agent](./Agent_Step15_DeepResearch/)|This sample demonstrates how to use the Deep Research Tool to perform comprehensive research on complex topics|
 |[Declarative agent](./Agent_Step16_Declarative/)|This sample demonstrates how to declaratively define an agent.|
 |[Providing additional AI Context to an agent using multiple AIContextProviders](./Agent_Step17_AdditionalAIContext/)|This sample demonstrates how to inject additional AI context into a ChatClientAgent using multiple custom AIContextProvider components that are attached to the agent.|
+|[Using compaction pipeline with an agent](./Agent_Step18_CompactionPipeline/)|This sample demonstrates how to use a compaction pipeline to efficiently limit the size of the conversation history for an agent.|
 
 ## Running the samples from the console
 

dotnet/samples/02-agents/FoundryAgents/FoundryAgents_Step22_MemorySearch/Program.cs

@@ -12,11 +12,8 @@ using OpenAI.Responses;
 
 string endpoint = Environment.GetEnvironmentVariable("AZURE_AI_PROJECT_ENDPOINT") ?? throw new InvalidOperationException("AZURE_AI_PROJECT_ENDPOINT is not set.");
 string deploymentName = Environment.GetEnvironmentVariable("AZURE_AI_MODEL_DEPLOYMENT_NAME") ?? "gpt-4o-mini";
-
-// Memory store configuration
-// NOTE: Memory stores must be created beforehand via Azure Portal or Python SDK.
-// The .NET SDK currently only supports using existing memory stores with agents.
-string memoryStoreName = Environment.GetEnvironmentVariable("AZURE_AI_MEMORY_STORE_ID") ?? throw new InvalidOperationException("AZURE_AI_MEMORY_STORE_ID is not set.");
+string embeddingModelName = Environment.GetEnvironmentVariable("AZURE_AI_EMBEDDING_DEPLOYMENT_NAME") ?? "text-embedding-ada-002";
+string memoryStoreName = Environment.GetEnvironmentVariable("AZURE_AI_MEMORY_STORE_ID") ?? $"foundry-memory-sample-{Guid.NewGuid():N}";
 
 const string AgentInstructions = """
     You are a helpful assistant that remembers past conversations.
@@ -32,71 +29,57 @@ const string AgentNameNative = "MemorySearchAgent-NATIVE";
 string userScope = $"user_{Environment.MachineName}";
 
 // Get a client to create/retrieve/delete server side agents with Azure Foundry Agents.
-AIProjectClient aiProjectClient = new(new Uri(endpoint), new AzureCliCredential());
+DefaultAzureCredential credential = new();
+AIProjectClient aiProjectClient = new(new Uri(endpoint), credential);
+
+// Ensure the memory store exists and has memories to retrieve.
+await EnsureMemoryStoreAsync();
 
 // Create the Memory Search tool configuration
-MemorySearchPreviewTool memorySearchTool = new(memoryStoreName, userScope)
-{
-    // Optional: Configure how quickly new memories are indexed (in seconds)
-    UpdateDelay = 1,
-
-    // Optional: Configure search behavior
-    SearchOptions = new MemorySearchToolOptions
-    {
-        // Additional search options can be configured here if needed
-    }
-};
+MemorySearchPreviewTool memorySearchTool = new(memoryStoreName, userScope) { UpdateDelay = 0 };
 
 // Create agent using Option 1 (MEAI) or Option 2 (Native SDK)
 AIAgent agent = await CreateAgentWithMEAI();
 // AIAgent agent = await CreateAgentWithNativeSDK();
 
-Console.WriteLine("Agent created with Memory Search tool. Starting conversation...\n");
-
-// Conversation 1: Share some personal information
-Console.WriteLine("User: My name is Alice and I love programming in C#.");
-AgentResponse response1 = await agent.RunAsync("My name is Alice and I love programming in C#.");
-Console.WriteLine($"Agent: {response1.Messages.LastOrDefault()?.Text}\n");
-
-// Allow time for memory to be indexed
-await Task.Delay(2000);
-
-// Conversation 2: Test if the agent remembers
-Console.WriteLine("User: What's my name and what programming language do I prefer?");
-AgentResponse response2 = await agent.RunAsync("What's my name and what programming language do I prefer?");
-Console.WriteLine($"Agent: {response2.Messages.LastOrDefault()?.Text}\n");
-
-// Inspect memory search results if available in raw response items
-// Note: Memory search tool call results appear as AgentResponseItem types
-foreach (var message in response2.Messages)
+try
 {
-    if (message.RawRepresentation is AgentResponseItem agentResponseItem &&
-        agentResponseItem is MemorySearchToolCallResponseItem memorySearchResult)
-    {
-        Console.WriteLine($"Memory Search Status: {memorySearchResult.Status}");
-        Console.WriteLine($"Memory Search Results Count: {memorySearchResult.Results.Count}");
+    Console.WriteLine("Agent created with Memory Search tool. Starting conversation...\n");
 
-        foreach (var result in memorySearchResult.Results)
+    // The agent uses the memory search tool to recall stored information.
+    Console.WriteLine("User: What's my name and what programming language do I prefer?");
+    AgentResponse response = await agent.RunAsync("What's my name and what programming language do I prefer?");
+    Console.WriteLine($"Agent: {response.Messages.LastOrDefault()?.Text}\n");
+
+    // Inspect memory search results if available in raw response items.
+    foreach (var message in response.Messages)
+    {
+        if (message.RawRepresentation is MemorySearchToolCallResponseItem memorySearchResult)
         {
-            var memoryItem = result.MemoryItem;
-            Console.WriteLine($"  - Memory ID: {memoryItem.MemoryId}");
-            Console.WriteLine($"    Scope: {memoryItem.Scope}");
-            Console.WriteLine($"    Content: {memoryItem.Content}");
-            Console.WriteLine($"    Updated: {memoryItem.UpdatedAt}");
+            Console.WriteLine($"Memory Search Status: {memorySearchResult.Status}");
+            Console.WriteLine($"Memory Search Results Count: {memorySearchResult.Results.Count}");
+
+            foreach (var result in memorySearchResult.Results)
+            {
+                var memoryItem = result.MemoryItem;
+                Console.WriteLine($"  - Memory ID: {memoryItem.MemoryId}");
+                Console.WriteLine($"    Scope: {memoryItem.Scope}");
+                Console.WriteLine($"    Content: {memoryItem.Content}");
+                Console.WriteLine($"    Updated: {memoryItem.UpdatedAt}");
+            }
         }
     }
 }
+finally
+{
+    // Cleanup: Delete the agent and memory store.
+    Console.WriteLine("\nCleaning up...");
+    await aiProjectClient.Agents.DeleteAgentAsync(agent.Name);
+    Console.WriteLine("Agent deleted.");
+    await aiProjectClient.MemoryStores.DeleteMemoryStoreAsync(memoryStoreName);
+    Console.WriteLine("Memory store deleted.");
+}
 
-// Cleanup: Delete the agent (memory store persists and should be cleaned up separately if needed)
-Console.WriteLine("\nCleaning up agent...");
-await aiProjectClient.Agents.DeleteAgentAsync(agent.Name);
-Console.WriteLine("Agent deleted successfully.");
-
-// NOTE: Memory stores are long-lived resources and are NOT deleted with the agent.
-// To delete a memory store, use the Azure Portal or Python SDK:
-// await project_client.memory_stores.delete(memory_store.name)
-
-// --- Agent Creation Options ---
 #pragma warning disable CS8321 // Local function is declared but never used
 
 // Option 1 - Using MemorySearchTool wrapped as MEAI AITool
@@ -122,3 +105,36 @@ async Task<AIAgent> CreateAgentWithNativeSDK()
             })
     );
 }
+
+// Helpers — kept at the bottom so the main agent flow above stays clean.
+async Task EnsureMemoryStoreAsync()
+{
+    Console.WriteLine($"Creating memory store '{memoryStoreName}'...");
+    try
+    {
+        await aiProjectClient.MemoryStores.GetMemoryStoreAsync(memoryStoreName);
+        Console.WriteLine("Memory store already exists.");
+    }
+    catch (System.ClientModel.ClientResultException ex) when (ex.Status == 404)
+    {
+        MemoryStoreDefaultDefinition definition = new(deploymentName, embeddingModelName);
+        await aiProjectClient.MemoryStores.CreateMemoryStoreAsync(memoryStoreName, definition, "Sample memory store for Memory Search demo");
+        Console.WriteLine("Memory store created.");
+    }
+
+    Console.WriteLine("Storing memories from a prior conversation...");
+    MemoryUpdateOptions memoryOptions = new(userScope) { UpdateDelay = 0 };
+    memoryOptions.Items.Add(ResponseItem.CreateUserMessageItem("My name is Alice and I love programming in C#."));
+
+    MemoryUpdateResult updateResult = await aiProjectClient.MemoryStores.WaitForMemoriesUpdateAsync(
+        memoryStoreName: memoryStoreName,
+        options: memoryOptions,
+        pollingInterval: 500);
+
+    if (updateResult.Status == MemoryStoreUpdateStatus.Failed)
+    {
+        throw new InvalidOperationException($"Memory update failed: {updateResult.ErrorDetails}");
+    }
+
+    Console.WriteLine($"Memory update completed (status: {updateResult.Status}).\n");
+}

dotnet/samples/05-end-to-end/AGUIClientServer/AGUIDojoServer/ChatClientAgentFactory.cs

@@ -10,7 +10,7 @@ using Azure.AI.OpenAI;
 using Azure.Identity;
 using Microsoft.Agents.AI;
 using Microsoft.Extensions.AI;
-using ChatClient = OpenAI.Chat.ChatClient;
+using OpenAI.Chat;
 
 namespace AGUIDojoServer;
 
@@ -36,7 +36,7 @@ internal static class ChatClientAgentFactory
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
 
-        return chatClient.AsIChatClient().AsAIAgent(
+        return chatClient.AsAIAgent(
             name: "AgenticChat",
             description: "A simple chat agent using Azure OpenAI");
     }
@@ -45,7 +45,7 @@ internal static class ChatClientAgentFactory
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
 
-        return chatClient.AsIChatClient().AsAIAgent(
+        return chatClient.AsAIAgent(
             name: "BackendToolRenderer",
             description: "An agent that can render backend tools using Azure OpenAI",
             tools: [AIFunctionFactory.Create(
@@ -59,7 +59,7 @@ internal static class ChatClientAgentFactory
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
 
-        return chatClient.AsIChatClient().AsAIAgent(
+        return chatClient.AsAIAgent(
             name: "HumanInTheLoopAgent",
             description: "An agent that involves human feedback in its decision-making process using Azure OpenAI");
     }
@@ -68,7 +68,7 @@ internal static class ChatClientAgentFactory
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
 
-        return chatClient.AsIChatClient().AsAIAgent(
+        return chatClient.AsAIAgent(
             name: "ToolBasedGenerativeUIAgent",
             description: "An agent that uses tools to generate user interfaces using Azure OpenAI");
     }
@@ -76,7 +76,7 @@ internal static class ChatClientAgentFactory
     public static AIAgent CreateAgenticUI(JsonSerializerOptions options)
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
-        var baseAgent = chatClient.AsIChatClient().AsAIAgent(new ChatClientAgentOptions
+        var baseAgent = chatClient.AsAIAgent(new ChatClientAgentOptions
         {
             Name = "AgenticUIAgent",
             Description = "An agent that generates agentic user interfaces using Azure OpenAI",
@@ -119,7 +119,7 @@ internal static class ChatClientAgentFactory
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
 
-        var baseAgent = chatClient.AsIChatClient().AsAIAgent(
+        var baseAgent = chatClient.AsAIAgent(
             name: "SharedStateAgent",
             description: "An agent that demonstrates shared state patterns using Azure OpenAI");
 
@@ -130,7 +130,7 @@ internal static class ChatClientAgentFactory
     {
         ChatClient chatClient = s_azureOpenAIClient!.GetChatClient(s_deploymentName!);
 
-        var baseAgent = chatClient.AsIChatClient().AsAIAgent(new ChatClientAgentOptions
+        var baseAgent = chatClient.AsAIAgent(new ChatClientAgentOptions
         {
             Name = "PredictiveStateUpdatesAgent",
             Description = "An agent that demonstrates predictive state updates using Azure OpenAI",

dotnet/samples/05-end-to-end/AGUIWebChat/README.md

@@ -74,7 +74,7 @@ AzureOpenAIClient azureOpenAIClient = new AzureOpenAIClient(
 ChatClient chatClient = azureOpenAIClient.GetChatClient(deploymentName);
 
 // Create AI agent
-ChatClientAgent agent = chatClient.AsIChatClient().AsAIAgent(
+ChatClientAgent agent = chatClient.AsAIAgent(
     name: "ChatAssistant",
     instructions: "You are a helpful assistant.");
 
@@ -162,7 +162,7 @@ dotnet run
 Edit the instructions in `Server/Program.cs`:
 
 ```csharp
-ChatClientAgent agent = chatClient.AsIChatClient().AsAIAgent(
+ChatClientAgent agent = chatClient.AsAIAgent(
     name: "ChatAssistant",
     instructions: "You are a helpful coding assistant specializing in C# and .NET.");
 ```

dotnet/samples/05-end-to-end/AGUIWebChat/Server/Program.cs

@@ -6,7 +6,6 @@ using Azure.AI.OpenAI;
 using Azure.Identity;
 using Microsoft.Agents.AI;
 using Microsoft.Agents.AI.Hosting.AGUI.AspNetCore;
-using Microsoft.Extensions.AI;
 using OpenAI.Chat;
 
 WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
@@ -28,7 +27,7 @@ AzureOpenAIClient azureOpenAIClient = new(
 
 ChatClient chatClient = azureOpenAIClient.GetChatClient(deploymentName);
 
-ChatClientAgent agent = chatClient.AsIChatClient().AsAIAgent(
+ChatClientAgent agent = chatClient.AsAIAgent(
     name: "ChatAssistant",
     instructions: "You are a helpful assistant.");
 

dotnet/samples/05-end-to-end/AspNetAgentAuthorization/Service/Program.cs

@@ -12,6 +12,7 @@ using Microsoft.AspNetCore.Authentication.JwtBearer;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.Extensions.AI;
 using OpenAI;
+using OpenAI.Chat;
 
 WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
 
@@ -89,7 +90,6 @@ builder.Services.AddSingleton<AIAgent>(sp =>
 
     return new OpenAIClient(apiKey)
         .GetChatClient(model)
-        .AsIChatClient()
         .AsAIAgent(
             name: "ExpenseApprovalAgent",
             instructions: "You are an expense approval assistant. You can list pending expenses "

dotnet/samples/05-end-to-end/AspNetAgentAuthorization/Service/Service.csproj

@@ -10,7 +10,6 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" />
-    <PackageReference Include="Microsoft.Extensions.AI.OpenAI" />
   </ItemGroup>
 
   <ItemGroup>

dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs

@@ -127,6 +127,7 @@ public sealed class A2AAgent : AIAgent
             {
                 AgentId = this.Id,
                 ResponseId = message.MessageId,
+                FinishReason = ChatFinishReason.Stop,
                 RawRepresentation = message,
                 Messages = [message.ToChatMessage()],
                 AdditionalProperties = message.Metadata?.ToAdditionalProperties(),
@@ -141,6 +142,7 @@ public sealed class A2AAgent : AIAgent
             {
                 AgentId = this.Id,
                 ResponseId = agentTask.Id,
+                FinishReason = MapTaskStateToFinishReason(agentTask.Status.State),
                 RawRepresentation = agentTask,
                 Messages = agentTask.ToChatMessages() ?? [],
                 ContinuationToken = CreateContinuationToken(agentTask.Id, agentTask.Status.State),
@@ -328,6 +330,7 @@ public sealed class A2AAgent : AIAgent
         {
             AgentId = this.Id,
             ResponseId = message.MessageId,
+            FinishReason = ChatFinishReason.Stop,
             RawRepresentation = message,
             Role = ChatRole.Assistant,
             MessageId = message.MessageId,
@@ -342,6 +345,7 @@ public sealed class A2AAgent : AIAgent
         {
             AgentId = this.Id,
             ResponseId = task.Id,
+            FinishReason = MapTaskStateToFinishReason(task.Status.State),
             RawRepresentation = task,
             Role = ChatRole.Assistant,
             Contents = task.ToAIContents(),
@@ -365,7 +369,16 @@ public sealed class A2AAgent : AIAgent
             responseUpdate.Contents = artifactUpdateEvent.Artifact.ToAIContents();
             responseUpdate.RawRepresentation = artifactUpdateEvent;
         }
+        else if (taskUpdateEvent is TaskStatusUpdateEvent statusUpdateEvent)
+        {
+            responseUpdate.FinishReason = MapTaskStateToFinishReason(statusUpdateEvent.Status.State);
+        }
 
         return responseUpdate;
     }
+
+    private static ChatFinishReason? MapTaskStateToFinishReason(TaskState state)
+    {
+        return state == TaskState.Completed ? ChatFinishReason.Stop : null;
+    }
 }

dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs

@@ -20,6 +20,19 @@ namespace Microsoft.Agents.AI;
 /// <see cref="AIAgent"/> serves as the foundational class for implementing AI agents that can participate in conversations
 /// and process user requests. An agent instance may participate in multiple concurrent conversations, and each conversation
 /// may involve multiple agents working together.
+/// <para>
+/// <strong>Security considerations:</strong> An <see cref="AIAgent"/> orchestrates data flow across trust boundaries —
+/// messages are sent to external AI services, context providers, chat history stores, and function tools. Agent Framework
+/// passes messages through as-is without validation or sanitization. Developers must be aware that:
+/// <list type="bullet">
+/// <item><description>User-supplied messages may contain prompt injection attempts designed to manipulate LLM behavior.</description></item>
+/// <item><description>LLM responses should be treated as untrusted output — they may contain hallucinations, malicious payloads (e.g., scripts, SQL),
+/// or content influenced by indirect prompt injection. Always validate and sanitize LLM output before rendering in HTML, executing as code,
+/// or using in database queries.</description></item>
+/// <item><description>Messages with different roles carry different trust levels: <c>system</c> messages have the highest trust and must be developer-controlled;
+/// <c>user</c>, <c>assistant</c>, and <c>tool</c> messages should be treated as untrusted.</description></item>
+/// </list>
+/// </para>
 /// </remarks>
 [DebuggerDisplay("{DebuggerDisplay,nq}")]
 public abstract partial class AIAgent
@@ -165,6 +178,11 @@ public abstract partial class AIAgent
     /// This method enables saving conversation sessions to persistent storage,
     /// allowing conversations to resume across application restarts or be migrated between
     /// different agent instances. Use <see cref="DeserializeSessionAsync"/> to restore the session.
+    /// <para>
+    /// <strong>Security consideration:</strong> Serialized sessions may contain conversation content, session identifiers,
+    /// and other potentially sensitive data including PII. Ensure that serialized session data is stored securely with
+    /// appropriate access controls and encryption at rest.
+    /// </para>
     /// </remarks>
     public ValueTask<JsonElement> SerializeSessionAsync(AgentSession session, JsonSerializerOptions? jsonSerializerOptions = null, CancellationToken cancellationToken = default)
         => this.SerializeSessionCoreAsync(session, jsonSerializerOptions, cancellationToken);
@@ -194,6 +212,12 @@ public abstract partial class AIAgent
     /// This method enables restoration of conversation sessions from previously saved state,
     /// allowing conversations to resume across application restarts or be migrated between
     /// different agent instances.
+    /// <para>
+    /// <strong>Security consideration:</strong> Restoring a session from an untrusted source is equivalent to accepting untrusted input.
+    /// Serialized sessions may contain conversation content, session identifiers, and potentially sensitive data. A compromised
+    /// storage backend could alter message roles to escalate trust, or inject adversarial content that influences LLM behavior.
+    /// Treat serialized session data as sensitive and ensure it is stored and transmitted securely.
+    /// </para>
     /// </remarks>
     public ValueTask<AgentSession> DeserializeSessionAsync(JsonElement serializedState, JsonSerializerOptions? jsonSerializerOptions = null, CancellationToken cancellationToken = default)
         => this.DeserializeSessionCoreAsync(serializedState, jsonSerializerOptions, cancellationToken);
@@ -301,6 +325,11 @@ public abstract partial class AIAgent
     /// The messages are processed in the order provided and become part of the conversation history.
     /// The agent's response will also be added to <paramref name="session"/> if one is provided.
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Agent Framework does not validate or sanitize message content — it is passed through
+    /// to the underlying AI service as-is. If input messages include untrusted user content, developers should be aware of prompt injection risks.
+    /// System-role messages must be developer-controlled and should never contain end-user input.
+    /// </para>
     /// </remarks>
     public Task<AgentResponse> RunAsync(
         IEnumerable<ChatMessage> messages,
@@ -426,6 +455,11 @@ public abstract partial class AIAgent
     /// Each <see cref="AgentResponseUpdate"/> represents a portion of the complete response, allowing consumers
     /// to display partial results, implement progressive loading, or provide immediate feedback to users.
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Agent Framework does not validate or sanitize message content — it is passed through
+    /// to the underlying AI service as-is. If input messages include untrusted user content, developers should be aware of prompt injection risks.
+    /// System-role messages must be developer-controlled and should never contain end-user input.
+    /// </para>
     /// </remarks>
     public async IAsyncEnumerable<AgentResponseUpdate> RunStreamingAsync(
         IEnumerable<ChatMessage> messages,

dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs

@@ -28,6 +28,14 @@ namespace Microsoft.Agents.AI;
 /// <see cref="InvokingAsync"/> to provide context, and optionally called at the end of invocation via
 /// <see cref="InvokedAsync"/> to process results.
 /// </para>
+/// <para>
+/// <strong>Security considerations:</strong> Context providers may inject messages with any role, including <c>system</c>, which
+/// has the highest trust level and directly shapes LLM behavior. Developers must ensure that all providers attached to an agent
+/// are trusted. Agent Framework does not validate or filter the data returned by providers — it is accepted as-is and merged into
+/// the request context. If a provider retrieves data from an external source (e.g., a vector database or memory service), be aware
+/// that a compromised data source could introduce adversarial content designed to manipulate LLM behavior via indirect prompt injection.
+/// Implementers should validate and sanitize data retrieved from external sources before returning it.
+/// </para>
 /// </remarks>
 public abstract class AIContextProvider
 {
@@ -96,6 +104,11 @@ public abstract class AIContextProvider
     /// <item><description>Injecting contextual messages from conversation history</description></item>
     /// </list>
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Data retrieved from external sources (e.g., vector databases, memory services, or
+    /// knowledge bases) may contain adversarial content designed to influence LLM behavior via indirect prompt injection.
+    /// Implementers should validate data integrity and consider the trustworthiness of the data source.
+    /// </para>
     /// </remarks>
     public ValueTask<AIContext> InvokingAsync(InvokingContext context, CancellationToken cancellationToken = default)
         => this.InvokingCoreAsync(Throw.IfNull(context), cancellationToken);
@@ -195,6 +208,11 @@ public abstract class AIContextProvider
     /// In contrast with <see cref="InvokingCoreAsync"/>, this method only returns additional context to be merged with the input,
     /// while <see cref="InvokingCoreAsync"/> is responsible for returning the full merged <see cref="AIContext"/> for the invocation.
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Any messages, tools, or instructions returned by this method will be merged into the
+    /// AI request context. If data is retrieved from external or untrusted sources, implementers should validate and sanitize it
+    /// to prevent indirect prompt injection attacks.
+    /// </para>
     /// </remarks>
     /// <param name="context">Contains the request context including the caller provided messages that will be used by the agent for this invocation.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
@@ -299,6 +317,10 @@ public abstract class AIContextProvider
     /// <para>
     /// The default implementation of <see cref="InvokedCoreAsync"/> only calls this method if the invocation succeeded.
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Messages being processed/stored may contain PII and sensitive conversation content.
+    /// Implementers should ensure appropriate encryption at rest and access controls for the storage backend.
+    /// </para>
     /// </remarks>
     protected virtual ValueTask StoreAIContextAsync(InvokedContext context, CancellationToken cancellationToken = default) =>
         default;

dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponse.cs

@@ -61,6 +61,7 @@ public class AgentResponse
 
         this.AdditionalProperties = response.AdditionalProperties;
         this.CreatedAt = response.CreatedAt;
+        this.FinishReason = response.FinishReason;
         this.Messages = response.Messages;
         this.RawRepresentation = response;
         this.ResponseId = response.ResponseId;
@@ -84,6 +85,7 @@ public class AgentResponse
 
         this.AdditionalProperties = response.AdditionalProperties;
         this.CreatedAt = response.CreatedAt;
+        this.FinishReason = response.FinishReason;
         this.Messages = response.Messages;
         this.RawRepresentation = response;
         this.ResponseId = response.ResponseId;
@@ -190,6 +192,21 @@ public class AgentResponse
     /// </remarks>
     public DateTimeOffset? CreatedAt { get; set; }
 
+    /// <summary>
+    /// Gets or sets the reason for the agent response finishing.
+    /// </summary>
+    /// <value>
+    /// A <see cref="ChatFinishReason"/> value indicating why the response finished (e.g., stop, length, content filter, tool calls),
+    /// or <see langword="null"/> if the finish reason is not available.
+    /// </value>
+    /// <remarks>
+    /// <para>
+    /// This property is particularly useful for detecting non-normal completions, such as content filtering
+    /// or token limit truncation, which may require special handling by the caller.
+    /// </para>
+    /// </remarks>
+    public ChatFinishReason? FinishReason { get; set; }
+
     /// <summary>
     /// Gets or sets the resource usage information for generating this response.
     /// </summary>
@@ -276,6 +293,7 @@ public class AgentResponse
                 RawRepresentation = message.RawRepresentation,
                 Role = message.Role,
 
+                FinishReason = this.FinishReason,
                 AgentId = this.AgentId,
                 ResponseId = this.ResponseId,
                 MessageId = message.MessageId,

dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponseExtensions.cs

@@ -38,6 +38,7 @@ public static class AgentResponseExtensions
             {
                 AdditionalProperties = response.AdditionalProperties,
                 CreatedAt = response.CreatedAt,
+                FinishReason = response.FinishReason,
                 Messages = response.Messages,
                 RawRepresentation = response,
                 ResponseId = response.ResponseId,
@@ -71,6 +72,7 @@ public static class AgentResponseExtensions
                 AuthorName = responseUpdate.AuthorName,
                 Contents = responseUpdate.Contents,
                 CreatedAt = responseUpdate.CreatedAt,
+                FinishReason = responseUpdate.FinishReason,
                 MessageId = responseUpdate.MessageId,
                 RawRepresentation = responseUpdate,
                 ResponseId = responseUpdate.ResponseId,

dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponseUpdate.cs

@@ -70,6 +70,7 @@ public class AgentResponseUpdate
         this.AuthorName = chatResponseUpdate.AuthorName;
         this.Contents = chatResponseUpdate.Contents;
         this.CreatedAt = chatResponseUpdate.CreatedAt;
+        this.FinishReason = chatResponseUpdate.FinishReason;
         this.MessageId = chatResponseUpdate.MessageId;
         this.RawRepresentation = chatResponseUpdate;
         this.ResponseId = chatResponseUpdate.ResponseId;
@@ -153,6 +154,15 @@ public class AgentResponseUpdate
     /// </remarks>
     public ResponseContinuationToken? ContinuationToken { get; set; }
 
+    /// <summary>
+    /// Gets or sets the reason for the agent response finishing.
+    /// </summary>
+    /// <value>
+    /// A <see cref="ChatFinishReason"/> value indicating why the response finished (e.g., stop, length, content filter, tool calls),
+    /// or <see langword="null"/> if the finish reason is not available or not yet determined (mid-stream).
+    /// </value>
+    public ChatFinishReason? FinishReason { get; set; }
+
     /// <inheritdoc/>
     public override string ToString() => this.Text;
 

dotnet/src/Microsoft.Agents.AI.Abstractions/AgentSession.cs

@@ -42,6 +42,15 @@ namespace Microsoft.Agents.AI;
 /// <see cref="JsonElement"/> and the <see cref="AIAgent.DeserializeSessionAsync(JsonElement, JsonSerializerOptions?, System.Threading.CancellationToken)"/> method
 /// can be used to deserialize the session.
 /// </para>
+/// <para>
+/// <strong>Security considerations:</strong> Serialized sessions may contain conversation content, session identifiers,
+/// and other potentially sensitive data including PII. Developers should:
+/// <list type="bullet">
+/// <item><description>Treat serialized session data as sensitive and store it securely with appropriate access controls and encryption at rest.</description></item>
+/// <item><description>Treat restoring a session from an untrusted source as equivalent to accepting untrusted input. A compromised storage backend
+/// could alter message roles to escalate trust, or inject adversarial content that influences LLM behavior.</description></item>
+/// </list>
+/// </para>
 /// </remarks>
 /// <seealso cref="AIAgent"/>
 /// <seealso cref="AIAgent.CreateSessionAsync(System.Threading.CancellationToken)"/>
@@ -67,6 +76,11 @@ public abstract class AgentSession
     /// <summary>
     /// Gets any arbitrary state associated with this session.
     /// </summary>
+    /// <remarks>
+    /// Data stored in the <see cref="StateBag"/> will be included when the session is serialized.
+    /// Avoid storing secrets, credentials, or highly sensitive data in the state bag without appropriate encryption,
+    /// as this data may be persisted to external storage.
+    /// </remarks>
     [JsonPropertyName("stateBag")]
     public AgentSessionStateBag StateBag { get; protected set; } = new();
 

dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProvider.cs

@@ -37,6 +37,14 @@ namespace Microsoft.Agents.AI;
 /// A <see cref="ChatHistoryProvider"/> is only relevant for scenarios where the underlying AI service that the agent is using
 /// does not use in-service chat history storage.
 /// </para>
+/// <para>
+/// <strong>Security considerations:</strong> Agent Framework does not validate or filter the messages returned by the provider
+/// during load — they are accepted as-is and treated identically to user-supplied messages. Implementers must ensure that only
+/// trusted data is returned. If the underlying storage is compromised, adversarial content could influence LLM behavior via
+/// indirect prompt injection — for example, injected messages could alter the conversation context or impersonate different roles.
+/// Messages stored in chat history may contain PII and sensitive conversation content; implementers should consider encryption
+/// at rest and appropriate access controls for the storage backend.
+/// </para>
 /// </remarks>
 public abstract class ChatHistoryProvider
 {
@@ -159,6 +167,11 @@ public abstract class ChatHistoryProvider
     /// Messages are returned in chronological order to maintain proper conversation flow and context for the agent.
     /// The oldest messages appear first in the collection, followed by more recent messages.
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Messages loaded from storage should be treated with the same caution as user-supplied
+    /// messages. A compromised storage backend could alter message roles to escalate trust (e.g., changing <c>user</c> messages to
+    /// <c>system</c> messages) or inject adversarial content that influences LLM behavior.
+    /// </para>
     /// </remarks>
     /// <param name="context">Contains the request context including the caller provided messages that will be used by the agent for this invocation.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
@@ -273,6 +286,10 @@ public abstract class ChatHistoryProvider
     /// <para>
     /// The default implementation of <see cref="InvokedCoreAsync"/> only calls this method if the invocation succeeded.
     /// </para>
+    /// <para>
+    /// <strong>Security consideration:</strong> Messages being stored may contain PII and sensitive conversation content.
+    /// Implementers should ensure appropriate encryption at rest and access controls for the storage backend.
+    /// </para>
     /// </remarks>
     protected virtual ValueTask StoreChatHistoryAsync(InvokedContext context, CancellationToken cancellationToken = default) =>
         default;

dotnet/src/Microsoft.Agents.AI.Abstractions/InMemoryChatHistoryProvider.cs

@@ -79,20 +79,21 @@ public sealed class InMemoryChatHistoryProvider : ChatHistoryProvider
     /// <exception cref="ArgumentNullException"><paramref name="messages"/> is <see langword="null"/>.</exception>
     public void SetMessages(AgentSession? session, List<ChatMessage> messages)
     {
-        _ = Throw.IfNull(messages);
+        Throw.IfNull(messages);
 
-        var state = this._sessionState.GetOrInitializeState(session);
+        State state = this._sessionState.GetOrInitializeState(session);
         state.Messages = messages;
     }
 
     /// <inheritdoc />
     protected override async ValueTask<IEnumerable<ChatMessage>> ProvideChatHistoryAsync(InvokingContext context, CancellationToken cancellationToken = default)
     {
-        var state = this._sessionState.GetOrInitializeState(context.Session);
+        State state = this._sessionState.GetOrInitializeState(context.Session);
 
         if (this.ReducerTriggerEvent is InMemoryChatHistoryProviderOptions.ChatReducerTriggerEvent.BeforeMessagesRetrieval && this.ChatReducer is not null)
         {
-            state.Messages = (await this.ChatReducer.ReduceAsync(state.Messages, cancellationToken).ConfigureAwait(false)).ToList();
+            // Apply pre-retrieval reduction if configured
+            await ReduceMessagesAsync(this.ChatReducer, state, cancellationToken).ConfigureAwait(false);
         }
 
         return state.Messages;
@@ -101,7 +102,7 @@ public sealed class InMemoryChatHistoryProvider : ChatHistoryProvider
     /// <inheritdoc />
     protected override async ValueTask StoreChatHistoryAsync(InvokedContext context, CancellationToken cancellationToken = default)
     {
-        var state = this._sessionState.GetOrInitializeState(context.Session);
+        State state = this._sessionState.GetOrInitializeState(context.Session);
 
         // Add request and response messages to the provider
         var allNewMessages = context.RequestMessages.Concat(context.ResponseMessages ?? []);
@@ -109,10 +110,16 @@ public sealed class InMemoryChatHistoryProvider : ChatHistoryProvider
 
         if (this.ReducerTriggerEvent is InMemoryChatHistoryProviderOptions.ChatReducerTriggerEvent.AfterMessageAdded && this.ChatReducer is not null)
         {
-            state.Messages = (await this.ChatReducer.ReduceAsync(state.Messages, cancellationToken).ConfigureAwait(false)).ToList();
+            // Apply pre-write reduction strategy if configured
+            await ReduceMessagesAsync(this.ChatReducer, state, cancellationToken).ConfigureAwait(false);
         }
     }
 
+    private static async Task ReduceMessagesAsync(IChatReducer reducer, State state, CancellationToken cancellationToken = default)
+    {
+        state.Messages = [.. await reducer.ReduceAsync(state.Messages, cancellationToken).ConfigureAwait(false)];
+    }
+
     /// <summary>
     /// Represents the state of a <see cref="InMemoryChatHistoryProvider"/> stored in the <see cref="AgentSession.StateBag"/>.
     /// </summary>

dotnet/src/Microsoft.Agents.AI.CosmosNoSql/CosmosChatHistoryProvider.cs

@@ -17,6 +17,24 @@ namespace Microsoft.Agents.AI;
 /// <summary>
 /// Provides a Cosmos DB implementation of the <see cref="ChatHistoryProvider"/> abstract class.
 /// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Security considerations:</strong>
+/// <list type="bullet">
+/// <item><description><strong>PII and sensitive data:</strong> Chat history stored in Cosmos DB may contain PII, sensitive conversation
+/// content, and system instructions. Ensure the Cosmos DB account is configured with appropriate access controls, encryption at rest,
+/// and network security (e.g., private endpoints, virtual network rules). The <see cref="MessageTtlSeconds"/> property can be used to
+/// automatically expire messages and limit data retention.</description></item>
+/// <item><description><strong>Compromised store risks:</strong> Agent Framework does not validate or filter messages loaded from the
+/// store — they are accepted as-is. If the Cosmos DB store is compromised, adversarial content could be injected into the conversation
+/// context, potentially influencing LLM behavior via indirect prompt injection. Altered message roles (e.g., changing <c>user</c> to
+/// <c>system</c>) could escalate trust levels.</description></item>
+/// <item><description><strong>Authentication:</strong> Agent Framework does not manage authentication or encryption for the Cosmos DB
+/// connection — these are the responsibility of the <see cref="CosmosClient"/> configuration. Use managed identity
+/// or token-based authentication where possible, and avoid embedding connection strings with keys in source code.</description></item>
+/// </list>
+/// </para>
+/// </remarks>
 [RequiresUnreferencedCode("The CosmosChatHistoryProvider uses JSON serialization which is incompatible with trimming.")]
 [RequiresDynamicCode("The CosmosChatHistoryProvider uses JSON serialization which is incompatible with NativeAOT.")]
 public sealed class CosmosChatHistoryProvider : ChatHistoryProvider, IDisposable

dotnet/src/Microsoft.Agents.AI.FoundryMemory/Microsoft.Agents.AI.FoundryMemory.csproj

@@ -13,10 +13,6 @@
   </PropertyGroup>
 
   <Import Project="$(RepoRoot)/dotnet/nuget/nuget-package.props" />
-  <PropertyGroup>
-    <!-- Disable packing until we are ready to release this as a nuget -->
-    <IsPackable>false</IsPackable>
-  </PropertyGroup>
 
   <ItemGroup>
     <ProjectReference Include="..\Microsoft.Agents.AI.Abstractions\Microsoft.Agents.AI.Abstractions.csproj" />

dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/AIAgentChatCompletionsProcessor.cs

@@ -72,9 +72,7 @@ internal static class AIAgentChatCompletionsProcessor
 
             await foreach (var agentResponseUpdate in agent.RunStreamingAsync(chatMessages, options: options, cancellationToken: cancellationToken).WithCancellation(cancellationToken))
             {
-                var finishReason = (agentResponseUpdate.RawRepresentation is ChatResponseUpdate { FinishReason: not null } chatResponseUpdate)
-                    ? chatResponseUpdate.FinishReason.ToString()
-                    : "stop";
+                var finishReason = agentResponseUpdate.FinishReason?.ToString() ?? "stop";
 
                 var choiceChunks = new List<ChatCompletionChoiceChunk>();
                 CompletionUsage? usageDetails = null;

dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/AgentResponseExtensions.cs

@@ -34,9 +34,7 @@ internal static class AgentResponseExtensions
         var chatCompletionChoices = new List<ChatCompletionChoice>();
         var index = 0;
 
-        var finishReason = (agentResponse.RawRepresentation is ChatResponse { FinishReason: not null } chatResponse)
-            ? chatResponse.FinishReason.ToString()
-            : "stop"; // "stop" is a natural stop point; returning this by-default
+        var finishReason = agentResponse.FinishReason?.ToString() ?? ChatFinishReason.Stop.Value; // "stop" is a natural stop point; returning this by-default
 
         foreach (var message in agentResponse.Messages)
         {

dotnet/src/Microsoft.Agents.AI.Mem0/Mem0Provider.cs

@@ -13,16 +13,38 @@ using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI.Mem0;
 
+#pragma warning disable IDE0001 // Simplify Names - Microsoft.Extensions.Logging.LogLevel.Trace doesn't get found in net472 when removing the namespace.
 /// <summary>
 /// Provides a Mem0 backed <see cref="MessageAIContextProvider"/> that persists conversation messages as memories
 /// and retrieves related memories to augment the agent invocation context.
 /// </summary>
 /// <remarks>
+/// <para>
 /// The provider stores user, assistant and system messages as Mem0 memories and retrieves relevant memories
 /// for new invocations using a semantic search endpoint. Retrieved memories are injected as user messages
 /// to the model, prefixed by a configurable context prompt.
+/// </para>
+/// <para>
+/// <strong>Security considerations:</strong>
+/// <list type="bullet">
+/// <item><description><strong>External service trust:</strong> This provider communicates with an external Mem0 service over HTTP.
+/// Agent Framework does not manage authentication, encryption, or connection details for this service — these are the responsibility
+/// of the <see cref="HttpClient"/> configuration. Ensure the HTTP client is configured with appropriate authentication
+/// and uses HTTPS to protect data in transit.</description></item>
+/// <item><description><strong>PII and sensitive data:</strong> Conversation messages (including user inputs, LLM responses, and system
+/// instructions) are sent to the external Mem0 service for storage. These messages may contain PII or sensitive information.
+/// Ensure the Mem0 service is configured with appropriate data retention policies and access controls.</description></item>
+/// <item><description><strong>Indirect prompt injection:</strong> Memories retrieved from the Mem0 service are injected into the LLM
+/// context as user messages. If the memory store is compromised, adversarial content could influence LLM behavior. The data
+/// returned from the service is accepted as-is without validation or sanitization.</description></item>
+/// <item><description><strong>Trace logging:</strong> When <see cref="Microsoft.Extensions.Logging.LogLevel.Trace"/> is enabled,
+/// full memory content (including search queries and results) may be logged. This data may contain PII and should not be enabled
+/// in production environments.</description></item>
+/// </list>
+/// </para>
 /// </remarks>
 public sealed class Mem0Provider : MessageAIContextProvider
+#pragma warning restore IDE0001 // Simplify Names
 {
     private const string DefaultContextPrompt = "## Memories\nConsider the following memories when answering user questions:";
 

dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs

@@ -3,6 +3,7 @@
 #pragma warning disable CS0618 // Type or member is obsolete - Internal use of obsolete types for backward compatibility
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -133,7 +134,25 @@ internal sealed class ExecutorProtocol(MessageRouter router, ISet<Type> sendType
 
     public bool CanHandle(Type type) => router.CanHandle(type);
 
-    public bool CanOutput(Type type) => this._yieldTypes.Contains(new(type));
+    private readonly ConcurrentDictionary<Type, bool> _canOutputCache = new();
+
+    public bool CanOutput(Type type)
+    {
+        return this._canOutputCache.GetOrAdd(type, this.CanOutputCore);
+    }
+
+    private bool CanOutputCore(Type type)
+    {
+        foreach (TypeId yieldType in this._yieldTypes)
+        {
+            if (yieldType.IsMatchPolymorphic(type))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
 
     public ProtocolDescriptor Describe() => new(this.Router.IncomingTypes, yieldTypes, sendTypes, this.Router.HasCatchAll);
 }

dotnet/src/Microsoft.Agents.AI.Workflows/MessageMerger.cs

@@ -124,6 +124,7 @@ internal sealed class MessageMerger
         List<ChatMessage> messages = [];
         Dictionary<string, AgentResponse> responses = [];
         HashSet<string> agentIds = [];
+        HashSet<ChatFinishReason> finishReasons = [];
 
         foreach (string responseId in this._mergeStates.Keys)
         {
@@ -156,6 +157,11 @@ internal sealed class MessageMerger
                 createdTimes.Add(response.CreatedAt.Value);
             }
 
+            if (response.FinishReason.HasValue)
+            {
+                finishReasons.Add(response.FinishReason.Value);
+            }
+
             usage = MergeUsage(usage, response.Usage);
             additionalProperties = MergeProperties(additionalProperties, response.AdditionalProperties);
         }
@@ -182,6 +188,7 @@ internal sealed class MessageMerger
             AgentId = primaryAgentId
                    ?? primaryAgentName
                    ?? (agentIds.Count == 1 ? agentIds.First() : null),
+            FinishReason = finishReasons.Count == 1 ? finishReasons.First() : null,
             CreatedAt = DateTimeOffset.UtcNow,
             Usage = usage,
             AdditionalProperties = additionalProperties
@@ -207,6 +214,7 @@ internal sealed class MessageMerger
                 AgentId = incoming.AgentId ?? current.AgentId,
                 AdditionalProperties = MergeProperties(current.AdditionalProperties, incoming.AdditionalProperties),
                 CreatedAt = incoming.CreatedAt ?? current.CreatedAt,
+                FinishReason = incoming.FinishReason ?? current.FinishReason,
                 Messages = current.Messages.Concat(incoming.Messages).ToList(),
                 ResponseId = current.ResponseId,
                 RawRepresentation = rawRepresentation,

dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs

@@ -17,6 +17,25 @@ namespace Microsoft.Agents.AI;
 /// <summary>
 /// Provides an <see cref="AIAgent"/> that delegates to an <see cref="IChatClient"/> implementation.
 /// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Security considerations:</strong> The <see cref="ChatClientAgent"/> orchestrates data flow across trust boundaries.
+/// The underlying AI service is an external endpoint and LLM responses should be treated as untrusted output. Developers should be aware of:
+/// <list type="bullet">
+/// <item><description><strong>Hallucination:</strong> LLMs may generate plausible-sounding but factually incorrect information.
+/// Do not treat LLM output as authoritative without verification.</description></item>
+/// <item><description><strong>Indirect prompt injection:</strong> Data retrieved by tools, AI context providers, or chat history providers may
+/// contain adversarial content designed to influence LLM behavior or exfiltrate data through tool calls.</description></item>
+/// <item><description><strong>Malicious payloads:</strong> LLM output may contain content that is harmful if rendered or executed without
+/// sanitization — for example, HTML/JavaScript for cross-site scripting, SQL for injection, or shell commands.</description></item>
+/// <item><description><strong>Tool invocation:</strong> By default, all tools provided to the agent are invoked without user approval.
+/// The AI selects which functions to call and with what arguments. Function arguments should be treated as untrusted input.
+/// Developers should require explicit approval for tools with side effects, data sensitivity, or irreversibility.</description></item>
+/// </list>
+/// Developers should validate and sanitize LLM output before rendering it in HTML, executing it as code, using it in database queries,
+/// or passing it to any security-sensitive context. Apply defense-in-depth by combining tool approval requirements with output validation.
+/// </para>
+/// </remarks>
 public sealed partial class ChatClientAgent : AIAgent
 {
     private readonly ChatClientAgentOptions? _agentOptions;
@@ -44,6 +63,9 @@ public sealed partial class ChatClientAgent : AIAgent
     /// Optional collection of tools that the agent can invoke during conversations.
     /// These tools augment any tools that may be provided to the agent via <see cref="ChatOptions.Tools"/> when
     /// the agent is run.
+    /// By default, all provided tools are invoked without user approval. The AI selects which functions to call and chooses
+    /// the arguments — these arguments should be treated as untrusted input. Developers should require explicit approval
+    /// for tools that have side effects, access sensitive data, or perform irreversible operations.
     /// </param>
     /// <param name="loggerFactory">
     /// Optional logger factory for creating loggers used by the agent and its components.

dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs

@@ -55,7 +55,7 @@ public static class ChatClientExtensions
 
         if (chatClient.GetService<FunctionInvokingChatClient>() is null)
         {
-            _ = chatBuilder.Use((innerClient, services) =>
+            chatBuilder.Use((innerClient, services) =>
             {
                 var loggerFactory = services.GetService<ILoggerFactory>();
 

dotnet/src/Microsoft.Agents.AI/Compaction/ChatMessageContentEquality.cs

@@ -0,0 +1,159 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Content-based equality comparison for <see cref="ChatMessage"/> instances.
+/// </summary>
+internal static class ChatMessageContentEquality
+{
+    /// <summary>
+    /// Determines whether two <see cref="ChatMessage"/> instances represent the same message by content.
+    /// </summary>
+    /// <remarks>
+    /// When both messages define a <see cref="ChatMessage.MessageId"/>, identity is determined solely
+    /// by that identifier.  Otherwise, the comparison falls through to <see cref="ChatMessage.Role"/>,
+    /// <see cref="ChatMessage.AuthorName"/>, and each item in <see cref="ChatMessage.Contents"/>.
+    /// </remarks>
+    internal static bool ContentEquals(this ChatMessage? message, ChatMessage? other)
+    {
+        if (ReferenceEquals(message, other))
+        {
+            return true;
+        }
+
+        if (message is null || other is null)
+        {
+            return false;
+        }
+
+        // A matching MessageId is sufficient.
+        if (message.MessageId is not null && other.MessageId is not null)
+        {
+            return string.Equals(message.MessageId, other.MessageId, StringComparison.Ordinal);
+        }
+
+        if (message.Role != other.Role)
+        {
+            return false;
+        }
+
+        if (!string.Equals(message.AuthorName, other.AuthorName, StringComparison.Ordinal))
+        {
+            return false;
+        }
+
+        return ContentsEqual(message.Contents, other.Contents);
+    }
+
+    private static bool ContentsEqual(IList<AIContent> left, IList<AIContent> right)
+    {
+        if (left.Count != right.Count)
+        {
+            return false;
+        }
+
+        for (int i = 0; i < left.Count; i++)
+        {
+            if (!ContentItemEquals(left[i], right[i]))
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private static bool ContentItemEquals(AIContent left, AIContent right)
+    {
+        if (ReferenceEquals(left, right))
+        {
+            return true;
+        }
+
+        if (left.GetType() != right.GetType())
+        {
+            return false;
+        }
+
+        return (left, right) switch
+        {
+            (TextContent a, TextContent b) => TextContentEquals(a, b),
+            (TextReasoningContent a, TextReasoningContent b) => TextReasoningContentEquals(a, b),
+            (DataContent a, DataContent b) => DataContentEquals(a, b),
+            (UriContent a, UriContent b) => UriContentEquals(a, b),
+            (ErrorContent a, ErrorContent b) => ErrorContentEquals(a, b),
+            (FunctionCallContent a, FunctionCallContent b) => FunctionCallContentEquals(a, b),
+            (FunctionResultContent a, FunctionResultContent b) => FunctionResultContentEquals(a, b),
+            (HostedFileContent a, HostedFileContent b) => HostedFileContentEquals(a, b),
+            (AIContent a, AIContent b) => a.GetType() == b.GetType(),
+        };
+    }
+
+    private static bool TextContentEquals(TextContent a, TextContent b) =>
+        string.Equals(a.Text, b.Text, StringComparison.Ordinal);
+
+    private static bool TextReasoningContentEquals(TextReasoningContent a, TextReasoningContent b) =>
+        string.Equals(a.Text, b.Text, StringComparison.Ordinal) &&
+        string.Equals(a.ProtectedData, b.ProtectedData, StringComparison.Ordinal);
+
+    private static bool DataContentEquals(DataContent a, DataContent b) =>
+        string.Equals(a.MediaType, b.MediaType, StringComparison.Ordinal) &&
+        string.Equals(a.Name, b.Name, StringComparison.Ordinal) &&
+        a.Data.Span.SequenceEqual(b.Data.Span);
+
+    private static bool UriContentEquals(UriContent a, UriContent b) =>
+        Equals(a.Uri, b.Uri) &&
+        string.Equals(a.MediaType, b.MediaType, StringComparison.Ordinal);
+
+    private static bool ErrorContentEquals(ErrorContent a, ErrorContent b) =>
+        string.Equals(a.Message, b.Message, StringComparison.Ordinal) &&
+        string.Equals(a.ErrorCode, b.ErrorCode, StringComparison.Ordinal) &&
+        Equals(a.Details, b.Details);
+
+    private static bool FunctionCallContentEquals(FunctionCallContent a, FunctionCallContent b) =>
+        string.Equals(a.CallId, b.CallId, StringComparison.Ordinal) &&
+        string.Equals(a.Name, b.Name, StringComparison.Ordinal) &&
+        ArgumentsEqual(a.Arguments, b.Arguments);
+
+    private static bool FunctionResultContentEquals(FunctionResultContent a, FunctionResultContent b) =>
+        string.Equals(a.CallId, b.CallId, StringComparison.Ordinal) &&
+        Equals(a.Result, b.Result);
+
+    private static bool ArgumentsEqual(IDictionary<string, object?>? left, IDictionary<string, object?>? right)
+    {
+        if (ReferenceEquals(left, right))
+        {
+            return true;
+        }
+
+        if (left is null || right is null)
+        {
+            return false;
+        }
+
+        if (left.Count != right.Count)
+        {
+            return false;
+        }
+
+        foreach (KeyValuePair<string, object?> entry in left)
+        {
+            if (!right.TryGetValue(entry.Key, out object? value) || !Equals(entry.Value, value))
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private static bool HostedFileContentEquals(HostedFileContent a, HostedFileContent b) =>
+        string.Equals(a.FileId, b.FileId, StringComparison.Ordinal) &&
+        string.Equals(a.MediaType, b.MediaType, StringComparison.Ordinal) &&
+        string.Equals(a.Name, b.Name, StringComparison.Ordinal);
+}

dotnet/src/Microsoft.Agents.AI/Compaction/ChatReducerCompactionStrategy.cs

@@ -0,0 +1,82 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A compaction strategy that delegates to an <see cref="IChatReducer"/> to reduce the conversation's
+/// included messages.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This strategy bridges the <see cref="IChatReducer"/> abstraction from <c>Microsoft.Extensions.AI</c>
+/// into the compaction pipeline. It collects the currently included messages from the
+/// <see cref="CompactionMessageIndex"/>, passes them to the reducer, and rebuilds the index from the
+/// reduced message list when the reducer produces fewer messages.
+/// </para>
+/// <para>
+/// The <see cref="CompactionTrigger"/> controls when reduction is attempted.
+/// Use <see cref="CompactionTriggers"/> for common trigger conditions such as token or message thresholds.
+/// </para>
+/// <para>
+/// Use this strategy when you have an existing <see cref="IChatReducer"/> implementation
+/// (such as <c>MessageCountingChatReducer</c>) and want to apply it as part of a
+/// <see cref="CompactionStrategy"/> pipeline or as an in-run compaction strategy.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class ChatReducerCompactionStrategy : CompactionStrategy
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ChatReducerCompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="chatReducer">
+    /// The <see cref="IChatReducer"/> that performs the message reduction.
+    /// </param>
+    /// <param name="trigger">
+    /// The <see cref="CompactionTrigger"/> that controls when compaction proceeds.
+    /// </param>
+    public ChatReducerCompactionStrategy(IChatReducer chatReducer, CompactionTrigger trigger)
+        : base(trigger)
+    {
+        this.ChatReducer = Throw.IfNull(chatReducer);
+    }
+
+    /// <summary>
+    /// Gets the chat reducer used to reduce messages.
+    /// </summary>
+    public IChatReducer ChatReducer { get; }
+
+    /// <inheritdoc/>
+    protected override async ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+    {
+        // No need to short-circuit on empty conversations, this is handled by <see cref="CompactionStrategy.CompactAsync"/>.
+        List<ChatMessage> includedMessages = [.. index.GetIncludedMessages()];
+
+        IEnumerable<ChatMessage> reduced = await this.ChatReducer.ReduceAsync(includedMessages, cancellationToken).ConfigureAwait(false);
+        IList<ChatMessage> reducedMessages = reduced as IList<ChatMessage> ?? [.. reduced];
+
+        if (reducedMessages.Count >= includedMessages.Count)
+        {
+            return false;
+        }
+
+        // Rebuild the index from the reduced messages
+        CompactionMessageIndex rebuilt = CompactionMessageIndex.Create(reducedMessages, index.Tokenizer);
+        index.Groups.Clear();
+        foreach (CompactionMessageGroup group in rebuilt.Groups)
+        {
+            index.Groups.Add(group);
+        }
+
+        return true;
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionGroupKind.cs

@@ -0,0 +1,55 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Identifies the kind of a <see cref="CompactionMessageGroup"/>.
+/// </summary>
+/// <remarks>
+/// Message groups are used to classify logically related messages that must be kept together
+/// during compaction operations. For example, an assistant message containing tool calls
+/// and its corresponding tool result messages form an atomic <see cref="ToolCall"/> group.
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public enum CompactionGroupKind
+{
+    /// <summary>
+    /// A system message group containing one or more system messages.
+    /// </summary>
+    System,
+
+    /// <summary>
+    /// A user message group containing a single user message.
+    /// </summary>
+    User,
+
+    /// <summary>
+    /// An assistant message group containing a single assistant text response (no tool calls).
+    /// </summary>
+    AssistantText,
+
+    /// <summary>
+    /// An atomic tool call group containing an assistant message with tool calls
+    /// followed by the corresponding tool result messages.
+    /// </summary>
+    /// <remarks>
+    /// This group must be treated as an atomic unit during compaction. Removing the assistant
+    /// message without its tool results (or vice versa) will cause LLM API errors.
+    /// </remarks>
+    ToolCall,
+
+#pragma warning disable IDE0001 // Simplify Names
+    /// <summary>
+    /// A summary message group produced by a compaction strategy (e.g., <c>SummarizationCompactionStrategy</c>).
+    /// </summary>
+    /// <remarks>
+    /// Summary groups replace previously compacted messages with a condensed representation.
+    /// They are identified by the <see cref="CompactionMessageGroup.SummaryPropertyKey"/> metadata entry
+    /// on the underlying <see cref="Microsoft.Extensions.AI.ChatMessage"/>.
+    /// </remarks>
+#pragma warning restore IDE0001 // Simplify Names
+    Summary,
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionLogMessages.cs

@@ -0,0 +1,112 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Extensions.Logging;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+#pragma warning disable SYSLIB1006 // Multiple logging methods cannot use the same event id within a class
+
+/// <summary>
+/// Extensions for logging compaction diagnostics.
+/// </summary>
+/// <remarks>
+/// This extension uses the <see cref="LoggerMessageAttribute"/> to
+/// generate logging code at compile time to achieve optimized code.
+/// </remarks>
+[ExcludeFromCodeCoverage]
+internal static partial class CompactionLogMessages
+{
+    /// <summary>
+    /// Logs when compaction is skipped because the trigger condition was not met.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Trace,
+        Message = "Compaction skipped for {StrategyName}: trigger condition not met or insufficient groups.")]
+    public static partial void LogCompactionSkipped(
+        this ILogger logger,
+        string strategyName);
+
+    /// <summary>
+    /// Logs compaction completion with before/after metrics.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Debug,
+        Message = "Compaction completed: {StrategyName} in {DurationMs}ms — Messages {BeforeMessages}→{AfterMessages}, Groups {BeforeGroups}→{AfterGroups}, Tokens {BeforeTokens}→{AfterTokens}")]
+    public static partial void LogCompactionCompleted(
+        this ILogger logger,
+        string strategyName,
+        long durationMs,
+        int beforeMessages,
+        int afterMessages,
+        int beforeGroups,
+        int afterGroups,
+        int beforeTokens,
+        int afterTokens);
+
+    /// <summary>
+    /// Logs when the compaction provider skips compaction.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Trace,
+        Message = "CompactionProvider skipped: {Reason}.")]
+    public static partial void LogCompactionProviderSkipped(
+        this ILogger logger,
+        string reason);
+
+    /// <summary>
+    /// Logs when the compaction provider begins applying a compaction strategy.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Debug,
+        Message = "CompactionProvider applying compaction to {MessageCount} messages using {StrategyName}.")]
+    public static partial void LogCompactionProviderApplying(
+        this ILogger logger,
+        int messageCount,
+        string strategyName);
+
+    /// <summary>
+    /// Logs when the compaction provider has applied compaction with result metrics.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Debug,
+        Message = "CompactionProvider compaction applied: messages {BeforeMessages}→{AfterMessages}.")]
+    public static partial void LogCompactionProviderApplied(
+        this ILogger logger,
+        int beforeMessages,
+        int afterMessages);
+
+    /// <summary>
+    /// Logs when a summarization LLM call is starting.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Debug,
+        Message = "Summarization starting for {GroupCount} groups ({MessageCount} messages) using {ChatClientType}.")]
+    public static partial void LogSummarizationStarting(
+        this ILogger logger,
+        int groupCount,
+        int messageCount,
+        string chatClientType);
+
+    /// <summary>
+    /// Logs when a summarization LLM call has completed.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Debug,
+        Message = "Summarization completed: summary length {SummaryLength} characters, inserted at index {InsertIndex}.")]
+    public static partial void LogSummarizationCompleted(
+        this ILogger logger,
+        int summaryLength,
+        int insertIndex);
+
+    /// <summary>
+    /// Logs when a summarization LLM call fails and groups are restored.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Warning,
+        Message = "Summarization failed for {GroupCount} groups; restoring excluded groups and continuing without compaction. Error: {ErrorMessage}")]
+    public static partial void LogSummarizationFailed(
+        this ILogger logger,
+        int groupCount,
+        string errorMessage);
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageGroup.cs

@@ -0,0 +1,116 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+using Microsoft.Extensions.AI;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Represents a logical group of <see cref="ChatMessage"/> instances that must be kept or removed together during compaction.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Message groups ensure atomic preservation of related messages. For example, an assistant message
+/// containing tool calls and its corresponding tool result messages form a <see cref="CompactionGroupKind.ToolCall"/>
+/// group — removing one without the other would cause LLM API errors.
+/// </para>
+/// <para>
+/// Groups also support exclusion semantics: a group can be marked as excluded (with an optional reason)
+/// to indicate it should not be included in the messages sent to the model, while still being preserved
+/// for diagnostics, storage, or later re-inclusion.
+/// </para>
+/// <para>
+/// Each group tracks its <see cref="MessageCount"/>, <see cref="ByteCount"/>, and <see cref="TokenCount"/>
+/// so that <see cref="CompactionMessageIndex"/> can efficiently aggregate totals across all or only included groups.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class CompactionMessageGroup
+{
+    /// <summary>
+    /// The <see cref="ChatMessage.AdditionalProperties"/> key used to identify a message as a compaction summary.
+    /// </summary>
+    /// <remarks>
+    /// When this key is present with a value of <see langword="true"/>, the message is classified as
+    /// <see cref="CompactionGroupKind.Summary"/> by <see cref="CompactionMessageIndex.Create"/>.
+    /// </remarks>
+    public static readonly string SummaryPropertyKey = "_is_summary";
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CompactionMessageGroup"/> class.
+    /// </summary>
+    /// <param name="kind">The kind of message group.</param>
+    /// <param name="messages">The messages in this group. The list is captured as a read-only snapshot.</param>
+    /// <param name="byteCount">The total UTF-8 byte count of the text content in the messages.</param>
+    /// <param name="tokenCount">The token count for the messages, computed by a tokenizer or estimated.</param>
+    /// <param name="turnIndex">
+    /// The user turn this group belongs to, or <see langword="null"/> for <see cref="CompactionGroupKind.System"/>.
+    /// </param>
+    [JsonConstructor]
+    internal CompactionMessageGroup(CompactionGroupKind kind, IReadOnlyList<ChatMessage> messages, int byteCount, int tokenCount, int? turnIndex = null)
+    {
+        this.Kind = kind;
+        this.Messages = messages;
+        this.MessageCount = messages.Count;
+        this.ByteCount = byteCount;
+        this.TokenCount = tokenCount;
+        this.TurnIndex = turnIndex;
+    }
+
+    /// <summary>
+    /// Gets the kind of this message group.
+    /// </summary>
+    public CompactionGroupKind Kind { get; }
+
+    /// <summary>
+    /// Gets the messages in this group.
+    /// </summary>
+    public IReadOnlyList<ChatMessage> Messages { get; }
+
+    /// <summary>
+    /// Gets the number of messages in this group.
+    /// </summary>
+    public int MessageCount { get; }
+
+    /// <summary>
+    /// Gets the total UTF-8 byte count of the text content in this group's messages.
+    /// </summary>
+    public int ByteCount { get; }
+
+    /// <summary>
+    /// Gets the estimated or actual token count for this group's messages.
+    /// </summary>
+    public int TokenCount { get; }
+
+    /// <summary>
+    /// Gets user turn index this group belongs to, or <see langword="null"/> for groups
+    /// that precede the first user message (e.g., system messages).  A turn index of 0
+    /// corresponds with any non-system message that precedes the first user message,
+    /// turn index 1 corresponds with the first user message and its subsequent non-user
+    /// messages, and so on...
+    /// </summary>
+    /// <remarks>
+    /// A turn starts with a <see cref="CompactionGroupKind.User"/> group and includes all subsequent
+    /// non-user, non-system groups until the next user group or end of conversation.  System messages
+    /// (<see cref="CompactionGroupKind.System"/>) are always assigned a <see langword="null"/> turn index
+    /// since they never belong to a user turn.
+    /// </remarks>
+    public int? TurnIndex { get; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether this group is excluded from the projected message list.
+    /// </summary>
+    /// <remarks>
+    /// Excluded groups are preserved in the collection for diagnostics or storage purposes
+    /// but are not included when calling <see cref="CompactionMessageIndex.GetIncludedMessages"/>.
+    /// </remarks>
+    public bool IsExcluded { get; set; }
+
+    /// <summary>
+    /// Gets or sets an optional reason explaining why this group was excluded.
+    /// </summary>
+    public string? ExcludeReason { get; set; }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageIndex.cs

@@ -0,0 +1,529 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Text;
+using Microsoft.Extensions.AI;
+using Microsoft.ML.Tokenizers;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A collection of <see cref="CompactionMessageGroup"/> instances and derived metrics based on a flat list of <see cref="ChatMessage"/> objects.
+/// </summary>
+/// <remarks>
+/// <see cref="CompactionMessageIndex"/> provides structural grouping of messages into logical <see cref="CompactionMessageGroup"/> units.  Individual
+/// groups can be marked as excluded without being removed, allowing compaction strategies to toggle visibility while preserving
+/// the full history for diagnostics or storage.  Metrics are provided both including and excluding excluded groups,
+/// allowing strategies to make informed decisions based on the impact of potential exclusions.
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class CompactionMessageIndex
+{
+    private int _currentTurn;
+    private ChatMessage? _lastProcessedMessage;
+
+    /// <summary>
+    /// Gets the list of message groups in this collection.
+    /// </summary>
+    public IList<CompactionMessageGroup> Groups { get; }
+
+    /// <summary>
+    /// Gets the tokenizer used for computing token counts, or <see langword="null"/> if token counts are estimated.
+    /// </summary>
+    public Tokenizer? Tokenizer { get; }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CompactionMessageIndex"/> class with the specified groups.
+    /// </summary>
+    /// <param name="groups">The message groups.</param>
+    /// <param name="tokenizer">An optional tokenizer retained for computing token counts when adding new groups.</param>
+    public CompactionMessageIndex(IList<CompactionMessageGroup> groups, Tokenizer? tokenizer = null)
+    {
+        this.Groups = Throw.IfNull(groups, nameof(groups));
+        this.Tokenizer = tokenizer;
+
+        // Restore turn counter and last processed message from the groups
+        for (int index = groups.Count - 1; index >= 0; --index)
+        {
+            if (this._lastProcessedMessage is null && this.Groups[index].Kind != CompactionGroupKind.Summary)
+            {
+                IReadOnlyList<ChatMessage> groupMessages = this.Groups[index].Messages;
+                this._lastProcessedMessage = groupMessages[^1];
+            }
+
+            if (this.Groups[index].TurnIndex.HasValue)
+            {
+                this._currentTurn = this.Groups[index].TurnIndex!.Value;
+
+                // Both values restored — no need to keep scanning
+                if (this._lastProcessedMessage is not null)
+                {
+                    break;
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Creates a <see cref="CompactionMessageIndex"/> from a flat list of <see cref="ChatMessage"/> instances.
+    /// </summary>
+    /// <param name="messages">The messages to group.</param>
+    /// <param name="tokenizer">
+    /// An optional <see cref="Tokenizer"/> for computing token counts on each group.
+    /// When <see langword="null"/>, token counts are estimated as <c>ByteCount / 4</c>.
+    /// </param>
+    /// <returns>A new <see cref="CompactionMessageIndex"/> with messages organized into logical groups.</returns>
+    /// <remarks>
+    /// The grouping algorithm:
+    /// <list type="bullet">
+    /// <item><description>System messages become <see cref="CompactionGroupKind.System"/> groups.</description></item>
+    /// <item><description>User messages become <see cref="CompactionGroupKind.User"/> groups.</description></item>
+    /// <item><description>Assistant messages with tool calls, followed by their corresponding tool result messages, become <see cref="CompactionGroupKind.ToolCall"/> groups.</description></item>
+    /// <item><description>Assistant messages marked with <see cref="CompactionMessageGroup.SummaryPropertyKey"/> become <see cref="CompactionGroupKind.Summary"/> groups.</description></item>
+    /// <item><description>Assistant messages without tool calls become <see cref="CompactionGroupKind.AssistantText"/> groups.</description></item>
+    /// </list>
+    /// </remarks>
+    internal static CompactionMessageIndex Create(IList<ChatMessage> messages, Tokenizer? tokenizer = null)
+    {
+        CompactionMessageIndex instance = new([], tokenizer);
+        instance.AppendFromMessages(messages, 0);
+        return instance;
+    }
+
+    /// <summary>
+    /// Incrementally updates the groups with new messages from the conversation.
+    /// </summary>
+    /// <param name="allMessages">
+    /// The full list of messages for the conversation. This must be the same list (or a replacement with the same
+    /// prefix) that was used to create or last update this instance.
+    /// </param>
+    /// <remarks>
+    /// <para>
+    /// Uses equality on the last processed message to detect changes.  Only the messages after that position are
+    /// processed and appended as new groups. Existing groups and their compaction state (exclusions) are preserved.
+    /// </para>
+    /// <para>
+    /// If the last processed message is not found (e.g., the message list was replaced entirely
+    /// or a sliding window shifted past it), all groups are cleared and rebuilt from scratch.
+    /// </para>
+    /// <para>
+    /// If the last message in <paramref name="allMessages"/> matches the last
+    /// processed message, no work is performed.
+    /// </para>
+    /// </remarks>
+    internal void Update(IList<ChatMessage> allMessages)
+    {
+        if (allMessages.Count == 0)
+        {
+            this.Groups.Clear();
+            this._currentTurn = 0;
+            this._lastProcessedMessage = null;
+            return;
+        }
+
+        // If the last message is unchanged and the list hasn't shrunk, there is nothing new to process.
+        if (this._lastProcessedMessage is not null &&
+            allMessages.Count >= this.RawMessageCount &&
+            allMessages[allMessages.Count - 1].ContentEquals(this._lastProcessedMessage))
+        {
+            return;
+        }
+
+        // Walk backwards to locate where we left off.
+        int foundIndex = -1;
+        if (this._lastProcessedMessage is not null)
+        {
+            for (int i = allMessages.Count - 1; i >= 0; --i)
+            {
+                if (allMessages[i].ContentEquals(this._lastProcessedMessage))
+                {
+                    foundIndex = i;
+                    break;
+                }
+            }
+        }
+
+        if (foundIndex < 0)
+        {
+            // Last processed message not found — total rebuild.
+            this.Groups.Clear();
+            this._currentTurn = 0;
+            this.AppendFromMessages(allMessages, 0);
+            return;
+        }
+
+        // Guard against a sliding window that removed messages from the front:
+        // the number of messages up to (and including) the found position must
+        // match the number of messages already represented by existing groups.
+        if (foundIndex + 1 < this.RawMessageCount)
+        {
+            // Front of the message list was trimmed — rebuild.
+            this.Groups.Clear();
+            this._currentTurn = 0;
+            this.AppendFromMessages(allMessages, 0);
+            return;
+        }
+
+        // Process only the delta messages.
+        this.AppendFromMessages(allMessages, foundIndex + 1);
+    }
+
+    private void AppendFromMessages(IList<ChatMessage> messages, int startIndex)
+    {
+        int index = startIndex;
+
+        while (index < messages.Count)
+        {
+            ChatMessage message = messages[index];
+
+            if (message.Role == ChatRole.System)
+            {
+                // System messages are not part of any turn
+                this.Groups.Add(CreateGroup(CompactionGroupKind.System, [message], this.Tokenizer, turnIndex: null));
+                index++;
+            }
+            else if (message.Role == ChatRole.User)
+            {
+                this._currentTurn++;
+                this.Groups.Add(CreateGroup(CompactionGroupKind.User, [message], this.Tokenizer, this._currentTurn));
+                index++;
+            }
+            else if (message.Role == ChatRole.Assistant && HasToolCalls(message))
+            {
+                List<ChatMessage> groupMessages = [message];
+                index++;
+
+                // Collect all subsequent tool result messages and reasoning-only assistant messages
+                while (index < messages.Count &&
+                       (messages[index].Role == ChatRole.Tool ||
+                        (messages[index].Role == ChatRole.Assistant && HasOnlyReasoning(messages[index]))))
+                {
+                    groupMessages.Add(messages[index]);
+                    index++;
+                }
+
+                this.Groups.Add(CreateGroup(CompactionGroupKind.ToolCall, groupMessages, this.Tokenizer, this._currentTurn));
+            }
+            else if (message.Role == ChatRole.Assistant && IsSummaryMessage(message))
+            {
+                this.Groups.Add(CreateGroup(CompactionGroupKind.Summary, [message], this.Tokenizer, this._currentTurn));
+                index++;
+            }
+            else if (message.Role == ChatRole.Assistant && HasOnlyReasoning(message))
+            {
+                // Reasoning-only assistant messages that precede a tool-call assistant message
+                // are part of the same atomic tool-call group. Look ahead past consecutive
+                // reasoning messages to find a possible tool-call message.
+                int lookahead = index + 1;
+                while (lookahead < messages.Count &&
+                       messages[lookahead].Role == ChatRole.Assistant &&
+                       HasOnlyReasoning(messages[lookahead]))
+                {
+                    lookahead++;
+                }
+
+                if (lookahead < messages.Count && messages[lookahead].Role == ChatRole.Assistant && HasToolCalls(messages[lookahead]))
+                {
+                    // Group all reasoning messages + the tool-call message together
+                    List<ChatMessage> groupMessages = [];
+                    for (int j = index; j <= lookahead; j++)
+                    {
+                        groupMessages.Add(messages[j]);
+                    }
+
+                    index = lookahead + 1;
+
+                    // Collect all subsequent tool result messages and reasoning-only assistant messages
+                    while (index < messages.Count &&
+                           (messages[index].Role == ChatRole.Tool ||
+                            (messages[index].Role == ChatRole.Assistant && HasOnlyReasoning(messages[index]))))
+                    {
+                        groupMessages.Add(messages[index]);
+                        index++;
+                    }
+
+                    this.Groups.Add(CreateGroup(CompactionGroupKind.ToolCall, groupMessages, this.Tokenizer, this._currentTurn));
+                }
+                else
+                {
+                    this.Groups.Add(CreateGroup(CompactionGroupKind.AssistantText, [message], this.Tokenizer, this._currentTurn));
+                    index++;
+                }
+            }
+            else
+            {
+                this.Groups.Add(CreateGroup(CompactionGroupKind.AssistantText, [message], this.Tokenizer, this._currentTurn));
+                index++;
+            }
+        }
+
+        if (messages.Count > 0)
+        {
+            this._lastProcessedMessage = messages[^1];
+        }
+    }
+
+    /// <summary>
+    /// Creates a new <see cref="CompactionMessageGroup"/> with byte and token counts computed using this collection's
+    /// <see cref="Tokenizer"/>, and adds it to the <see cref="Groups"/> list at the specified index.
+    /// </summary>
+    /// <param name="index">The zero-based index at which the group should be inserted.</param>
+    /// <param name="kind">The kind of message group.</param>
+    /// <param name="messages">The messages in the group.</param>
+    /// <param name="turnIndex">The optional turn index to assign to the new group.</param>
+    /// <returns>The newly created <see cref="CompactionMessageGroup"/>.</returns>
+    public CompactionMessageGroup InsertGroup(int index, CompactionGroupKind kind, IReadOnlyList<ChatMessage> messages, int? turnIndex = null)
+    {
+        CompactionMessageGroup group = CreateGroup(kind, messages, this.Tokenizer, turnIndex);
+        this.Groups.Insert(index, group);
+        return group;
+    }
+
+    /// <summary>
+    /// Creates a new <see cref="CompactionMessageGroup"/> with byte and token counts computed using this collection's
+    /// <see cref="Tokenizer"/>, and appends it to the end of the <see cref="Groups"/> list.
+    /// </summary>
+    /// <param name="kind">The kind of message group.</param>
+    /// <param name="messages">The messages in the group.</param>
+    /// <param name="turnIndex">The optional turn index to assign to the new group.</param>
+    /// <returns>The newly created <see cref="CompactionMessageGroup"/>.</returns>
+    public CompactionMessageGroup AddGroup(CompactionGroupKind kind, IReadOnlyList<ChatMessage> messages, int? turnIndex = null)
+    {
+        CompactionMessageGroup group = CreateGroup(kind, messages, this.Tokenizer, turnIndex);
+        this.Groups.Add(group);
+        return group;
+    }
+
+    /// <summary>
+    /// Returns only the messages from groups that are not excluded.
+    /// </summary>
+    /// <returns>A list of <see cref="ChatMessage"/> instances from included groups, in order.</returns>
+    public IEnumerable<ChatMessage> GetIncludedMessages() =>
+        this.Groups.Where(group => !group.IsExcluded).SelectMany(group => group.Messages);
+
+    /// <summary>
+    /// Returns all messages from all groups, including excluded ones.
+    /// </summary>
+    /// <returns>A list of all <see cref="ChatMessage"/> instances, in order.</returns>
+    public IEnumerable<ChatMessage> GetAllMessages() => this.Groups.SelectMany(group => group.Messages);
+
+    /// <summary>
+    /// Gets the total number of groups, including excluded ones.
+    /// </summary>
+    public int TotalGroupCount => this.Groups.Count;
+
+    /// <summary>
+    /// Gets the total number of messages across all groups, including excluded ones.
+    /// </summary>
+    public int TotalMessageCount => this.Groups.Sum(group => group.MessageCount);
+
+    /// <summary>
+    /// Gets the total UTF-8 byte count across all groups, including excluded ones.
+    /// </summary>
+    public int TotalByteCount => this.Groups.Sum(group => group.ByteCount);
+
+    /// <summary>
+    /// Gets the total token count across all groups, including excluded ones.
+    /// </summary>
+    public int TotalTokenCount => this.Groups.Sum(group => group.TokenCount);
+
+    /// <summary>
+    /// Gets the total number of groups that are not excluded.
+    /// </summary>
+    public int IncludedGroupCount => this.Groups.Count(group => !group.IsExcluded);
+
+    /// <summary>
+    /// Gets the total number of messages across all included (non-excluded) groups.
+    /// </summary>
+    public int IncludedMessageCount => this.Groups.Where(group => !group.IsExcluded).Sum(group => group.MessageCount);
+
+    /// <summary>
+    /// Gets the total UTF-8 byte count across all included (non-excluded) groups.
+    /// </summary>
+    public int IncludedByteCount => this.Groups.Where(group => !group.IsExcluded).Sum(group => group.ByteCount);
+
+    /// <summary>
+    /// Gets the total token count across all included (non-excluded) groups.
+    /// </summary>
+    public int IncludedTokenCount => this.Groups.Where(group => !group.IsExcluded).Sum(group => group.TokenCount);
+
+    /// <summary>
+    /// Gets the total number of user turns across all groups (including those with excluded groups).
+    /// </summary>
+    public int TotalTurnCount => this.Groups.Select(group => group.TurnIndex).Distinct().Count(turnIndex => turnIndex is not null && turnIndex > 0);
+
+    /// <summary>
+    /// Gets the number of user turns that have at least one non-excluded group.
+    /// </summary>
+    public int IncludedTurnCount => this.Groups.Where(group => !group.IsExcluded && group.TurnIndex is not null && group.TurnIndex > 0).Select(group => group.TurnIndex).Distinct().Count();
+
+    /// <summary>
+    /// Gets the total number of groups across all included (non-excluded) groups that are not <see cref="CompactionGroupKind.System"/>.
+    /// </summary>
+    public int IncludedNonSystemGroupCount => this.Groups.Count(group => !group.IsExcluded && group.Kind != CompactionGroupKind.System);
+
+    /// <summary>
+    /// Gets the total number of original messages (that are not summaries).
+    /// </summary>
+    public int RawMessageCount => this.Groups.Where(group => group.Kind != CompactionGroupKind.Summary).Sum(group => group.MessageCount);
+
+    /// <summary>
+    /// Returns all groups that belong to the specified user turn.
+    /// </summary>
+    /// <param name="turnIndex">The desired turn index.</param>
+    /// <returns>The groups belonging to the turn, in order.</returns>
+    public IEnumerable<CompactionMessageGroup> GetTurnGroups(int turnIndex) => this.Groups.Where(group => group.TurnIndex == turnIndex);
+
+    /// <summary>
+    /// Computes the UTF-8 byte count for a set of messages across all content types.
+    /// </summary>
+    /// <param name="messages">The messages to compute byte count for.</param>
+    /// <returns>The total UTF-8 byte count of all message content.</returns>
+    internal static int ComputeByteCount(IReadOnlyList<ChatMessage> messages)
+    {
+        int total = 0;
+        for (int i = 0; i < messages.Count; i++)
+        {
+            IList<AIContent> contents = messages[i].Contents;
+            for (int j = 0; j < contents.Count; j++)
+            {
+                total += ComputeContentByteCount(contents[j]);
+            }
+        }
+
+        return total;
+    }
+
+    /// <summary>
+    /// Computes the token count for a set of messages using the specified tokenizer.
+    /// </summary>
+    /// <param name="messages">The messages to compute token count for.</param>
+    /// <param name="tokenizer">The tokenizer to use for counting tokens.</param>
+    /// <returns>The total token count across all message content.</returns>
+    /// <remarks>
+    /// Text-bearing content (<see cref="TextContent"/> and <see cref="TextReasoningContent"/>)
+    /// is tokenized directly. All other content types estimate tokens as <c>byteCount / 4</c>.
+    /// </remarks>
+    internal static int ComputeTokenCount(IReadOnlyList<ChatMessage> messages, Tokenizer tokenizer)
+    {
+        int total = 0;
+        for (int i = 0; i < messages.Count; i++)
+        {
+            IList<AIContent> contents = messages[i].Contents;
+            for (int j = 0; j < contents.Count; j++)
+            {
+                AIContent content = contents[j];
+                switch (content)
+                {
+                    case TextContent text:
+                        if (text.Text is { Length: > 0 } t)
+                        {
+                            total += tokenizer.CountTokens(t);
+                        }
+
+                        break;
+
+                    case TextReasoningContent reasoning:
+                        if (reasoning.Text is { Length: > 0 } rt)
+                        {
+                            total += tokenizer.CountTokens(rt);
+                        }
+
+                        if (reasoning.ProtectedData is { Length: > 0 } pd)
+                        {
+                            total += tokenizer.CountTokens(pd);
+                        }
+
+                        break;
+
+                    default:
+                        total += ComputeContentByteCount(content) / 4;
+                        break;
+                }
+            }
+        }
+
+        return total;
+    }
+
+    private static int ComputeContentByteCount(AIContent content)
+    {
+        switch (content)
+        {
+            case TextContent text:
+                return GetStringByteCount(text.Text);
+
+            case TextReasoningContent reasoning:
+                return GetStringByteCount(reasoning.Text) + GetStringByteCount(reasoning.ProtectedData);
+
+            case DataContent data:
+                return data.Data.Length + GetStringByteCount(data.MediaType) + GetStringByteCount(data.Name);
+
+            case UriContent uri:
+                return (uri.Uri is Uri uriValue ? GetStringByteCount(uriValue.OriginalString) : 0) + GetStringByteCount(uri.MediaType);
+
+            case FunctionCallContent call:
+                int callBytes = GetStringByteCount(call.CallId) + GetStringByteCount(call.Name);
+                if (call.Arguments is not null)
+                {
+                    foreach (KeyValuePair<string, object?> arg in call.Arguments)
+                    {
+                        callBytes += GetStringByteCount(arg.Key);
+                        callBytes += GetStringByteCount(arg.Value?.ToString());
+                    }
+                }
+
+                return callBytes;
+
+            case FunctionResultContent result:
+                return GetStringByteCount(result.CallId) + GetStringByteCount(result.Result?.ToString());
+
+            case ErrorContent error:
+                return GetStringByteCount(error.Message) + GetStringByteCount(error.ErrorCode) + GetStringByteCount(error.Details);
+
+            case HostedFileContent file:
+                return GetStringByteCount(file.FileId) + GetStringByteCount(file.MediaType) + GetStringByteCount(file.Name);
+
+            default:
+                return 0;
+        }
+    }
+
+    private static int GetStringByteCount(string? value) =>
+        value is { Length: > 0 } ? Encoding.UTF8.GetByteCount(value) : 0;
+
+    private static CompactionMessageGroup CreateGroup(CompactionGroupKind kind, IReadOnlyList<ChatMessage> messages, Tokenizer? tokenizer, int? turnIndex)
+    {
+        int byteCount = ComputeByteCount(messages);
+        int tokenCount = tokenizer is not null
+            ? ComputeTokenCount(messages, tokenizer)
+            : byteCount / 4;
+
+        return new CompactionMessageGroup(kind, messages, byteCount, tokenCount, turnIndex);
+    }
+
+    private static bool HasToolCalls(ChatMessage message)
+    {
+        foreach (AIContent content in message.Contents)
+        {
+            if (content is FunctionCallContent)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private static bool HasOnlyReasoning(ChatMessage message) =>
+        message.Contents.All(content => content is TextReasoningContent);
+
+    private static bool IsSummaryMessage(ChatMessage message) =>
+        message.AdditionalProperties?.TryGetValue(CompactionMessageGroup.SummaryPropertyKey, out object? value) is true
+            && value is true;
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionProvider.cs

@@ -0,0 +1,186 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A <see cref="AIContextProvider"/> that applies a <see cref="CompactionStrategy"/> to compact
+/// the message list before each agent invocation.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This provider performs in-run compaction by organizing messages into atomic groups (preserving
+/// tool-call/result pairings) before applying compaction logic. Only included messages are forwarded
+/// to the agent's underlying chat client.
+/// </para>
+/// <para>
+/// The <see cref="CompactionProvider"/> can be added to an agent's context provider pipeline
+/// via <see cref="ChatClientAgentOptions.AIContextProviders"/> or via <c>UseAIContextProviders</c>
+/// on a <see cref="ChatClientBuilder"/> or <see cref="AIAgentBuilder"/>.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class CompactionProvider : AIContextProvider
+{
+    private readonly CompactionStrategy _compactionStrategy;
+    private readonly ProviderSessionState<State> _sessionState;
+    private readonly ILoggerFactory? _loggerFactory;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CompactionProvider"/> class.
+    /// </summary>
+    /// <param name="compactionStrategy">The compaction strategy to apply before each invocation.</param>
+    /// <param name="stateKey">
+    /// An optional key used to store the provider state in the <see cref="AgentSession.StateBag"/>.  Provide
+    /// an explicit value if configuring multiple agents with different compaction strategies that will interact
+    /// in the same session.
+    /// </param>
+    /// <param name="loggerFactory">
+    /// An optional <see cref="ILoggerFactory"/> used to create a logger for provider diagnostics.
+    /// When <see langword="null"/>, logging is disabled.
+    /// </param>
+    /// <exception cref="ArgumentNullException"><paramref name="compactionStrategy"/> is <see langword="null"/>.</exception>
+    public CompactionProvider(CompactionStrategy compactionStrategy, string? stateKey = null, ILoggerFactory? loggerFactory = null)
+    {
+        this._compactionStrategy = Throw.IfNull(compactionStrategy);
+        stateKey ??= this._compactionStrategy.GetType().Name;
+        this.StateKeys = [stateKey];
+        this._sessionState = new ProviderSessionState<State>(
+            _ => new State(),
+            stateKey,
+            AgentJsonUtilities.DefaultOptions);
+        this._loggerFactory = loggerFactory;
+    }
+
+    /// <inheritdoc />
+    public override IReadOnlyList<string> StateKeys { get; }
+
+    /// <summary>
+    /// Applies compaction strategy to the provided message list and returns the compacted messages.
+    /// This can be used for ad-hoc compaction outside of the provider pipeline.
+    /// </summary>
+    /// <param name="compactionStrategy">The compaction strategy to apply before each invocation.</param>
+    /// <param name="messages">The messages to compact</param>
+    /// <param name="logger">An optional <see cref="ILogger"/> for emitting compaction diagnostics.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
+    /// <returns>An enumeration of the compacted <see cref="ChatMessage"/> instances.</returns>
+    public static async Task<IEnumerable<ChatMessage>> CompactAsync(CompactionStrategy compactionStrategy, IEnumerable<ChatMessage> messages, ILogger? logger = null, CancellationToken cancellationToken = default)
+    {
+        Throw.IfNull(compactionStrategy);
+        Throw.IfNull(messages);
+
+        List<ChatMessage> messageList = messages as List<ChatMessage> ?? [.. messages];
+        CompactionMessageIndex messageIndex = CompactionMessageIndex.Create(messageList);
+
+        await compactionStrategy.CompactAsync(messageIndex, logger, cancellationToken).ConfigureAwait(false);
+
+        return messageIndex.GetIncludedMessages();
+    }
+
+    /// <summary>
+    /// Applies the compaction strategy to the accumulated message list before forwarding it to the agent.
+    /// </summary>
+    /// <param name="context">Contains the request context including all accumulated messages.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
+    /// <returns>
+    /// A task that represents the asynchronous operation. The task result contains an <see cref="AIContext"/>
+    /// with the compacted message list.
+    /// </returns>
+    protected override async ValueTask<AIContext> InvokingCoreAsync(InvokingContext context, CancellationToken cancellationToken = default)
+    {
+        using Activity? activity = CompactionTelemetry.ActivitySource.StartActivity(CompactionTelemetry.ActivityNames.CompactionProviderInvoke);
+
+        ILoggerFactory loggerFactory = this.GetLoggerFactory(context.Agent);
+        ILogger logger = loggerFactory.CreateLogger<CompactionProvider>();
+
+        AgentSession? session = context.Session;
+        IEnumerable<ChatMessage>? allMessages = context.AIContext.Messages;
+
+        if (session is null || allMessages is null)
+        {
+            logger.LogCompactionProviderSkipped("no session or no messages");
+            return context.AIContext;
+        }
+
+        ChatClientAgentSession? chatClientSession = session.GetService<ChatClientAgentSession>();
+        if (chatClientSession is not null &&
+            !string.IsNullOrWhiteSpace(chatClientSession.ConversationId))
+        {
+            logger.LogCompactionProviderSkipped("session managed by remote service");
+            return context.AIContext;
+        }
+
+        List<ChatMessage> messageList = allMessages as List<ChatMessage> ?? [.. allMessages];
+
+        State state = this._sessionState.GetOrInitializeState(session);
+
+        CompactionMessageIndex messageIndex;
+        if (state.MessageGroups.Count > 0)
+        {
+            // Update existing index with any new messages appended since the last call.
+            messageIndex = new([.. state.MessageGroups]);
+            messageIndex.Update(messageList);
+        }
+        else
+        {
+            // First pass — initialize the message index from scratch.
+            messageIndex = CompactionMessageIndex.Create(messageList);
+        }
+
+        string strategyName = this._compactionStrategy.GetType().Name;
+        int beforeMessages = messageIndex.IncludedMessageCount;
+        logger.LogCompactionProviderApplying(beforeMessages, strategyName);
+
+        // Apply compaction
+        await this._compactionStrategy.CompactAsync(
+            messageIndex,
+            loggerFactory.CreateLogger(this._compactionStrategy.GetType()),
+            cancellationToken).ConfigureAwait(false);
+
+        int afterMessages = messageIndex.IncludedMessageCount;
+        if (afterMessages < beforeMessages)
+        {
+            logger.LogCompactionProviderApplied(beforeMessages, afterMessages);
+        }
+
+        // Persist the index
+        state.MessageGroups.Clear();
+        state.MessageGroups.AddRange(messageIndex.Groups);
+
+        return new AIContext
+        {
+            Instructions = context.AIContext.Instructions,
+            Messages = messageIndex.GetIncludedMessages(),
+            Tools = context.AIContext.Tools
+        };
+    }
+
+    private ILoggerFactory GetLoggerFactory(AIAgent agent) =>
+        this._loggerFactory ??
+        agent.GetService<IChatClient>()?.GetService<ILoggerFactory>() ??
+        NullLoggerFactory.Instance;
+
+    /// <summary>
+    /// Represents the persisted state of a <see cref="CompactionProvider"/> stored in the <see cref="AgentSession.StateBag"/>.
+    /// </summary>
+    internal sealed class State
+    {
+        /// <summary>
+        /// Gets or sets the message index groups used for incremental compaction updates.
+        /// </summary>
+        [JsonPropertyName("messagegroups")]
+        public List<CompactionMessageGroup> MessageGroups { get; set; } = [];
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionStrategy.cs

@@ -0,0 +1,164 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Base class for strategies that compact a <see cref="CompactionMessageIndex"/> to reduce context size.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Compaction strategies operate on <see cref="CompactionMessageIndex"/> instances, which organize messages
+/// into atomic groups that respect the tool-call/result pairing constraint. Strategies mutate the collection
+/// in place by marking groups as excluded, removing groups, or replacing message content (e.g., with summaries).
+/// </para>
+/// <para>
+/// Every strategy requires a <see cref="CompactionTrigger"/> that determines whether compaction should
+/// proceed based on current <see cref="CompactionMessageIndex"/> metrics (token count, message count, turn count, etc.).
+/// The base class evaluates this trigger at the start of <see cref="CompactAsync"/> and skips compaction when
+/// the trigger returns <see langword="false"/>.
+/// </para>
+/// <para>
+/// An optional <b>target</b> condition controls when compaction stops. Strategies incrementally exclude
+/// groups and re-evaluate the target after each exclusion, stopping as soon as the target returns
+/// <see langword="true"/>. When no target is specified, it defaults to the inverse of the trigger —
+/// meaning compaction stops when the trigger condition would no longer fire.
+/// </para>
+/// <para>
+/// Strategies can be applied at three lifecycle points:
+/// <list type="bullet">
+/// <item><description><b>In-run</b>: During the tool loop, before each LLM call, to keep context within token limits.</description></item>
+/// <item><description><b>Pre-write</b>: Before persisting messages to storage via <see cref="ChatHistoryProvider"/>.</description></item>
+/// <item><description><b>On existing storage</b>: As a maintenance operation to compact stored history.</description></item>
+/// </list>
+/// </para>
+/// <para>
+/// Multiple strategies can be composed by applying them sequentially to the same <see cref="CompactionMessageIndex"/>
+/// via <see cref="PipelineCompactionStrategy"/>.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public abstract class CompactionStrategy
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="trigger">
+    /// The <see cref="CompactionTrigger"/> that determines whether compaction should proceed.
+    /// </param>
+    /// <param name="target">
+    /// An optional target condition that controls when compaction stops. Strategies re-evaluate
+    /// this predicate after each incremental exclusion and stop when it returns <see langword="true"/>.
+    /// When <see langword="null"/>, defaults to the inverse of the <paramref name="trigger"/> — compaction
+    /// stops as soon as the trigger condition would no longer fire.
+    /// </param>
+    protected CompactionStrategy(CompactionTrigger trigger, CompactionTrigger? target = null)
+    {
+        this.Trigger = Throw.IfNull(trigger);
+        this.Target = target ?? (index => !trigger(index));
+    }
+
+    /// <summary>
+    /// Gets the trigger predicate that controls when compaction proceeds.
+    /// </summary>
+    protected CompactionTrigger Trigger { get; }
+
+    /// <summary>
+    /// Gets the target predicate that controls when compaction stops.
+    /// Strategies re-evaluate this after each incremental exclusion and stop when it returns <see langword="true"/>.
+    /// </summary>
+    protected CompactionTrigger Target { get; }
+
+    /// <summary>
+    /// Applies the strategy-specific compaction logic to the specified message index.
+    /// </summary>
+    /// <remarks>
+    /// This method is called by <see cref="CompactAsync"/> only when the <see cref="Trigger"/>
+    /// returns <see langword="true"/>. Implementations do not need to evaluate the trigger or
+    /// report metrics — the base class handles both. Implementations should use <see cref="Target"/>
+    /// to determine when to stop compacting incrementally.
+    /// </remarks>
+    /// <param name="index">The message index to compact. The strategy mutates this collection in place.</param>
+    /// <param name="logger">The <see cref="ILogger"/> for emitting compaction diagnostics.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
+    /// <returns>A task whose result is <see langword="true"/> if any compaction was performed, <see langword="false"/> otherwise.</returns>
+    protected abstract ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken);
+
+    /// <summary>
+    /// Evaluates the <see cref="Trigger"/> and, when it fires, delegates to
+    /// <see cref="CompactCoreAsync"/> and reports compaction metrics.
+    /// </summary>
+    /// <param name="index">The message index to compact. The strategy mutates this collection in place.</param>
+    /// <param name="logger">An optional <see cref="ILogger"/> for emitting compaction diagnostics. When <see langword="null"/>, logging is disabled.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
+    /// <returns>A task representing the asynchronous operation. The task result is <see langword="true"/> if compaction occurred, <see langword="false"/> otherwise.</returns>
+    public async ValueTask<bool> CompactAsync(CompactionMessageIndex index, ILogger? logger = null, CancellationToken cancellationToken = default)
+    {
+        string strategyName = this.GetType().Name;
+        logger ??= NullLogger.Instance;
+
+        using Activity? activity = CompactionTelemetry.ActivitySource.StartActivity(CompactionTelemetry.ActivityNames.Compact);
+        activity?.SetTag(CompactionTelemetry.Tags.Strategy, strategyName);
+
+        if (index.IncludedNonSystemGroupCount <= 1 || !this.Trigger(index))
+        {
+            activity?.SetTag(CompactionTelemetry.Tags.Triggered, false);
+            logger.LogCompactionSkipped(strategyName);
+            return false;
+        }
+
+        activity?.SetTag(CompactionTelemetry.Tags.Triggered, true);
+
+        int beforeTokens = index.IncludedTokenCount;
+        int beforeGroups = index.IncludedGroupCount;
+        int beforeMessages = index.IncludedMessageCount;
+
+        Stopwatch stopwatch = Stopwatch.StartNew();
+
+        bool compacted = await this.CompactCoreAsync(index, logger, cancellationToken).ConfigureAwait(false);
+
+        stopwatch.Stop();
+
+        activity?.SetTag(CompactionTelemetry.Tags.Compacted, compacted);
+
+        if (compacted)
+        {
+            activity?
+                .SetTag(CompactionTelemetry.Tags.BeforeTokens, beforeTokens)
+                .SetTag(CompactionTelemetry.Tags.AfterTokens, index.IncludedTokenCount)
+                .SetTag(CompactionTelemetry.Tags.BeforeMessages, beforeMessages)
+                .SetTag(CompactionTelemetry.Tags.AfterMessages, index.IncludedMessageCount)
+                .SetTag(CompactionTelemetry.Tags.BeforeGroups, beforeGroups)
+                .SetTag(CompactionTelemetry.Tags.AfterGroups, index.IncludedGroupCount)
+                .SetTag(CompactionTelemetry.Tags.DurationMs, stopwatch.ElapsedMilliseconds);
+
+            logger.LogCompactionCompleted(
+                strategyName,
+                stopwatch.ElapsedMilliseconds,
+                beforeMessages,
+                index.IncludedMessageCount,
+                beforeGroups,
+                index.IncludedGroupCount,
+                beforeTokens,
+                index.IncludedTokenCount);
+        }
+
+        return compacted;
+    }
+
+    /// <summary>
+    /// Ensures the provided value is not a negative number.
+    /// </summary>
+    /// <param name="value">The target value.</param>
+    /// <returns>0 if negative; otherwise the value</returns>
+    protected static int EnsureNonNegative(int value) => Math.Max(0, value);
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTelemetry.cs

@@ -0,0 +1,45 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Provides shared telemetry infrastructure for compaction operations.
+/// </summary>
+internal static class CompactionTelemetry
+{
+    /// <summary>
+    /// The <see cref="ActivitySource"/> used to create activities for compaction operations.
+    /// </summary>
+    public static readonly ActivitySource ActivitySource = new(OpenTelemetryConsts.DefaultSourceName);
+
+    /// <summary>
+    /// Activity names used by compaction tracing.
+    /// </summary>
+    public static class ActivityNames
+    {
+        public const string Compact = "compaction.compact";
+        public const string CompactionProviderInvoke = "compaction.provider.invoke";
+        public const string Summarize = "compaction.summarize";
+    }
+
+    /// <summary>
+    /// Tag names used on compaction activities.
+    /// </summary>
+    public static class Tags
+    {
+        public const string Strategy = "compaction.strategy";
+        public const string Triggered = "compaction.triggered";
+        public const string Compacted = "compaction.compacted";
+        public const string BeforeTokens = "compaction.before.tokens";
+        public const string AfterTokens = "compaction.after.tokens";
+        public const string BeforeMessages = "compaction.before.messages";
+        public const string AfterMessages = "compaction.after.messages";
+        public const string BeforeGroups = "compaction.before.groups";
+        public const string AfterGroups = "compaction.after.groups";
+        public const string DurationMs = "compaction.duration_ms";
+        public const string GroupsSummarized = "compaction.groups_summarized";
+        public const string SummaryLength = "compaction.summary_length";
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTrigger.cs

@@ -0,0 +1,15 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Defines a condition based on <see cref="CompactionMessageIndex"/> metrics used by a <see cref="CompactionStrategy"/>
+/// to determine when to trigger compaction and when the target compaction threshold has been met.
+/// </summary>
+/// <param name="index">An index over conversation messages that provides group, token, message, and turn metrics.</param>
+/// <returns><see langword="true"/> to indicate the condition has been met; otherwise <see langword="false"/>.</returns>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public delegate bool CompactionTrigger(CompactionMessageIndex index);

dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTriggers.cs

@@ -0,0 +1,134 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Factory to create <see cref="CompactionTrigger"/> predicates.
+/// </summary>
+/// <remarks>
+/// <para>
+/// A <see cref="CompactionTrigger"/> defines a condition based on <see cref="CompactionMessageIndex"/> metrics used
+/// by a <see cref="CompactionStrategy"/> to determine when to trigger compaction and when the target
+/// compaction threshold has been met.
+/// </para>
+/// <para>
+/// Combine triggers with <see cref="All"/> or <see cref="Any"/> for compound conditions.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public static class CompactionTriggers
+{
+    /// <summary>
+    /// Always trigger, regardless of the message index state.
+    /// </summary>
+    public static readonly CompactionTrigger Always =
+        _ => true;
+
+    /// <summary>
+    /// Never trigger, regardless of the message index state.
+    /// </summary>
+    public static readonly CompactionTrigger Never =
+        _ => false;
+
+    /// <summary>
+    /// Creates a trigger that fires when the included token count is below the specified maximum.
+    /// </summary>
+    /// <param name="maxTokens">The token threshold.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that evaluates included token count.</returns>
+    public static CompactionTrigger TokensBelow(int maxTokens) =>
+        index => index.IncludedTokenCount < maxTokens;
+
+    /// <summary>
+    /// Creates a trigger that fires when the included token count exceeds the specified maximum.
+    /// </summary>
+    /// <param name="maxTokens">The token threshold.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that evaluates included token count.</returns>
+    public static CompactionTrigger TokensExceed(int maxTokens) =>
+        index => index.IncludedTokenCount > maxTokens;
+
+    /// <summary>
+    /// Creates a trigger that fires when the included message count exceeds the specified maximum.
+    /// </summary>
+    /// <param name="maxMessages">The message threshold.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that evaluates included message count.</returns>
+    public static CompactionTrigger MessagesExceed(int maxMessages) =>
+        index => index.IncludedMessageCount > maxMessages;
+
+    /// <summary>
+    /// Creates a trigger that fires when the included user turn count exceeds the specified maximum.
+    /// </summary>
+    /// <param name="maxTurns">The turn threshold.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that evaluates included turn count.</returns>
+    /// <remarks>
+    /// <para>
+    /// A user turn starts with a <see cref="CompactionGroupKind.User"/> group and includes all subsequent
+    /// non-user, non-system groups until the next user group or end of conversation.  Each group is assigned
+    /// a <see cref="CompactionMessageGroup.TurnIndex"/> indicating which user turn it belongs to.
+    /// System messages (<see cref="CompactionGroupKind.System"/>) are always assigned a <see langword="null"/>
+    /// <see cref="CompactionMessageGroup.TurnIndex"/> since they never belong to a user turn.
+    /// </para>
+    /// <para>
+    /// The turn count is the number of distinct values defined by <see cref="CompactionMessageGroup.TurnIndex"/>.
+    /// </para>
+    /// </remarks>
+    public static CompactionTrigger TurnsExceed(int maxTurns) =>
+        index => index.IncludedTurnCount > maxTurns;
+
+    /// <summary>
+    /// Creates a trigger that fires when the included group count exceeds the specified maximum.
+    /// </summary>
+    /// <param name="maxGroups">The group threshold.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that evaluates included group count.</returns>
+    public static CompactionTrigger GroupsExceed(int maxGroups) =>
+        index => index.IncludedGroupCount > maxGroups;
+
+    /// <summary>
+    /// Creates a trigger that fires when the included message index contains at least one
+    /// non-excluded <see cref="CompactionGroupKind.ToolCall"/> group.
+    /// </summary>
+    /// <returns>A <see cref="CompactionTrigger"/> that evaluates included tool call presence.</returns>
+    public static CompactionTrigger HasToolCalls() =>
+        index => index.Groups.Any(g => !g.IsExcluded && g.Kind == CompactionGroupKind.ToolCall);
+
+    /// <summary>
+    /// Creates a compound trigger that fires only when <b>all</b> of the specified triggers fire.
+    /// </summary>
+    /// <param name="triggers">The triggers to combine with logical AND.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that requires all conditions to be met.</returns>
+    public static CompactionTrigger All(params CompactionTrigger[] triggers) =>
+        index =>
+        {
+            for (int i = 0; i < triggers.Length; i++)
+            {
+                if (!triggers[i](index))
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        };
+
+    /// <summary>
+    /// Creates a compound trigger that fires when <b>any</b> of the specified triggers fire.
+    /// </summary>
+    /// <param name="triggers">The triggers to combine with logical OR.</param>
+    /// <returns>A <see cref="CompactionTrigger"/> that requires at least one condition to be met.</returns>
+    public static CompactionTrigger Any(params CompactionTrigger[] triggers) =>
+        index =>
+        {
+            for (int i = 0; i < triggers.Length; i++)
+            {
+                if (triggers[i](index))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        };
+}

dotnet/src/Microsoft.Agents.AI/Compaction/PipelineCompactionStrategy.cs

@@ -0,0 +1,62 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A compaction strategy that executes a sequential pipeline of <see cref="CompactionStrategy"/> instances
+/// against the same <see cref="CompactionMessageIndex"/>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Each strategy in the pipeline operates on the result of the previous one, enabling composed behaviors
+/// such as summarizing older messages first and then truncating to fit a token budget.
+/// </para>
+/// <para>
+/// The pipeline itself always executes while each child strategy evaluates its own
+/// <see cref="CompactionStrategy.Trigger"/> independently to decide whether it should compact.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class PipelineCompactionStrategy : CompactionStrategy
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="PipelineCompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="strategies">The ordered sequence of strategies to execute.</param>
+    public PipelineCompactionStrategy(params IEnumerable<CompactionStrategy> strategies)
+        : base(CompactionTriggers.Always)
+    {
+        this.Strategies = [.. Throw.IfNull(strategies)];
+    }
+
+    /// <summary>
+    /// Gets the ordered list of strategies in this pipeline.
+    /// </summary>
+    public IReadOnlyList<CompactionStrategy> Strategies { get; }
+
+    /// <inheritdoc/>
+    protected override async ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+    {
+        bool anyCompacted = false;
+
+        foreach (CompactionStrategy strategy in this.Strategies)
+        {
+            bool compacted = await strategy.CompactAsync(index, logger, cancellationToken).ConfigureAwait(false);
+
+            if (compacted)
+            {
+                anyCompacted = true;
+            }
+        }
+
+        return anyCompacted;
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/SlidingWindowCompactionStrategy.cs

@@ -0,0 +1,140 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A compaction strategy that removes the oldest user turns and their associated response groups
+/// to bound conversation length.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This strategy always preserves system messages. It identifies user turns in the
+/// conversation (via <see cref="CompactionMessageGroup.TurnIndex"/>) and excludes the oldest turns
+/// one at a time until the <see cref="CompactionStrategy.Target"/> condition is met.
+/// </para>
+/// <para>
+/// <see cref="MinimumPreservedTurns"/> is a hard floor: even if the <see cref="CompactionStrategy.Target"/>
+/// has not been reached, compaction will not touch the last <see cref="MinimumPreservedTurns"/> turns
+/// (by <see cref="CompactionMessageGroup.TurnIndex"/>). Groups with a <see cref="CompactionMessageGroup.TurnIndex"/>
+/// of <c>0</c> or <see langword="null"/> are always preserved regardless of this setting.
+/// </para>
+/// <para>
+/// This strategy is more predictable than token-based truncation for bounding conversation
+/// length, since it operates on logical turn boundaries rather than estimated token counts.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class SlidingWindowCompactionStrategy : CompactionStrategy
+{
+    /// <summary>
+    /// The default minimum number of most-recent turns to preserve.
+    /// </summary>
+    public const int DefaultMinimumPreserved = 1;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SlidingWindowCompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="trigger">
+    /// The <see cref="CompactionTrigger"/> that controls when compaction proceeds.
+    /// Use <see cref="CompactionTriggers.TurnsExceed"/> for turn-based thresholds.
+    /// </param>
+    /// <param name="minimumPreservedTurns">
+    /// The minimum number of most-recent turns (by <see cref="CompactionMessageGroup.TurnIndex"/>) to preserve.
+    /// This is a hard floor — compaction will not exclude turns within this range, regardless of the target condition.
+    /// Groups with <see cref="CompactionMessageGroup.TurnIndex"/> of <c>0</c> or <see langword="null"/> are always preserved.
+    /// </param>
+    /// <param name="target">
+    /// An optional target condition that controls when compaction stops. When <see langword="null"/>,
+    /// defaults to the inverse of the <paramref name="trigger"/> — compaction stops as soon as the trigger would no longer fire.
+    /// </param>
+    public SlidingWindowCompactionStrategy(CompactionTrigger trigger, int minimumPreservedTurns = DefaultMinimumPreserved, CompactionTrigger? target = null)
+        : base(trigger, target)
+    {
+        this.MinimumPreservedTurns = EnsureNonNegative(minimumPreservedTurns);
+    }
+
+    /// <summary>
+    /// Gets the minimum number of most-recent turns (by <see cref="CompactionMessageGroup.TurnIndex"/>) that are always preserved.
+    /// This is a hard floor that compaction cannot exceed, regardless of the target condition.
+    /// Groups with <see cref="CompactionMessageGroup.TurnIndex"/> of <c>0</c> or <see langword="null"/> are always preserved
+    /// independently of this value.
+    /// </summary>
+    public int MinimumPreservedTurns { get; }
+
+    /// <inheritdoc/>
+    protected override ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+    {
+        // Forward pass: pre-index non-system included groups by TurnIndex.
+        Dictionary<int, List<int>> turnGroups = [];
+        List<int> turnOrder = [];
+
+        for (int i = 0; i < index.Groups.Count; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (!group.IsExcluded && group.Kind != CompactionGroupKind.System && group.TurnIndex is int turnIndex)
+            {
+                if (!turnGroups.TryGetValue(turnIndex, out List<int>? indices))
+                {
+                    indices = [];
+                    turnGroups[turnIndex] = indices;
+                    turnOrder.Add(turnIndex);
+                }
+
+                indices.Add(i);
+            }
+        }
+
+        // Backward pass: identify protected turns by TurnIndex.
+        // TurnIndex = 0 is always protected (non-system messages before first user message).
+        // TurnIndex = null is always protected (system messages, already excluded from turn tracking).
+        HashSet<int> protectedTurnIndices = [];
+        if (turnGroups.ContainsKey(0))
+        {
+            protectedTurnIndices.Add(0);
+        }
+
+        // Protect the last MinimumPreservedTurns distinct turns.
+        int turnsToProtect = Math.Min(this.MinimumPreservedTurns, turnOrder.Count);
+        for (int i = turnOrder.Count - turnsToProtect; i < turnOrder.Count; i++)
+        {
+            protectedTurnIndices.Add(turnOrder[i]);
+        }
+
+        // Exclude turns oldest-first, skipping protected turns, checking target after each turn.
+        bool compacted = false;
+
+        for (int t = 0; t < turnOrder.Count; t++)
+        {
+            int currentTurnIndex = turnOrder[t];
+            if (protectedTurnIndices.Contains(currentTurnIndex))
+            {
+                continue;
+            }
+
+            List<int> groupIndices = turnGroups[currentTurnIndex];
+            for (int g = 0; g < groupIndices.Count; g++)
+            {
+                int idx = groupIndices[g];
+                index.Groups[idx].IsExcluded = true;
+                index.Groups[idx].ExcludeReason = $"Excluded by {nameof(SlidingWindowCompactionStrategy)}";
+            }
+
+            compacted = true;
+
+            if (this.Target(index))
+            {
+                break;
+            }
+        }
+
+        return new ValueTask<bool>(compacted);
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/SummarizationCompactionStrategy.cs

@@ -0,0 +1,207 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A compaction strategy that uses an LLM to summarize older portions of the conversation,
+/// replacing them with a single summary message that preserves key facts and context.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This strategy protects system messages and the most recent <see cref="MinimumPreservedGroups"/>
+/// non-system groups. All older groups are collected and sent to the <see cref="IChatClient"/>
+/// for summarization. The resulting summary replaces those messages as a single assistant message
+/// with <see cref="CompactionGroupKind.Summary"/>.
+/// </para>
+/// <para>
+/// <see cref="MinimumPreservedGroups"/> is a hard floor: even if the <see cref="CompactionStrategy.Target"/>
+/// has not been reached, compaction will not touch the last <see cref="MinimumPreservedGroups"/> non-system groups.
+/// </para>
+/// <para>
+/// The <see cref="CompactionTrigger"/> predicate controls when compaction proceeds. Use
+/// <see cref="CompactionTriggers"/> for common trigger conditions such as token thresholds.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class SummarizationCompactionStrategy : CompactionStrategy
+{
+    /// <summary>
+    /// The default summarization prompt used when none is provided.
+    /// </summary>
+    public const string DefaultSummarizationPrompt =
+        """
+        You are a conversation summarizer. Produce a concise summary of the conversation that preserves:
+
+        - Key facts, decisions, and user preferences
+        - Important context needed for future turns
+        - Tool call outcomes and their significance
+
+        Omit pleasantries and redundant exchanges. Be factual and brief.
+        """;
+
+    /// <summary>
+    /// The default minimum number of most-recent non-system groups to preserve.
+    /// </summary>
+    public const int DefaultMinimumPreserved = 8;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SummarizationCompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="chatClient">The <see cref="IChatClient"/> to use for generating summaries. A smaller, faster model is recommended.</param>
+    /// <param name="trigger">
+    /// The <see cref="CompactionTrigger"/> that controls when compaction proceeds.
+    /// </param>
+    /// <param name="minimumPreservedGroups">
+    /// The minimum number of most-recent non-system message groups to preserve.
+    /// This is a hard floor — compaction will not summarize groups beyond this limit,
+    /// regardless of the target condition. Defaults to 8, preserving the current and recent exchanges.
+    /// </param>
+    /// <param name="summarizationPrompt">
+    /// An optional custom system prompt for the summarization LLM call. When <see langword="null"/>,
+    /// <see cref="DefaultSummarizationPrompt"/> is used.
+    /// </param>
+    /// <param name="target">
+    /// An optional target condition that controls when compaction stops. When <see langword="null"/>,
+    /// defaults to the inverse of the <paramref name="trigger"/> — compaction stops as soon as the trigger would no longer fire.
+    /// </param>
+    public SummarizationCompactionStrategy(
+        IChatClient chatClient,
+        CompactionTrigger trigger,
+        int minimumPreservedGroups = DefaultMinimumPreserved,
+        string? summarizationPrompt = null,
+        CompactionTrigger? target = null)
+        : base(trigger, target)
+    {
+        this.ChatClient = Throw.IfNull(chatClient);
+        this.MinimumPreservedGroups = EnsureNonNegative(minimumPreservedGroups);
+        this.SummarizationPrompt = summarizationPrompt ?? DefaultSummarizationPrompt;
+    }
+
+    /// <summary>
+    /// Gets the chat client used for generating summaries.
+    /// </summary>
+    public IChatClient ChatClient { get; }
+
+    /// <summary>
+    /// Gets the minimum number of most-recent non-system groups that are always preserved.
+    /// This is a hard floor that compaction cannot exceed, regardless of the target condition.
+    /// </summary>
+    public int MinimumPreservedGroups { get; }
+
+    /// <summary>
+    /// Gets the prompt used when requesting summaries from the chat client.
+    /// </summary>
+    public string SummarizationPrompt { get; }
+
+    /// <inheritdoc/>
+    protected override async ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+    {
+        // Count non-system, non-excluded groups to determine which are protected
+        int nonSystemIncludedCount = 0;
+        for (int i = 0; i < index.Groups.Count; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (!group.IsExcluded && group.Kind != CompactionGroupKind.System)
+            {
+                nonSystemIncludedCount++;
+            }
+        }
+
+        int protectedFromEnd = Math.Min(this.MinimumPreservedGroups, nonSystemIncludedCount);
+        int maxSummarizable = nonSystemIncludedCount - protectedFromEnd;
+
+        if (maxSummarizable <= 0)
+        {
+            return false;
+        }
+
+        // Mark oldest non-system groups for summarization one at a time until the target is met.
+        // Track which groups were excluded so we can restore them if the LLM call fails.
+        List<ChatMessage> summarizationMessages = [new ChatMessage(ChatRole.System, this.SummarizationPrompt)];
+        List<CompactionMessageGroup> excludedGroups = [];
+        int insertIndex = -1;
+
+        for (int i = 0; i < index.Groups.Count && excludedGroups.Count < maxSummarizable; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (group.IsExcluded || group.Kind == CompactionGroupKind.System)
+            {
+                continue;
+            }
+
+            if (insertIndex < 0)
+            {
+                insertIndex = i;
+            }
+
+            // Collect messages from this group for summarization
+            summarizationMessages.AddRange(group.Messages);
+
+            group.IsExcluded = true;
+            group.ExcludeReason = $"Summarized by {nameof(SummarizationCompactionStrategy)}";
+            excludedGroups.Add(group);
+
+            // Stop marking when target condition is met
+            if (this.Target(index))
+            {
+                break;
+            }
+        }
+
+        // Generate summary using the chat client (single LLM call for all marked groups)
+        int summarized = excludedGroups.Count;
+        if (logger.IsEnabled(LogLevel.Debug))
+        {
+            logger.LogSummarizationStarting(summarized, summarizationMessages.Count - 1, this.ChatClient.GetType().Name);
+        }
+
+        using Activity? summarizeActivity = CompactionTelemetry.ActivitySource.StartActivity(CompactionTelemetry.ActivityNames.Summarize);
+        summarizeActivity?.SetTag(CompactionTelemetry.Tags.GroupsSummarized, summarized);
+
+        ChatResponse response;
+        try
+        {
+            response = await this.ChatClient.GetResponseAsync(
+                summarizationMessages,
+                cancellationToken: cancellationToken).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (ex is not OperationCanceledException)
+        {
+            // Restore excluded groups so the conversation is not left in an inconsistent state
+            for (int i = 0; i < excludedGroups.Count; i++)
+            {
+                excludedGroups[i].IsExcluded = false;
+                excludedGroups[i].ExcludeReason = null;
+            }
+
+            logger.LogSummarizationFailed(summarized, ex.Message);
+
+            return false;
+        }
+
+        string summaryText = string.IsNullOrWhiteSpace(response.Text) ? "[Summary unavailable]" : response.Text;
+
+        summarizeActivity?.SetTag(CompactionTelemetry.Tags.SummaryLength, summaryText.Length);
+
+        // Insert a summary group at the position of the first summarized group
+        ChatMessage summaryMessage = new(ChatRole.Assistant, $"[Summary]\n{summaryText}");
+        (summaryMessage.AdditionalProperties ??= [])[CompactionMessageGroup.SummaryPropertyKey] = true;
+
+        index.InsertGroup(insertIndex, CompactionGroupKind.Summary, [summaryMessage]);
+
+        logger.LogSummarizationCompleted(summaryText.Length, insertIndex);
+
+        return true;
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/ToolResultCompactionStrategy.cs

@@ -0,0 +1,234 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A compaction strategy that collapses old tool call groups into single concise assistant
+/// messages, removing the detailed tool results while preserving a record of which tools were called
+/// and what they returned.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This is the gentlest compaction strategy — it does not remove any user messages or
+/// plain assistant responses. It only targets <see cref="CompactionGroupKind.ToolCall"/>
+/// groups outside the protected recent window, replacing each multi-message group
+/// (assistant call + tool results) with a single assistant message in a YAML-like format:
+/// <code>
+/// [Tool Calls]
+/// get_weather:
+///   - Sunny and 72°F
+/// search_docs:
+///   - Found 3 docs
+/// </code>
+/// </para>
+/// <para>
+/// <see cref="MinimumPreservedGroups"/> is a hard floor: even if the <see cref="CompactionStrategy.Target"/>
+/// has not been reached, compaction will not touch the last <see cref="MinimumPreservedGroups"/> non-system groups.
+/// </para>
+/// <para>
+/// The <see cref="CompactionTrigger"/> predicate controls when compaction proceeds. Use
+/// <see cref="CompactionTriggers"/> for common trigger conditions such as token thresholds.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class ToolResultCompactionStrategy : CompactionStrategy
+{
+    /// <summary>
+    /// The default minimum number of most-recent non-system groups to preserve.
+    /// </summary>
+    public const int DefaultMinimumPreserved = 16;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ToolResultCompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="trigger">
+    /// The <see cref="CompactionTrigger"/> that controls when compaction proceeds.
+    /// </param>
+    /// <param name="minimumPreservedGroups">
+    /// The minimum number of most-recent non-system message groups to preserve.
+    /// This is a hard floor — compaction will not collapse groups beyond this limit,
+    /// regardless of the target condition.
+    /// Defaults to <see cref="DefaultMinimumPreserved"/>, ensuring the current turn's tool interactions remain visible.
+    /// </param>
+    /// <param name="target">
+    /// An optional target condition that controls when compaction stops. When <see langword="null"/>,
+    /// defaults to the inverse of the <paramref name="trigger"/> — compaction stops as soon as the trigger would no longer fire.
+    /// </param>
+    public ToolResultCompactionStrategy(CompactionTrigger trigger, int minimumPreservedGroups = DefaultMinimumPreserved, CompactionTrigger? target = null)
+        : base(trigger, target)
+    {
+        this.MinimumPreservedGroups = EnsureNonNegative(minimumPreservedGroups);
+    }
+
+    /// <summary>
+    /// Gets the minimum number of most-recent non-system groups that are always preserved.
+    /// This is a hard floor that compaction cannot exceed, regardless of the target condition.
+    /// </summary>
+    public int MinimumPreservedGroups { get; }
+
+    /// <inheritdoc/>
+    protected override ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+    {
+        // Identify protected groups: the N most-recent non-system, non-excluded groups
+        List<int> nonSystemIncludedIndices = [];
+        for (int i = 0; i < index.Groups.Count; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (!group.IsExcluded && group.Kind != CompactionGroupKind.System)
+            {
+                nonSystemIncludedIndices.Add(i);
+            }
+        }
+
+        int protectedStart = EnsureNonNegative(nonSystemIncludedIndices.Count - this.MinimumPreservedGroups);
+        HashSet<int> protectedGroupIndices = [];
+        for (int i = protectedStart; i < nonSystemIncludedIndices.Count; i++)
+        {
+            protectedGroupIndices.Add(nonSystemIncludedIndices[i]);
+        }
+
+        // Collect eligible tool groups in order (oldest first)
+        List<int> eligibleIndices = [];
+        for (int i = 0; i < index.Groups.Count; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (!group.IsExcluded && group.Kind == CompactionGroupKind.ToolCall && !protectedGroupIndices.Contains(i))
+            {
+                eligibleIndices.Add(i);
+            }
+        }
+
+        if (eligibleIndices.Count == 0)
+        {
+            return new ValueTask<bool>(false);
+        }
+
+        // Collapse one tool group at a time from oldest, re-checking target after each
+        bool compacted = false;
+        int offset = 0;
+
+        for (int e = 0; e < eligibleIndices.Count; e++)
+        {
+            int idx = eligibleIndices[e] + offset;
+            CompactionMessageGroup group = index.Groups[idx];
+
+            string summary = BuildToolCallSummary(group);
+
+            // Exclude the original group and insert a collapsed replacement
+            group.IsExcluded = true;
+            group.ExcludeReason = $"Collapsed by {nameof(ToolResultCompactionStrategy)}";
+
+            ChatMessage summaryMessage = new(ChatRole.Assistant, summary);
+            (summaryMessage.AdditionalProperties ??= [])[CompactionMessageGroup.SummaryPropertyKey] = true;
+
+            index.InsertGroup(idx + 1, CompactionGroupKind.Summary, [summaryMessage], group.TurnIndex);
+            offset++; // Each insertion shifts subsequent indices by 1
+
+            compacted = true;
+
+            // Stop when target condition is met
+            if (this.Target(index))
+            {
+                break;
+            }
+        }
+
+        return new ValueTask<bool>(compacted);
+    }
+
+    /// <summary>
+    /// Builds a concise summary string for a tool call group, including tool names,
+    /// results, and deduplication counts for repeated tool names.
+    /// </summary>
+    private static string BuildToolCallSummary(CompactionMessageGroup group)
+    {
+        // Collect function calls (callId, name) and results (callId → result text)
+        List<(string CallId, string Name)> functionCalls = [];
+        Dictionary<string, string> resultsByCallId = new();
+        List<string> plainTextResults = [];
+
+        foreach (ChatMessage message in group.Messages)
+        {
+            if (message.Contents is null)
+            {
+                continue;
+            }
+
+            bool hasFunctionResult = false;
+            foreach (AIContent content in message.Contents)
+            {
+                if (content is FunctionCallContent fcc)
+                {
+                    functionCalls.Add((fcc.CallId, fcc.Name));
+                }
+                else if (content is FunctionResultContent frc && frc.CallId is not null)
+                {
+                    resultsByCallId[frc.CallId] = frc.Result?.ToString() ?? string.Empty;
+                    hasFunctionResult = true;
+                }
+            }
+
+            // Collect plain text from Tool-role messages that lack FunctionResultContent
+            if (!hasFunctionResult && message.Role == ChatRole.Tool && message.Text is string text)
+            {
+                plainTextResults.Add(text);
+            }
+        }
+
+        // Match function calls to their results using CallId or positional fallback,
+        // grouping by tool name while preserving first-seen order.
+        int plainTextIdx = 0;
+        List<string> orderedNames = [];
+        Dictionary<string, List<string>> groupedResults = new();
+
+        foreach ((string callId, string name) in functionCalls)
+        {
+            if (!groupedResults.TryGetValue(name, out _))
+            {
+                orderedNames.Add(name);
+                groupedResults[name] = [];
+            }
+
+            string? result = null;
+            if (resultsByCallId.TryGetValue(callId, out string? matchedResult))
+            {
+                result = matchedResult;
+            }
+            else if (plainTextIdx < plainTextResults.Count)
+            {
+                result = plainTextResults[plainTextIdx++];
+            }
+
+            if (!string.IsNullOrEmpty(result))
+            {
+                groupedResults[name].Add(result);
+            }
+        }
+
+        // Format as YAML-like block with [Tool Calls] header
+        List<string> lines = ["[Tool Calls]"];
+        foreach (string name in orderedNames)
+        {
+            List<string> results = groupedResults[name];
+
+            lines.Add($"{name}:");
+            if (results.Count > 0)
+            {
+                foreach (string result in results)
+                {
+                    lines.Add($"  - {result}");
+                }
+            }
+        }
+
+        return string.Join("\n", lines);
+    }
+}

dotnet/src/Microsoft.Agents.AI/Compaction/TruncationCompactionStrategy.cs

@@ -0,0 +1,110 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// A compaction strategy that removes the oldest non-system message groups,
+/// keeping at least <see cref="MinimumPreservedGroups"/> most-recent groups intact.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This strategy preserves system messages and removes the oldest non-system message groups first.
+/// It respects atomic group boundaries — an assistant message with tool calls and its
+/// corresponding tool result messages are always removed together.
+/// </para>
+/// <para>
+/// <see cref="MinimumPreservedGroups"/> is a hard floor: even if the <see cref="CompactionStrategy.Target"/>
+/// has not been reached, compaction will not touch the last <see cref="MinimumPreservedGroups"/> non-system groups.
+/// </para>
+/// <para>
+/// The <see cref="CompactionTrigger"/> controls when compaction proceeds.
+/// Use <see cref="CompactionTriggers"/> for common trigger conditions such as token or group thresholds.
+/// </para>
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public sealed class TruncationCompactionStrategy : CompactionStrategy
+{
+    /// <summary>
+    /// The default minimum number of most-recent non-system groups to preserve.
+    /// </summary>
+    public const int DefaultMinimumPreserved = 32;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TruncationCompactionStrategy"/> class.
+    /// </summary>
+    /// <param name="trigger">
+    /// The <see cref="CompactionTrigger"/> that controls when compaction proceeds.
+    /// </param>
+    /// <param name="minimumPreservedGroups">
+    /// The minimum number of most-recent non-system message groups to preserve.
+    /// This is a hard floor — compaction will not remove groups beyond this limit,
+    /// regardless of the target condition.
+    /// </param>
+    /// <param name="target">
+    /// An optional target condition that controls when compaction stops. When <see langword="null"/>,
+    /// defaults to the inverse of the <paramref name="trigger"/> — compaction stops as soon as the trigger would no longer fire.
+    /// </param>
+    public TruncationCompactionStrategy(CompactionTrigger trigger, int minimumPreservedGroups = DefaultMinimumPreserved, CompactionTrigger? target = null)
+        : base(trigger, target)
+    {
+        this.MinimumPreservedGroups = EnsureNonNegative(minimumPreservedGroups);
+    }
+
+    /// <summary>
+    /// Gets the minimum number of most-recent non-system message groups that are always preserved.
+    /// This is a hard floor that compaction cannot exceed, regardless of the target condition.
+    /// </summary>
+    public int MinimumPreservedGroups { get; }
+
+    /// <inheritdoc/>
+    protected override ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+    {
+        // Count removable (non-system, non-excluded) groups
+        int removableCount = 0;
+        for (int i = 0; i < index.Groups.Count; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (!group.IsExcluded && group.Kind != CompactionGroupKind.System)
+            {
+                removableCount++;
+            }
+        }
+
+        int maxRemovable = removableCount - this.MinimumPreservedGroups;
+        if (maxRemovable <= 0)
+        {
+            return new ValueTask<bool>(false);
+        }
+
+        // Exclude oldest non-system groups one at a time, re-checking target after each
+        bool compacted = false;
+        int removed = 0;
+        for (int i = 0; i < index.Groups.Count && removed < maxRemovable; i++)
+        {
+            CompactionMessageGroup group = index.Groups[i];
+            if (group.IsExcluded || group.Kind == CompactionGroupKind.System)
+            {
+                continue;
+            }
+
+            group.IsExcluded = true;
+            group.ExcludeReason = $"Truncated by {nameof(TruncationCompactionStrategy)}";
+            removed++;
+            compacted = true;
+
+            // Stop when target condition is met
+            if (this.Target(index))
+            {
+                break;
+            }
+        }
+
+        return new ValueTask<bool>(compacted);
+    }
+}

dotnet/src/Microsoft.Agents.AI/Memory/ChatHistoryMemoryProvider.cs

@@ -13,6 +13,7 @@ using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI;
 
+#pragma warning disable IDE0001 // Simplify Names - Microsoft.Extensions.Logging.LogLevel.Trace doesn't get found in net472 when removing the namespace.
 /// <summary>
 /// A context provider that stores all chat history in a vector store and is able to
 /// retrieve related chat history later to augment the current conversation.
@@ -33,8 +34,25 @@ namespace Microsoft.Agents.AI;
 /// exposes a function tool that the model can invoke to retrieve relevant memories on demand instead of
 /// injecting them automatically on each invocation.
 /// </para>
+/// <para>
+/// <strong>Security considerations:</strong>
+/// <list type="bullet">
+/// <item><description><strong>Indirect prompt injection:</strong> Messages retrieved from the vector store via semantic search
+/// are injected into the LLM context. If the vector store is compromised, adversarial content could influence LLM behavior.
+/// The data returned from the store is accepted as-is without validation or sanitization.</description></item>
+/// <item><description><strong>PII and sensitive data:</strong> Conversation messages (including user inputs and LLM responses)
+/// are stored as vectors in the underlying store. These messages may contain PII or sensitive information. Ensure the vector
+/// store is configured with appropriate access controls and encryption at rest.</description></item>
+/// <item><description><strong>On-demand search tool:</strong> When using <see cref="ChatHistoryMemoryProviderOptions.SearchBehavior.OnDemandFunctionCalling"/>,
+/// the AI model controls when and what to search for. The search query is AI-generated and should be treated as untrusted input
+/// by the vector store implementation.</description></item>
+/// <item><description><strong>Trace logging:</strong> When <see cref="Microsoft.Extensions.Logging.LogLevel.Trace"/> is enabled,
+/// full search queries and results may be logged. This data may contain PII.</description></item>
+/// </list>
+/// </para>
 /// </remarks>
 public sealed class ChatHistoryMemoryProvider : MessageAIContextProvider, IDisposable
+#pragma warning restore IDE0001 // Simplify Names
 {
     private const string DefaultContextPrompt = "## Memories\nConsider the following memories when answering user questions:";
     private const int DefaultMaxResults = 3;

dotnet/src/Microsoft.Agents.AI/Microsoft.Agents.AI.csproj

@@ -18,10 +18,14 @@
 
   <ItemGroup>
     <ProjectReference Include="..\Microsoft.Agents.AI.Abstractions\Microsoft.Agents.AI.Abstractions.csproj" />
+  </ItemGroup>
+  
+  <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.AI" />
     <PackageReference Include="Microsoft.Extensions.VectorData.Abstractions" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" />
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
+    <PackageReference Include="Microsoft.ML.Tokenizers" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />
   </ItemGroup>
 
@@ -36,7 +40,7 @@
 
     <InternalsVisibleTo Include="Microsoft.Agents.AI.UnitTests" />
     <InternalsVisibleTo Include="Microsoft.Agents.AI.Declarative.UnitTests" />
-    <InternalsVisibleTo Include="Microsoft.Agents.AI.Hosting.UnitTests"/>
+    <InternalsVisibleTo Include="Microsoft.Agents.AI.Hosting.UnitTests" />
   </ItemGroup>
 
 </Project>

dotnet/src/Microsoft.Agents.AI/OpenTelemetryAgent.cs

@@ -70,6 +70,12 @@ public sealed class OpenTelemetryAgent : DelegatingAIAgent, IDisposable
     /// and outputs, such as message content, function call arguments, and function call results.
     /// The default value can be overridden by setting the <c>OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT</c>
     /// environment variable to "true". Explicitly setting this property will override the environment variable.
+    /// <para>
+    /// <strong>Security consideration:</strong> When sensitive data capture is enabled, the full text of chat messages —
+    /// including user inputs, LLM responses, function call arguments, and function results — is emitted as telemetry.
+    /// This data may contain PII or other sensitive information. Ensure that your telemetry pipeline is configured
+    /// with appropriate access controls and data retention policies.
+    /// </para>
     /// </remarks>
     public bool EnableSensitiveData
     {

dotnet/src/Microsoft.Agents.AI/TextSearchProvider.cs

@@ -31,6 +31,18 @@ namespace Microsoft.Agents.AI;
 /// to the current request messages when forming the search input. This can improve search relevance by providing
 /// multi-turn context to the retrieval layer without permanently altering the conversation history.
 /// </para>
+/// <para>
+/// <strong>Security considerations:</strong> Search results retrieved from external sources are injected into the LLM context and may
+/// contain adversarial content designed to manipulate LLM behavior via indirect prompt injection. Developers should be aware that:
+/// <list type="bullet">
+/// <item><description>The search query may be constructed from user input or LLM-generated content, both of which are untrusted.
+/// Implementers of the search delegate should validate search inputs and apply appropriate access controls to search results.</description></item>
+/// <item><description>Retrieved documents are formatted and injected as messages in the AI request context. If the external data source
+/// is compromised, adversarial content could influence the LLM's responses.</description></item>
+/// <item><description>When using <see cref="TextSearchProviderOptions.TextSearchBehavior.OnDemandFunctionCalling"/>, the AI model controls
+/// when and what to search for — the search query text is AI-generated and should be treated as untrusted input by the search implementation.</description></item>
+/// </list>
+/// </para>
 /// </remarks>
 public sealed class TextSearchProvider : MessageAIContextProvider
 {

dotnet/tests/AzureAIAgentsPersistent.IntegrationTests/AzureAIAgentsPersistentCreateTests.cs

@@ -14,6 +14,8 @@ namespace AzureAIAgentsPersistent.IntegrationTests;
 
 public class AzureAIAgentsPersistentCreateTests
 {
+    private const string SkipCodeInterpreterReason = "Azure AI Code Interpreter intermittently fails to execute uploaded files in CI";
+
     private readonly PersistentAgentsClient _persistentAgentsClient = new(TestConfiguration.GetRequiredValue(TestSettings.AzureAIProjectEndpoint), TestAzureCliCredentials.CreateAzureCliCredential());
 
     [Theory]
@@ -131,11 +133,11 @@ public class AzureAIAgentsPersistentCreateTests
         }
     }
 
-    [Fact]
+    [Fact(Skip = SkipCodeInterpreterReason)]
     public Task CreateAgent_CreatesAgentWithCodeInterpreter_ChatClientAgentOptionsAsync()
         => this.CreateAgent_CreatesAgentWithCodeInterpreterAsync("CreateWithChatClientAgentOptionsAsync");
 
-    [RetryFact(Constants.RetryCount, Constants.RetryDelay)]
+    [Fact(Skip = SkipCodeInterpreterReason)]
     public Task CreateAgent_CreatesAgentWithCodeInterpreter_FoundryOptionsAsync()
         => this.CreateAgent_CreatesAgentWithCodeInterpreterAsync("CreateWithFoundryOptionsAsync");
 

dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs

@@ -126,6 +126,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.Single(result.Messages);
         Assert.Equal(ChatRole.Assistant, result.Messages[0].Role);
         Assert.Equal("Hello! How can I help you today?", result.Messages[0].Text);
+        Assert.Equal(ChatFinishReason.Stop, result.FinishReason);
     }
 
     [Fact]
@@ -249,8 +250,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.Equal("stream-1", updates[0].MessageId);
         Assert.Equal(this._agent.Id, updates[0].AgentId);
         Assert.Equal("stream-1", updates[0].ResponseId);
-
-        Assert.NotNull(updates[0].RawRepresentation);
+        Assert.Equal(ChatFinishReason.Stop, updates[0].FinishReason);
         Assert.IsType<AgentMessage>(updates[0].RawRepresentation);
         Assert.Equal("stream-1", ((AgentMessage)updates[0].RawRepresentation!).MessageId);
     }
@@ -501,8 +501,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.NotNull(result);
         Assert.Equal(this._agent.Id, result.AgentId);
         Assert.Equal("task-789", result.ResponseId);
-
-        Assert.NotNull(result.RawRepresentation);
+        Assert.Null(result.FinishReason);
         Assert.IsType<AgentTask>(result.RawRepresentation);
         Assert.Equal("task-789", ((AgentTask)result.RawRepresentation).Id);
 
@@ -552,6 +551,15 @@ public sealed class A2AAgentTests : IDisposable
         {
             Assert.Null(result.ContinuationToken);
         }
+
+        if (taskState is TaskState.Completed)
+        {
+            Assert.Equal(ChatFinishReason.Stop, result.FinishReason);
+        }
+        else
+        {
+            Assert.Null(result.FinishReason);
+        }
     }
 
     [Fact]
@@ -661,6 +669,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.Equal(MessageId, update0.ResponseId);
         Assert.Equal(this._agent.Id, update0.AgentId);
         Assert.Equal(MessageText, update0.Text);
+        Assert.Equal(ChatFinishReason.Stop, update0.FinishReason);
         Assert.IsType<AgentMessage>(update0.RawRepresentation);
         Assert.Equal(MessageId, ((AgentMessage)update0.RawRepresentation!).MessageId);
     }
@@ -702,6 +711,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.Equal(ChatRole.Assistant, update0.Role);
         Assert.Equal(TaskId, update0.ResponseId);
         Assert.Equal(this._agent.Id, update0.AgentId);
+        Assert.Null(update0.FinishReason);
         Assert.IsType<AgentTask>(update0.RawRepresentation);
         Assert.Equal(TaskId, ((AgentTask)update0.RawRepresentation!).Id);
 
@@ -741,6 +751,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.Equal(ChatRole.Assistant, update0.Role);
         Assert.Equal(TaskId, update0.ResponseId);
         Assert.Equal(this._agent.Id, update0.AgentId);
+        Assert.Null(update0.FinishReason);
         Assert.IsType<TaskStatusUpdateEvent>(update0.RawRepresentation);
 
         // Assert - session should be updated with context and task IDs
@@ -784,6 +795,7 @@ public sealed class A2AAgentTests : IDisposable
         Assert.Equal(ChatRole.Assistant, update0.Role);
         Assert.Equal(TaskId, update0.ResponseId);
         Assert.Equal(this._agent.Id, update0.AgentId);
+        Assert.Null(update0.FinishReason);
         Assert.IsType<TaskArtifactUpdateEvent>(update0.RawRepresentation);
 
         // Assert - artifact content should be in the update

dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseTests.cs

@@ -53,6 +53,7 @@ public class AgentResponseTests
         {
             AdditionalProperties = [],
             CreatedAt = new DateTimeOffset(2022, 1, 1, 0, 0, 0, TimeSpan.Zero),
+            FinishReason = ChatFinishReason.ContentFilter,
             Messages = [new(ChatRole.Assistant, "This is a test message.")],
             RawRepresentation = new object(),
             ResponseId = "responseId",
@@ -63,6 +64,7 @@ public class AgentResponseTests
         AgentResponse response = new(chatResponse);
         Assert.Same(chatResponse.AdditionalProperties, response.AdditionalProperties);
         Assert.Equal(chatResponse.CreatedAt, response.CreatedAt);
+        Assert.Equal(chatResponse.FinishReason, response.FinishReason);
         Assert.Same(chatResponse.Messages, response.Messages);
         Assert.Equal(chatResponse.ResponseId, response.ResponseId);
         Assert.Same(chatResponse, response.RawRepresentation as ChatResponse);
@@ -105,6 +107,10 @@ public class AgentResponseTests
         Assert.Null(response.ContinuationToken);
         response.ContinuationToken = ResponseContinuationToken.FromBytes(new byte[] { 1, 2, 3 });
         Assert.Equivalent(ResponseContinuationToken.FromBytes(new byte[] { 1, 2, 3 }), response.ContinuationToken);
+
+        Assert.Null(response.FinishReason);
+        response.FinishReason = ChatFinishReason.Length;
+        Assert.Equal(ChatFinishReason.Length, response.FinishReason);
     }
 
     [Fact]
@@ -188,6 +194,7 @@ public class AgentResponseTests
             ResponseId = "12345",
             CreatedAt = new DateTimeOffset(2024, 11, 10, 9, 20, 0, TimeSpan.Zero),
             AdditionalProperties = new() { ["key1"] = "value1", ["key2"] = 42 },
+            FinishReason = ChatFinishReason.ContentFilter,
             Usage = new UsageDetails
             {
                 TotalTokenCount = 100
@@ -205,6 +212,7 @@ public class AgentResponseTests
         Assert.Equal(new DateTimeOffset(2024, 11, 10, 9, 20, 0, TimeSpan.Zero), update0.CreatedAt);
         Assert.Equal("customRole", update0.Role?.Value);
         Assert.Equal("Text", update0.Text);
+        Assert.Equal(ChatFinishReason.ContentFilter, update0.FinishReason);
 
         AgentResponseUpdate update1 = updates[1];
         Assert.Equal("value1", update1.AdditionalProperties?["key1"]);

dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateExtensionsTests.cs

@@ -334,6 +334,7 @@ public class AgentResponseUpdateExtensionsTests
         {
             ResponseId = "test-response-id",
             CreatedAt = new DateTimeOffset(2024, 1, 1, 12, 0, 0, TimeSpan.Zero),
+            FinishReason = ChatFinishReason.ContentFilter,
             Usage = new UsageDetails { TotalTokenCount = 50 },
             AdditionalProperties = new() { ["key"] = "value" },
             ContinuationToken = ResponseContinuationToken.FromBytes(new byte[] { 1, 2, 3 }),
@@ -346,6 +347,7 @@ public class AgentResponseUpdateExtensionsTests
         Assert.NotNull(result);
         Assert.Equal("test-response-id", result.ResponseId);
         Assert.Equal(new DateTimeOffset(2024, 1, 1, 12, 0, 0, TimeSpan.Zero), result.CreatedAt);
+        Assert.Equal(ChatFinishReason.ContentFilter, result.FinishReason);
         Assert.Same(agentResponse.Messages, result.Messages);
         Assert.Same(agentResponse, result.RawRepresentation);
         Assert.Same(agentResponse.Usage, result.Usage);
@@ -392,6 +394,7 @@ public class AgentResponseUpdateExtensionsTests
             ResponseId = "update-id",
             MessageId = "message-id",
             CreatedAt = new DateTimeOffset(2024, 1, 1, 12, 0, 0, TimeSpan.Zero),
+            FinishReason = ChatFinishReason.ToolCalls,
             AdditionalProperties = new() { ["key"] = "value" },
             ContinuationToken = ResponseContinuationToken.FromBytes(new byte[] { 1, 2, 3 }),
         };
@@ -405,6 +408,7 @@ public class AgentResponseUpdateExtensionsTests
         Assert.Equal("update-id", result.ResponseId);
         Assert.Equal("message-id", result.MessageId);
         Assert.Equal(new DateTimeOffset(2024, 1, 1, 12, 0, 0, TimeSpan.Zero), result.CreatedAt);
+        Assert.Equal(ChatFinishReason.ToolCalls, result.FinishReason);
         Assert.Equal(ChatRole.Assistant, result.Role);
         Assert.Same(agentResponseUpdate.Contents, result.Contents);
         Assert.Same(agentResponseUpdate, result.RawRepresentation);

dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateTests.cs

@@ -24,6 +24,7 @@ public class AgentResponseUpdateTests
         Assert.Null(update.CreatedAt);
         Assert.Equal(string.Empty, update.ToString());
         Assert.Null(update.ContinuationToken);
+        Assert.Null(update.FinishReason);
     }
 
     [Fact]
@@ -50,6 +51,7 @@ public class AgentResponseUpdateTests
         Assert.Equal(chatResponseUpdate.AuthorName, response.AuthorName);
         Assert.Same(chatResponseUpdate.Contents, response.Contents);
         Assert.Equal(chatResponseUpdate.CreatedAt, response.CreatedAt);
+        Assert.Equal(chatResponseUpdate.FinishReason, response.FinishReason);
         Assert.Equal(chatResponseUpdate.MessageId, response.MessageId);
         Assert.Same(chatResponseUpdate, response.RawRepresentation as ChatResponseUpdate);
         Assert.Equal(chatResponseUpdate.ResponseId, response.ResponseId);
@@ -109,6 +111,10 @@ public class AgentResponseUpdateTests
         Assert.Null(update.ContinuationToken);
         update.ContinuationToken = ResponseContinuationToken.FromBytes(new byte[] { 1, 2, 3 });
         Assert.Equivalent(ResponseContinuationToken.FromBytes(new byte[] { 1, 2, 3 }), update.ContinuationToken);
+
+        Assert.Null(update.FinishReason);
+        update.FinishReason = ChatFinishReason.ToolCalls;
+        Assert.Equal(ChatFinishReason.ToolCalls, update.FinishReason);
     }
 
     [Fact]

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ChatMessageContentEqualityTests.cs

@@ -0,0 +1,518 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="ChatMessageContentEquality"/> extension methods.
+/// </summary>
+public class ChatMessageContentEqualityTests
+{
+    #region Null and reference handling
+
+    [Fact]
+    public void BothNullReturnsTrue()
+    {
+        ChatMessage? a = null;
+        ChatMessage? b = null;
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void LeftNullReturnsFalse()
+    {
+        ChatMessage? a = null;
+        ChatMessage b = new(ChatRole.User, "Hello");
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void RightNullReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+        ChatMessage? b = null;
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void SameReferenceReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+
+        Assert.True(a.ContentEquals(a));
+    }
+
+    #endregion
+
+    #region MessageId shortcut
+
+    [Fact]
+    public void MatchingMessageIdReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello") { MessageId = "msg-1" };
+        ChatMessage b = new(ChatRole.User, "Hello") { MessageId = "msg-1" };
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void MatchingMessageIdSufficientDespiteDifferentContent()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello") { MessageId = "msg-1" };
+        ChatMessage b = new(ChatRole.Assistant, "Goodbye") { MessageId = "msg-1" };
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentMessageIdReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello") { MessageId = "msg-1" };
+        ChatMessage b = new(ChatRole.User, "Hello") { MessageId = "msg-2" };
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void OnlyLeftHasMessageIdFallsThroughToContentComparison()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello") { MessageId = "msg-1" };
+        ChatMessage b = new(ChatRole.User, "Hello");
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void OnlyRightHasMessageIdFallsThroughToContentComparison()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+        ChatMessage b = new(ChatRole.User, "Hello") { MessageId = "msg-1" };
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region Role and AuthorName
+
+    [Fact]
+    public void DifferentRoleReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+        ChatMessage b = new(ChatRole.Assistant, "Hello");
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentAuthorNameReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello") { AuthorName = "Alice" };
+        ChatMessage b = new(ChatRole.User, "Hello") { AuthorName = "Bob" };
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void BothNullAuthorNamesAreEqual()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+        ChatMessage b = new(ChatRole.User, "Hello");
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region TextContent
+
+    [Fact]
+    public void EqualTextContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello world");
+        ChatMessage b = new(ChatRole.User, "Hello world");
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentTextContentReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+        ChatMessage b = new(ChatRole.User, "Goodbye");
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void TextContentIsCaseSensitive()
+    {
+        ChatMessage a = new(ChatRole.User, "Hello");
+        ChatMessage b = new(ChatRole.User, "hello");
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region TextReasoningContent
+
+    [Fact]
+    public void EqualTextReasoningContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new TextReasoningContent("thinking...") { ProtectedData = "opaque" }]);
+        ChatMessage b = new(ChatRole.Assistant, [new TextReasoningContent("thinking...") { ProtectedData = "opaque" }]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentReasoningTextReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new TextReasoningContent("alpha")]);
+        ChatMessage b = new(ChatRole.Assistant, [new TextReasoningContent("beta")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentProtectedDataReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new TextReasoningContent("same") { ProtectedData = "x" }]);
+        ChatMessage b = new(ChatRole.Assistant, [new TextReasoningContent("same") { ProtectedData = "y" }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region DataContent
+
+    [Fact]
+    public void EqualDataContentReturnsTrue()
+    {
+        byte[] data = Encoding.UTF8.GetBytes("payload");
+        ChatMessage a = new(ChatRole.User, [new DataContent(data, "application/octet-stream") { Name = "file.bin" }]);
+        ChatMessage b = new(ChatRole.User, [new DataContent(data, "application/octet-stream") { Name = "file.bin" }]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentDataBytesReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new DataContent(Encoding.UTF8.GetBytes("aaa"), "text/plain")]);
+        ChatMessage b = new(ChatRole.User, [new DataContent(Encoding.UTF8.GetBytes("bbb"), "text/plain")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentMediaTypeReturnsFalse()
+    {
+        byte[] data = [1, 2, 3];
+        ChatMessage a = new(ChatRole.User, [new DataContent(data, "image/png")]);
+        ChatMessage b = new(ChatRole.User, [new DataContent(data, "image/jpeg")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentDataContentNameReturnsFalse()
+    {
+        byte[] data = [1, 2, 3];
+        ChatMessage a = new(ChatRole.User, [new DataContent(data, "image/png") { Name = "a.png" }]);
+        ChatMessage b = new(ChatRole.User, [new DataContent(data, "image/png") { Name = "b.png" }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region UriContent
+
+    [Fact]
+    public void EqualUriContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.User, [new UriContent(new Uri("https://example.com/image.png"), "image/png")]);
+        ChatMessage b = new(ChatRole.User, [new UriContent(new Uri("https://example.com/image.png"), "image/png")]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentUriReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new UriContent(new Uri("https://a.com/x"), "image/png")]);
+        ChatMessage b = new(ChatRole.User, [new UriContent(new Uri("https://b.com/x"), "image/png")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentUriMediaTypeReturnsFalse()
+    {
+        Uri uri = new("https://example.com/file");
+        ChatMessage a = new(ChatRole.User, [new UriContent(uri, "image/png")]);
+        ChatMessage b = new(ChatRole.User, [new UriContent(uri, "image/jpeg")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region ErrorContent
+
+    [Fact]
+    public void EqualErrorContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new ErrorContent("fail") { ErrorCode = "E001" }]);
+        ChatMessage b = new(ChatRole.Assistant, [new ErrorContent("fail") { ErrorCode = "E001" }]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentErrorMessageReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new ErrorContent("fail")]);
+        ChatMessage b = new(ChatRole.Assistant, [new ErrorContent("crash")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentErrorCodeReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new ErrorContent("fail") { ErrorCode = "E001" }]);
+        ChatMessage b = new(ChatRole.Assistant, [new ErrorContent("fail") { ErrorCode = "E002" }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region FunctionCallContent
+
+    [Fact]
+    public void EqualFunctionCallContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "get_weather") { Arguments = new Dictionary<string, object?> { ["city"] = "Seattle" } }]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "get_weather") { Arguments = new Dictionary<string, object?> { ["city"] = "Seattle" } }]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentCallIdReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "get_weather")]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-2", "get_weather")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentFunctionNameReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "get_weather")]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "get_time")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentArgumentsReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn") { Arguments = new Dictionary<string, object?> { ["x"] = "1" } }]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn") { Arguments = new Dictionary<string, object?> { ["x"] = "2" } }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void NullArgumentsBothSidesReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn")]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn")]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void OneNullArgumentsReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn")]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn") { Arguments = new Dictionary<string, object?> { ["x"] = "1" } }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentArgumentCountReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn") { Arguments = new Dictionary<string, object?> { ["x"] = "1" } }]);
+        ChatMessage b = new(ChatRole.Assistant, [new FunctionCallContent("call-1", "fn") { Arguments = new Dictionary<string, object?> { ["x"] = "1", ["y"] = "2" } }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region FunctionResultContent
+
+    [Fact]
+    public void EqualFunctionResultContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.Tool, [new FunctionResultContent("call-1", "sunny")]);
+        ChatMessage b = new(ChatRole.Tool, [new FunctionResultContent("call-1", "sunny")]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentResultCallIdReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Tool, [new FunctionResultContent("call-1", "sunny")]);
+        ChatMessage b = new(ChatRole.Tool, [new FunctionResultContent("call-2", "sunny")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentResultValueReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Tool, [new FunctionResultContent("call-1", "sunny")]);
+        ChatMessage b = new(ChatRole.Tool, [new FunctionResultContent("call-1", "rainy")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region HostedFileContent
+
+    [Fact]
+    public void EqualHostedFileContentReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.User, [new HostedFileContent("file-abc") { MediaType = "text/csv", Name = "data.csv" }]);
+        ChatMessage b = new(ChatRole.User, [new HostedFileContent("file-abc") { MediaType = "text/csv", Name = "data.csv" }]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentFileIdReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new HostedFileContent("file-abc")]);
+        ChatMessage b = new(ChatRole.User, [new HostedFileContent("file-xyz")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentHostedFileMediaTypeReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new HostedFileContent("file-abc") { MediaType = "text/csv" }]);
+        ChatMessage b = new(ChatRole.User, [new HostedFileContent("file-abc") { MediaType = "text/plain" }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentHostedFileNameReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new HostedFileContent("file-abc") { Name = "a.csv" }]);
+        ChatMessage b = new(ChatRole.User, [new HostedFileContent("file-abc") { Name = "b.csv" }]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region Content list structure
+
+    [Fact]
+    public void DifferentContentCountReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new TextContent("one"), new TextContent("two")]);
+        ChatMessage b = new(ChatRole.User, [new TextContent("one")]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void MixedContentTypesInSameOrderReturnsTrue()
+    {
+        ChatMessage a = new(ChatRole.Assistant, new AIContent[] { new TextContent("reply"), new FunctionCallContent("c1", "fn") });
+        ChatMessage b = new(ChatRole.Assistant, new AIContent[] { new TextContent("reply"), new FunctionCallContent("c1", "fn") });
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void MismatchedContentTypeOrderReturnsFalse()
+    {
+        ChatMessage a = new(ChatRole.Assistant, new AIContent[] { new TextContent("reply"), new FunctionCallContent("c1", "fn") });
+        ChatMessage b = new(ChatRole.Assistant, new AIContent[] { new FunctionCallContent("c1", "fn"), new TextContent("reply") });
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void EmptyContentsListsAreEqual()
+    {
+        ChatMessage a = new() { Role = ChatRole.User, Contents = [] };
+        ChatMessage b = new() { Role = ChatRole.User, Contents = [] };
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void SameContentItemReferenceReturnsTrue()
+    {
+        // Exercises the ReferenceEquals fast-path on individual AIContent items.
+        TextContent shared = new("Hello");
+        ChatMessage a = new(ChatRole.User, [shared]);
+        ChatMessage b = new(ChatRole.User, [shared]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    #endregion
+
+    #region Unknown AIContent subtype
+
+    [Fact]
+    public void UnknownContentSubtypeSameTypeReturnsTrue()
+    {
+        // Unknown subtypes with the same concrete type are considered equal.
+        ChatMessage a = new(ChatRole.User, [new StubContent()]);
+        ChatMessage b = new(ChatRole.User, [new StubContent()]);
+
+        Assert.True(a.ContentEquals(b));
+    }
+
+    [Fact]
+    public void DifferentUnknownContentSubtypesReturnFalse()
+    {
+        ChatMessage a = new(ChatRole.User, [new StubContent()]);
+        ChatMessage b = new(ChatRole.User, [new OtherStubContent()]);
+
+        Assert.False(a.ContentEquals(b));
+    }
+
+    private sealed class StubContent : AIContent;
+
+    private sealed class OtherStubContent : AIContent;
+
+    #endregion
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ChatReducerCompactionStrategyTests.cs

@@ -0,0 +1,255 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="ChatReducerCompactionStrategy"/> class.
+/// </summary>
+public class ChatReducerCompactionStrategyTests
+{
+    [Fact]
+    public void ConstructorNullReducerThrows()
+    {
+        // Act & Assert
+        Assert.Throws<ArgumentNullException>(() => new ChatReducerCompactionStrategy(null!, CompactionTriggers.Always));
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerNotMetReturnsFalseAsync()
+    {
+        // Arrange — trigger never fires
+        TestChatReducer reducer = new(messages => messages.Take(1));
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Never);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(0, reducer.CallCount);
+        Assert.Equal(2, index.IncludedGroupCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncReducerReturnsFewerMessagesRebuildsIndexAsync()
+    {
+        // Arrange — reducer keeps only the last message
+        TestChatReducer reducer = new(messages => messages.Skip(messages.Count() - 1));
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "First"),
+            new ChatMessage(ChatRole.Assistant, "Response 1"),
+            new ChatMessage(ChatRole.User, "Second"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.True(result);
+        Assert.Equal(1, reducer.CallCount);
+        Assert.Equal(1, index.IncludedGroupCount);
+        Assert.Equal("Second", index.Groups[0].Messages[0].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncReducerReturnsSameCountReturnsFalseAsync()
+    {
+        // Arrange — reducer returns all messages (no reduction)
+        TestChatReducer reducer = new(messages => messages);
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(1, reducer.CallCount);
+        Assert.Equal(2, index.IncludedGroupCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncEmptyIndexReturnsFalseAsync()
+    {
+        // Arrange — no included messages
+        TestChatReducer reducer = new(messages => messages);
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create([]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(0, reducer.CallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesSystemMessagesWhenReducerKeepsThemAsync()
+    {
+        // Arrange — reducer keeps system + last user message
+        TestChatReducer reducer = new(messages =>
+        {
+            var nonSystem = messages.Where(m => m.Role != ChatRole.System).ToList();
+            return messages.Where(m => m.Role == ChatRole.System)
+                .Concat(nonSystem.Skip(nonSystem.Count - 1));
+        });
+
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "First"),
+            new ChatMessage(ChatRole.Assistant, "Response 1"),
+            new ChatMessage(ChatRole.User, "Second"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.True(result);
+        Assert.Equal(2, index.IncludedGroupCount);
+        Assert.Equal(CompactionGroupKind.System, index.Groups[0].Kind);
+        Assert.Equal("You are helpful.", index.Groups[0].Messages[0].Text);
+        Assert.Equal(CompactionGroupKind.User, index.Groups[1].Kind);
+        Assert.Equal("Second", index.Groups[1].Messages[0].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncRebuildsToolCallGroupsCorrectlyAsync()
+    {
+        // Arrange — reducer keeps last 3 messages (assistant tool call + tool result + user)
+        TestChatReducer reducer = new(messages => messages.Skip(messages.Count() - 3));
+
+        ChatMessage assistantToolCall = new(ChatRole.Assistant, [new FunctionCallContent("call1", "get_weather")]);
+        ChatMessage toolResult = new(ChatRole.Tool, "Sunny");
+
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Old question"),
+            new ChatMessage(ChatRole.Assistant, "Old answer"),
+            assistantToolCall,
+            toolResult,
+            new ChatMessage(ChatRole.User, "New question"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.True(result);
+        // Should have 2 groups: ToolCall group (assistant + tool result) + User group
+        Assert.Equal(2, index.IncludedGroupCount);
+        Assert.Equal(CompactionGroupKind.ToolCall, index.Groups[0].Kind);
+        Assert.Equal(2, index.Groups[0].Messages.Count);
+        Assert.Equal(CompactionGroupKind.User, index.Groups[1].Kind);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSkipsAlreadyExcludedGroupsAsync()
+    {
+        // Arrange — one group is pre-excluded, reducer keeps last message
+        TestChatReducer reducer = new(messages => messages.Skip(messages.Count() - 1));
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Excluded"),
+            new ChatMessage(ChatRole.User, "Included 1"),
+            new ChatMessage(ChatRole.User, "Included 2"),
+        ]);
+        index.Groups[0].IsExcluded = true;
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — reducer only saw 2 included messages, kept 1
+        Assert.True(result);
+        Assert.Equal(1, index.IncludedGroupCount);
+        Assert.Equal("Included 2", index.Groups[0].Messages[0].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncExposesReducerPropertyAsync()
+    {
+        // Arrange
+        TestChatReducer reducer = new(messages => messages);
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+
+        // Assert
+        Assert.Same(reducer, strategy.ChatReducer);
+        await Task.CompletedTask;
+    }
+
+    [Fact]
+    public async Task CompactAsyncPassesCancellationTokenToReducerAsync()
+    {
+        // Arrange
+        using CancellationTokenSource cancellationSource = new();
+        CancellationToken capturedToken = default;
+        TestChatReducer reducer = new((messages, cancellationToken) =>
+        {
+            capturedToken = cancellationToken;
+            return Task.FromResult<IEnumerable<ChatMessage>>(messages.Skip(messages.Count() - 1).ToList());
+        });
+
+        ChatReducerCompactionStrategy strategy = new(reducer, CompactionTriggers.Always);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "First"),
+            new ChatMessage(ChatRole.User, "Second"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index, logger: null, cancellationSource.Token);
+
+        // Assert
+        Assert.Equal(cancellationSource.Token, capturedToken);
+    }
+
+    /// <summary>
+    /// A test implementation of <see cref="IChatReducer"/> that applies a configurable reduction function.
+    /// </summary>
+    private sealed class TestChatReducer : IChatReducer
+    {
+        private readonly Func<IEnumerable<ChatMessage>, CancellationToken, Task<IEnumerable<ChatMessage>>> _reduceFunc;
+
+        public TestChatReducer(Func<IEnumerable<ChatMessage>, IEnumerable<ChatMessage>> reduceFunc)
+        {
+            this._reduceFunc = (messages, _) => Task.FromResult(reduceFunc(messages));
+        }
+
+        public TestChatReducer(Func<IEnumerable<ChatMessage>, CancellationToken, Task<IEnumerable<ChatMessage>>> reduceFunc)
+        {
+            this._reduceFunc = reduceFunc;
+        }
+
+        public int CallCount { get; private set; }
+
+        public async Task<IEnumerable<ChatMessage>> ReduceAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken = default)
+        {
+            this.CallCount++;
+            return await this._reduceFunc(messages, cancellationToken).ConfigureAwait(false);
+        }
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionProviderTests.cs

@@ -0,0 +1,366 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+using Moq;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="CompactionProvider"/> class.
+/// </summary>
+public sealed class CompactionProviderTests
+{
+    [Fact]
+    public void ConstructorThrowsOnNullStrategy()
+    {
+        Assert.Throws<ArgumentNullException>(() => new CompactionProvider(null!));
+    }
+
+    [Fact]
+    public void StateKeysReturnsExpectedKey()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy);
+
+        // Act & Assert — default state key is the strategy type name
+        Assert.Single(provider.StateKeys);
+        Assert.Equal(nameof(TruncationCompactionStrategy), provider.StateKeys[0]);
+    }
+
+    [Fact]
+    public void StateKeysAreStableAcrossEquivalentInstances()
+    {
+        // Arrange — two providers with equivalent (but distinct) strategies
+        CompactionProvider provider1 = new(new TruncationCompactionStrategy(CompactionTriggers.TokensExceed(100000)));
+        CompactionProvider provider2 = new(new TruncationCompactionStrategy(CompactionTriggers.TokensExceed(100000)));
+
+        // Act & Assert — default keys must be identical for session state stability
+        Assert.Equal(provider1.StateKeys[0], provider2.StateKeys[0]);
+    }
+
+    [Fact]
+    public void StateKeysReturnsCustomKeyWhenProvided()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy, stateKey: "my-custom-key");
+
+        // Act & Assert
+        Assert.Single(provider.StateKeys);
+        Assert.Equal("my-custom-key", provider.StateKeys[0]);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncNoSessionPassesThroughAsync()
+    {
+        // Arrange — no session → passthrough
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+        ];
+
+        AIContextProvider.InvokingContext context = new(
+            mockAgent.Object,
+            session: null,
+            new AIContext { Messages = messages });
+
+        // Act
+        AIContext result = await provider.InvokingAsync(context);
+
+        // Assert — original context returned unchanged
+        Assert.Same(messages, result.Messages);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncNullMessagesPassesThroughAsync()
+    {
+        // Arrange — messages is null → passthrough
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        TestAgentSession session = new();
+        AIContextProvider.InvokingContext context = new(
+            mockAgent.Object,
+            session,
+            new AIContext { Messages = null });
+
+        // Act
+        AIContext result = await provider.InvokingAsync(context);
+
+        // Assert — original context returned unchanged
+        Assert.Null(result.Messages);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncAppliesCompactionWhenTriggeredAsync()
+    {
+        // Arrange — strategy that always triggers and keeps only 1 group
+        TruncationCompactionStrategy strategy = new(_ => true, minimumPreservedGroups: 1);
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        TestAgentSession session = new();
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ];
+
+        AIContextProvider.InvokingContext context = new(
+            mockAgent.Object,
+            session,
+            new AIContext { Messages = messages });
+
+        // Act
+        AIContext result = await provider.InvokingAsync(context);
+
+        // Assert — compaction should have reduced the message count
+        Assert.NotNull(result.Messages);
+        List<ChatMessage> resultList = [.. result.Messages!];
+        Assert.True(resultList.Count < messages.Count);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncNoCompactionNeededReturnsOriginalMessagesAsync()
+    {
+        // Arrange — trigger never fires → no compaction
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        TestAgentSession session = new();
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+        ];
+
+        AIContextProvider.InvokingContext context = new(
+            mockAgent.Object,
+            session,
+            new AIContext { Messages = messages });
+
+        // Act
+        AIContext result = await provider.InvokingAsync(context);
+
+        // Assert — original messages passed through
+        Assert.NotNull(result.Messages);
+        List<ChatMessage> resultList = [.. result.Messages!];
+        Assert.Single(resultList);
+        Assert.Equal("Hello", resultList[0].Text);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncPreservesInstructionsAndToolsAsync()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        TestAgentSession session = new();
+        List<ChatMessage> messages = [new ChatMessage(ChatRole.User, "Hello")];
+        AITool[] tools = [AIFunctionFactory.Create(() => "tool", "MyTool")];
+
+        AIContextProvider.InvokingContext context = new(
+            mockAgent.Object,
+            session,
+            new AIContext
+            {
+                Instructions = "Be helpful",
+                Messages = messages,
+                Tools = tools
+            });
+
+        // Act
+        AIContext result = await provider.InvokingAsync(context);
+
+        // Assert — instructions and tools are preserved
+        Assert.Equal("Be helpful", result.Instructions);
+        Assert.Same(tools, result.Tools);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncWithExistingIndexUpdatesAsync()
+    {
+        // Arrange — call twice to exercise the "existing index" path
+        TruncationCompactionStrategy strategy = new(_ => true, minimumPreservedGroups: 1);
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        TestAgentSession session = new();
+
+        List<ChatMessage> messages1 =
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ];
+
+        AIContextProvider.InvokingContext context1 = new(
+            mockAgent.Object,
+            session,
+            new AIContext { Messages = messages1 });
+
+        // First call — initializes state
+        await provider.InvokingAsync(context1);
+
+        List<ChatMessage> messages2 =
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ];
+
+        AIContextProvider.InvokingContext context2 = new(
+            mockAgent.Object,
+            session,
+            new AIContext { Messages = messages2 });
+
+        // Act — second call exercises the update path
+        AIContext result = await provider.InvokingAsync(context2);
+
+        // Assert
+        Assert.NotNull(result.Messages);
+    }
+
+    [Fact]
+    public async Task InvokingAsyncWithNonListEnumerableCreatesListCopyAsync()
+    {
+        // Arrange — pass IEnumerable (not List<ChatMessage>) to exercise the list copy branch
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        CompactionProvider provider = new(strategy);
+
+        Mock<AIAgent> mockAgent = new() { CallBase = true };
+        TestAgentSession session = new();
+
+        // Use an IEnumerable (not a List) to trigger the copy path
+        IEnumerable<ChatMessage> messages = [new ChatMessage(ChatRole.User, "Hello")];
+
+        AIContextProvider.InvokingContext context = new(
+            mockAgent.Object,
+            session,
+            new AIContext { Messages = messages });
+
+        // Act
+        AIContext result = await provider.InvokingAsync(context);
+
+        // Assert
+        Assert.NotNull(result.Messages);
+        List<ChatMessage> resultList = [.. result.Messages!];
+        Assert.Single(resultList);
+        Assert.Equal("Hello", resultList[0].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncThrowsOnNullStrategyAsync()
+    {
+        List<ChatMessage> messages = [new ChatMessage(ChatRole.User, "Hello")];
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => CompactionProvider.CompactAsync(null!, messages));
+    }
+
+    [Fact]
+    public async Task CompactAsyncReturnsAllMessagesWhenTriggerDoesNotFireAsync()
+    {
+        // Arrange — trigger never fires → no compaction
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ];
+
+        // Act
+        IEnumerable<ChatMessage> result = await CompactionProvider.CompactAsync(strategy, messages);
+
+        // Assert — all messages preserved
+        List<ChatMessage> resultList = [.. result];
+        Assert.Equal(messages.Count, resultList.Count);
+        Assert.Equal("Q1", resultList[0].Text);
+        Assert.Equal("A1", resultList[1].Text);
+        Assert.Equal("Q2", resultList[2].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncReducesMessagesWhenTriggeredAsync()
+    {
+        // Arrange — strategy that always triggers and keeps only 1 group
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ];
+
+        // Act
+        IEnumerable<ChatMessage> result = await CompactionProvider.CompactAsync(strategy, messages);
+
+        // Assert — compaction should have reduced the message count
+        List<ChatMessage> resultList = [.. result];
+        Assert.True(resultList.Count < messages.Count);
+    }
+
+    [Fact]
+    public async Task CompactAsyncHandlesEmptyMessageListAsync()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        List<ChatMessage> messages = [];
+
+        // Act
+        IEnumerable<ChatMessage> result = await CompactionProvider.CompactAsync(strategy, messages);
+
+        // Assert
+        Assert.Empty(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncWorksWithNonListEnumerableAsync()
+    {
+        // Arrange — IEnumerable (not a List<ChatMessage>) to exercise the list copy branch
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(100000));
+        IEnumerable<ChatMessage> messages = [new ChatMessage(ChatRole.User, "Hello")];
+
+        // Act
+        IEnumerable<ChatMessage> result = await CompactionProvider.CompactAsync(strategy, messages);
+
+        // Assert
+        List<ChatMessage> resultList = [.. result];
+        Assert.Single(resultList);
+        Assert.Equal("Hello", resultList[0].Text);
+    }
+
+    [Fact]
+    public void CompactionStateAssignment()
+    {
+        // Arrange
+        CompactionProvider.State state = new();
+
+        // Assert
+        Assert.NotNull(state.MessageGroups);
+        Assert.Empty(state.MessageGroups);
+
+        // Act
+        state.MessageGroups = [new CompactionMessageGroup(CompactionGroupKind.User, [], 0, 0, 0)];
+
+        // Assert
+        Assert.Single(state.MessageGroups);
+    }
+
+    private sealed class TestAgentSession : AgentSession;
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionStrategyTests.cs

@@ -0,0 +1,236 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="CompactionStrategy"/> abstract base class.
+/// </summary>
+public class CompactionStrategyTests
+{
+    [Fact]
+    public void ConstructorNullTriggerThrows()
+    {
+        // Act & Assert
+        Assert.Throws<ArgumentNullException>(() => new TestStrategy(null!));
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerNotMetReturnsFalseAsync()
+    {
+        // Arrange — trigger never fires, but enough non-system groups to pass short-circuit
+        TestStrategy strategy = new(_ => false);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(0, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerMetCallsApplyAsync()
+    {
+        // Arrange — trigger always fires, enough non-system groups
+        TestStrategy strategy = new(_ => true, applyFunc: _ => true);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.True(result);
+        Assert.Equal(1, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncReturnsFalseWhenApplyReturnsFalseAsync()
+    {
+        // Arrange — trigger fires but Apply does nothing
+        TestStrategy strategy = new(_ => true, applyFunc: _ => false);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(1, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSingleNonSystemGroupShortCircuitsAsync()
+    {
+        // Arrange — trigger would fire, but only 1 non-system group → short-circuit
+        TestStrategy strategy = new(_ => true, applyFunc: _ => true);
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "Hello")]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — short-circuited before trigger or Apply
+        Assert.False(result);
+        Assert.Equal(0, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSingleNonSystemGroupWithSystemShortCircuitsAsync()
+    {
+        // Arrange — system group + 1 non-system group → still short-circuits
+        TestStrategy strategy = new(_ => true, applyFunc: _ => true);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "Hello"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — system groups don't count, still only 1 non-system group
+        Assert.False(result);
+        Assert.Equal(0, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncTwoNonSystemGroupsProceedsToTriggerAsync()
+    {
+        // Arrange — exactly 2 non-system groups: boundary passes, trigger fires
+        TestStrategy strategy = new(_ => true, applyFunc: _ => true);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — not short-circuited, Apply was called
+        Assert.True(result);
+        Assert.Equal(1, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncDefaultTargetIsInverseOfTriggerAsync()
+    {
+        // Arrange — trigger fires when groups > 2
+        // Default target should be: stop when groups <= 2 (i.e., !trigger)
+        CompactionTrigger trigger = CompactionTriggers.GroupsExceed(2);
+        TestStrategy strategy = new(trigger, applyFunc: index =>
+        {
+            // Exclude oldest non-system group one at a time
+            foreach (CompactionMessageGroup group in index.Groups)
+            {
+                if (!group.IsExcluded && group.Kind != CompactionGroupKind.System)
+                {
+                    group.IsExcluded = true;
+                    // Target (default = !trigger) returns true when groups <= 2
+                    // So the strategy would check Target after this exclusion
+                    break;
+                }
+            }
+
+            return true;
+        });
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — trigger fires (4 > 2), Apply is called
+        Assert.True(result);
+        Assert.Equal(1, strategy.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncCustomTargetIsPassedToStrategyAsync()
+    {
+        // Arrange — custom target that always signals stop
+        bool targetCalled = false;
+        bool CustomTarget(CompactionMessageIndex _)
+        {
+            targetCalled = true;
+            return true;
+        }
+
+        TestStrategy strategy = new(_ => true, CustomTarget, _ =>
+        {
+            // Access the target from within the strategy
+            return true;
+        });
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — the custom target is accessible (verified by TestStrategy checking it)
+        Assert.Equal(1, strategy.ApplyCallCount);
+        // The target is accessible to derived classes via the protected property
+        Assert.True(strategy.InvokeTarget(index));
+        Assert.True(targetCalled);
+    }
+
+    /// <summary>
+    /// A concrete test implementation of <see cref="CompactionStrategy"/> for testing the base class.
+    /// </summary>
+    private sealed class TestStrategy : CompactionStrategy
+    {
+        private readonly Func<CompactionMessageIndex, bool>? _applyFunc;
+
+        public TestStrategy(
+            CompactionTrigger trigger,
+            CompactionTrigger? target = null,
+            Func<CompactionMessageIndex, bool>? applyFunc = null)
+            : base(trigger, target)
+        {
+            this._applyFunc = applyFunc;
+        }
+
+        public int ApplyCallCount { get; private set; }
+
+        /// <summary>
+        /// Exposes the protected Target property for test verification.
+        /// </summary>
+        public bool InvokeTarget(CompactionMessageIndex index) => this.Target(index);
+
+        protected override ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+        {
+            this.ApplyCallCount++;
+            bool result = this._applyFunc?.Invoke(index) ?? false;
+            return new(result);
+        }
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionTriggersTests.cs

@@ -0,0 +1,180 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for <see cref="CompactionTrigger"/> and <see cref="CompactionTriggers"/>.
+/// </summary>
+public class CompactionTriggersTests
+{
+    [Fact]
+    public void TokensExceedReturnsTrueWhenAboveThreshold()
+    {
+        // Arrange — use a long message to guarantee tokens > 0
+        CompactionTrigger trigger = CompactionTriggers.TokensExceed(0);
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "Hello world")]);
+
+        // Act & Assert
+        Assert.True(trigger(index));
+    }
+
+    [Fact]
+    public void TokensExceedReturnsFalseWhenBelowThreshold()
+    {
+        CompactionTrigger trigger = CompactionTriggers.TokensExceed(999_999);
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "Hi")]);
+
+        Assert.False(trigger(index));
+    }
+
+    [Fact]
+    public void MessagesExceedReturnsExpectedResult()
+    {
+        CompactionTrigger trigger = CompactionTriggers.MessagesExceed(2);
+        CompactionMessageIndex small = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "A"),
+            new ChatMessage(ChatRole.User, "B"),
+        ]);
+        CompactionMessageIndex large = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "A"),
+            new ChatMessage(ChatRole.User, "B"),
+            new ChatMessage(ChatRole.User, "C"),
+        ]);
+
+        Assert.False(trigger(small));
+        Assert.True(trigger(large));
+    }
+
+    [Fact]
+    public void TurnsExceedReturnsExpectedResult()
+    {
+        CompactionTrigger trigger = CompactionTriggers.TurnsExceed(1);
+        CompactionMessageIndex oneTurn = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+        ]);
+        CompactionMessageIndex twoTurns = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        Assert.False(trigger(oneTurn));
+        Assert.True(trigger(twoTurns));
+    }
+
+    [Fact]
+    public void GroupsExceedReturnsExpectedResult()
+    {
+        CompactionTrigger trigger = CompactionTriggers.GroupsExceed(2);
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "A"),
+            new ChatMessage(ChatRole.Assistant, "B"),
+            new ChatMessage(ChatRole.User, "C"),
+        ]);
+
+        Assert.True(trigger(index));
+    }
+
+    [Fact]
+    public void HasToolCallsReturnsTrueWhenToolCallGroupExists()
+    {
+        CompactionTrigger trigger = CompactionTriggers.HasToolCalls();
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+            new ChatMessage(ChatRole.Tool, "result"),
+        ]);
+
+        Assert.True(trigger(index));
+    }
+
+    [Fact]
+    public void HasToolCallsReturnsFalseWhenNoToolCallGroup()
+    {
+        CompactionTrigger trigger = CompactionTriggers.HasToolCalls();
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        Assert.False(trigger(index));
+    }
+
+    [Fact]
+    public void AllRequiresAllConditions()
+    {
+        CompactionTrigger trigger = CompactionTriggers.All(
+            CompactionTriggers.TokensExceed(0),
+            CompactionTriggers.MessagesExceed(5));
+
+        CompactionMessageIndex small = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "A")]);
+
+        // Tokens > 0 is true, but messages > 5 is false
+        Assert.False(trigger(small));
+    }
+
+    [Fact]
+    public void AnyRequiresAtLeastOneCondition()
+    {
+        CompactionTrigger trigger = CompactionTriggers.Any(
+            CompactionTriggers.TokensExceed(999_999),
+            CompactionTriggers.MessagesExceed(0));
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "A")]);
+
+        // Tokens not exceeded, but messages > 0 is true
+        Assert.True(trigger(index));
+    }
+
+    [Fact]
+    public void AllEmptyTriggersReturnsTrue()
+    {
+        CompactionTrigger trigger = CompactionTriggers.All();
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "A")]);
+        Assert.True(trigger(index));
+    }
+
+    [Fact]
+    public void AnyEmptyTriggersReturnsFalse()
+    {
+        CompactionTrigger trigger = CompactionTriggers.Any();
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "A")]);
+        Assert.False(trigger(index));
+    }
+
+    [Fact]
+    public void TokensBelowReturnsTrueWhenBelowThreshold()
+    {
+        CompactionTrigger trigger = CompactionTriggers.TokensBelow(999_999);
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "Hi")]);
+
+        Assert.True(trigger(index));
+    }
+
+    [Fact]
+    public void TokensBelowReturnsFalseWhenAboveThreshold()
+    {
+        CompactionTrigger trigger = CompactionTriggers.TokensBelow(0);
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "Hello world")]);
+
+        Assert.False(trigger(index));
+    }
+
+    [Fact]
+    public void AlwaysReturnsTrue()
+    {
+        CompactionMessageIndex index = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "A")]);
+        Assert.True(CompactionTriggers.Always(index));
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/PipelineCompactionStrategyTests.cs

@@ -0,0 +1,208 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="PipelineCompactionStrategy"/> class.
+/// </summary>
+public class PipelineCompactionStrategyTests
+{
+    [Fact]
+    public async Task CompactAsyncExecutesAllStrategiesInOrderAsync()
+    {
+        // Arrange
+        List<string> executionOrder = [];
+        TestCompactionStrategy strategy1 = new(
+            _ =>
+            {
+                executionOrder.Add("first");
+                return false;
+            });
+
+        TestCompactionStrategy strategy2 = new(
+            _ =>
+            {
+                executionOrder.Add("second");
+                return false;
+            });
+
+        PipelineCompactionStrategy pipeline = new(strategy1, strategy2);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        await pipeline.CompactAsync(groups);
+
+        // Assert
+        Assert.Equal(["first", "second"], executionOrder);
+    }
+
+    [Fact]
+    public async Task CompactAsyncReturnsFalseWhenNoStrategyCompactsAsync()
+    {
+        // Arrange
+        TestCompactionStrategy strategy1 = new(_ => false);
+
+        PipelineCompactionStrategy pipeline = new(strategy1);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await pipeline.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncReturnsTrueWhenAnyStrategyCompactsAsync()
+    {
+        // Arrange
+        TestCompactionStrategy strategy1 = new(_ => false);
+        TestCompactionStrategy strategy2 = new(_ => true);
+
+        PipelineCompactionStrategy pipeline = new(strategy1, strategy2);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await pipeline.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncContinuesAfterFirstCompactionAsync()
+    {
+        // Arrange
+        TestCompactionStrategy strategy1 = new(_ => true);
+        TestCompactionStrategy strategy2 = new(_ => false);
+
+        PipelineCompactionStrategy pipeline = new(strategy1, strategy2);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        await pipeline.CompactAsync(groups);
+
+        // Assert — both strategies were called
+        Assert.Equal(1, strategy1.ApplyCallCount);
+        Assert.Equal(1, strategy2.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncComposesStrategiesEndToEndAsync()
+    {
+        // Arrange — pipeline: first exclude oldest 2 non-system groups, then exclude 2 more
+        static void ExcludeOldest2(CompactionMessageIndex index)
+        {
+            int excluded = 0;
+            foreach (CompactionMessageGroup group in index.Groups)
+            {
+                if (!group.IsExcluded && group.Kind != CompactionGroupKind.System && excluded < 2)
+                {
+                    group.IsExcluded = true;
+                    excluded++;
+                }
+            }
+        }
+
+        TestCompactionStrategy phase1 = new(
+            index =>
+            {
+                ExcludeOldest2(index);
+                return true;
+            });
+
+        TestCompactionStrategy phase2 = new(
+            index =>
+            {
+                ExcludeOldest2(index);
+                return true;
+            });
+
+        PipelineCompactionStrategy pipeline = new(phase1, phase2);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act
+        bool result = await pipeline.CompactAsync(groups);
+
+        // Assert — system is preserved, phase1 excluded Q1+A1, phase2 excluded Q2+A2 → System + Q3
+        Assert.True(result);
+        Assert.Equal(2, groups.IncludedGroupCount);
+
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        Assert.Equal(2, included.Count);
+        Assert.Equal("You are helpful.", included[0].Text);
+        Assert.Equal("Q3", included[1].Text);
+
+        Assert.Equal(1, phase1.ApplyCallCount);
+        Assert.Equal(1, phase2.ApplyCallCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncEmptyPipelineReturnsFalseAsync()
+    {
+        // Arrange
+        PipelineCompactionStrategy pipeline = new(new List<CompactionStrategy>());
+        CompactionMessageIndex groups = CompactionMessageIndex.Create([new ChatMessage(ChatRole.User, "Hello")]);
+
+        // Act
+        bool result = await pipeline.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    /// <summary>
+    /// A simple test implementation of <see cref="CompactionStrategy"/> that delegates to a synchronous callback.
+    /// </summary>
+    private sealed class TestCompactionStrategy : CompactionStrategy
+    {
+        private readonly Func<CompactionMessageIndex, bool> _applyFunc;
+
+        public TestCompactionStrategy(Func<CompactionMessageIndex, bool> applyFunc)
+            : base(CompactionTriggers.Always)
+        {
+            this._applyFunc = applyFunc;
+        }
+
+        public int ApplyCallCount { get; private set; }
+
+        protected override ValueTask<bool> CompactCoreAsync(CompactionMessageIndex index, ILogger logger, CancellationToken cancellationToken)
+        {
+            this.ApplyCallCount++;
+            return new(this._applyFunc(index));
+        }
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SlidingWindowCompactionStrategyTests.cs

@@ -0,0 +1,311 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="SlidingWindowCompactionStrategy"/> class.
+/// </summary>
+public class SlidingWindowCompactionStrategyTests
+{
+    [Fact]
+    public async Task CompactAsyncBelowMaxTurnsReturnsFalseAsync()
+    {
+        // Arrange — trigger requires > 3 turns, conversation has 2
+        SlidingWindowCompactionStrategy strategy = new(CompactionTriggers.TurnsExceed(3));
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncExceedsMaxTurnsExcludesOldestTurnsAsync()
+    {
+        // Arrange — trigger on > 2 turns, conversation has 3
+        SlidingWindowCompactionStrategy strategy = new(CompactionTriggers.TurnsExceed(2));
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+            new ChatMessage(ChatRole.Assistant, "A3"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        // Turn 1 (Q1 + A1) should be excluded
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.True(groups.Groups[1].IsExcluded);
+        // Turn 2 and 3 should remain
+        Assert.False(groups.Groups[2].IsExcluded);
+        Assert.False(groups.Groups[3].IsExcluded);
+        Assert.False(groups.Groups[4].IsExcluded);
+        Assert.False(groups.Groups[5].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesSystemMessagesAsync()
+    {
+        // Arrange — trigger on > 1 turn
+        SlidingWindowCompactionStrategy strategy = new(CompactionTriggers.TurnsExceed(1));
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        Assert.False(groups.Groups[0].IsExcluded); // System preserved
+        Assert.True(groups.Groups[1].IsExcluded);  // Turn 1 excluded
+        Assert.True(groups.Groups[2].IsExcluded);  // Turn 1 response excluded
+        Assert.False(groups.Groups[3].IsExcluded); // Turn 2 kept
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesToolCallGroupsInKeptTurnsAsync()
+    {
+        // Arrange — trigger on > 1 turn
+        SlidingWindowCompactionStrategy strategy = new(CompactionTriggers.TurnsExceed(1));
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("call1", "search")]),
+            new ChatMessage(ChatRole.Tool, "Results"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        // Turn 1 excluded
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.True(groups.Groups[1].IsExcluded);
+        // Turn 2 kept (user + tool call group)
+        Assert.False(groups.Groups[2].IsExcluded);
+        Assert.False(groups.Groups[3].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerNotMetReturnsFalseAsync()
+    {
+        // Arrange — trigger requires > 99 turns
+        SlidingWindowCompactionStrategy strategy = new(CompactionTriggers.TurnsExceed(99));
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncIncludedMessagesContainOnlyKeptTurnsAsync()
+    {
+        // Arrange — trigger on > 1 turn
+        SlidingWindowCompactionStrategy strategy = new(CompactionTriggers.TurnsExceed(1));
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "System"),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        Assert.Equal(3, included.Count);
+        Assert.Equal("System", included[0].Text);
+        Assert.Equal("Q2", included[1].Text);
+        Assert.Equal("A2", included[2].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncCustomTargetStopsExcludingEarlyAsync()
+    {
+        // Arrange — trigger on > 1 turn, custom target stops after removing 1 turn
+        int removeCount = 0;
+        bool TargetAfterOne(CompactionMessageIndex _) => ++removeCount >= 1;
+
+        SlidingWindowCompactionStrategy strategy = new(
+            CompactionTriggers.TurnsExceed(1),
+            minimumPreservedTurns: 0,
+            target: TargetAfterOne);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+            new ChatMessage(ChatRole.Assistant, "A3"),
+            new ChatMessage(ChatRole.User, "Q4"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — only turn 1 excluded (target stopped after 1 removal)
+        Assert.True(result);
+        Assert.True(index.Groups[0].IsExcluded);   // Q1 (turn 1)
+        Assert.True(index.Groups[1].IsExcluded);   // A1 (turn 1)
+        Assert.False(index.Groups[2].IsExcluded);  // Q2 (turn 2) — kept
+        Assert.False(index.Groups[3].IsExcluded);  // A2 (turn 2)
+    }
+
+    [Fact]
+    public async Task CompactAsyncMinimumPreservedStopsCompactionAsync()
+    {
+        // Arrange — always trigger with never-satisfied target, but MinimumPreserved = 2 is hard floor
+        SlidingWindowCompactionStrategy strategy = new(
+            CompactionTriggers.TurnsExceed(1),
+            minimumPreservedTurns: 2,
+            target: _ => false);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+            new ChatMessage(ChatRole.Assistant, "A3"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — target never says stop, but MinimumPreserved=2 protects the last 2 turns
+        Assert.True(result);
+        Assert.Equal(4, index.IncludedGroupCount);
+        // Turn 1 excluded
+        Assert.True(index.Groups[0].IsExcluded);   // Q1
+        Assert.True(index.Groups[1].IsExcluded);   // A1
+        // Last 2 turns must be preserved
+        Assert.False(index.Groups[2].IsExcluded);  // Q2
+        Assert.False(index.Groups[3].IsExcluded);  // A2
+        Assert.False(index.Groups[4].IsExcluded);  // Q3
+        Assert.False(index.Groups[5].IsExcluded);  // A3
+    }
+
+    [Fact]
+    public async Task CompactAsyncSkipsExcludedAndSystemGroupsInEnumerationAsync()
+    {
+        // Arrange — includes system and pre-excluded groups that must be skipped
+        SlidingWindowCompactionStrategy strategy = new(
+            CompactionTriggers.TurnsExceed(1),
+            minimumPreservedTurns: 0);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "System prompt"),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+        // Pre-exclude one group
+        index.Groups[1].IsExcluded = true;
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — system preserved, pre-excluded skipped
+        Assert.True(result);
+        Assert.False(index.Groups[0].IsExcluded); // System preserved
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesTurnIndexZeroAsync()
+    {
+        // Arrange — assistant message before first user turn gets TurnIndex = 0
+        SlidingWindowCompactionStrategy strategy = new(
+            CompactionTriggers.TurnsExceed(1),
+            minimumPreservedTurns: 0,
+            target: _ => false);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.Assistant, "Welcome!"),  // TurnIndex = 0
+            new ChatMessage(ChatRole.User, "Q1"),             // TurnIndex = 1
+            new ChatMessage(ChatRole.Assistant, "A1"),        // TurnIndex = 1
+            new ChatMessage(ChatRole.User, "Q2"),             // TurnIndex = 2
+            new ChatMessage(ChatRole.Assistant, "A2"),        // TurnIndex = 2
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — TurnIndex = 0 is always preserved even with minimumPreservedTurns = 0
+        Assert.True(result);
+        Assert.False(index.Groups[0].IsExcluded);  // Welcome (TurnIndex 0) preserved
+        Assert.True(index.Groups[1].IsExcluded);   // Q1 (TurnIndex 1) excluded
+        Assert.True(index.Groups[2].IsExcluded);   // A1 (TurnIndex 1) excluded
+        Assert.True(index.Groups[3].IsExcluded);   // Q2 (TurnIndex 2) excluded
+        Assert.True(index.Groups[4].IsExcluded);   // A2 (TurnIndex 2) excluded
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesNullTurnIndexAsync()
+    {
+        // Arrange — system messages (TurnIndex = null) should never be removed
+        SlidingWindowCompactionStrategy strategy = new(
+            CompactionTriggers.TurnsExceed(0),
+            minimumPreservedTurns: 0,
+            target: _ => false);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — system message (TurnIndex null) always preserved
+        Assert.True(result);
+        Assert.False(index.Groups[0].IsExcluded);  // System (TurnIndex null) preserved
+        Assert.True(index.Groups[1].IsExcluded);   // Q1 excluded
+        Assert.True(index.Groups[2].IsExcluded);   // A1 excluded
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SummarizationCompactionStrategyTests.cs

@@ -0,0 +1,613 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+using Moq;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="SummarizationCompactionStrategy"/> class.
+/// </summary>
+public class SummarizationCompactionStrategyTests
+{
+    /// <summary>
+    /// Creates a mock <see cref="IChatClient"/> that returns the specified summary text.
+    /// </summary>
+    private static IChatClient CreateMockChatClient(string summaryText = "Summary of conversation.")
+    {
+        Mock<IChatClient> mock = new();
+        mock.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new ChatResponse([new ChatMessage(ChatRole.Assistant, summaryText)]));
+        return mock.Object;
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerNotMetReturnsFalseAsync()
+    {
+        // Arrange — trigger requires > 100000 tokens
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient(),
+            CompactionTriggers.TokensExceed(100000),
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(2, index.IncludedGroupCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSummarizesOldGroupsAsync()
+    {
+        // Arrange — always trigger, preserve 1 recent group
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient("Key facts from earlier."),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "First question"),
+            new ChatMessage(ChatRole.Assistant, "First answer"),
+            new ChatMessage(ChatRole.User, "Second question"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.True(result);
+
+        List<ChatMessage> included = [.. index.GetIncludedMessages()];
+
+        // Should have: summary + preserved recent group (Second question)
+        Assert.Equal(2, included.Count);
+        Assert.Contains("[Summary]", included[0].Text);
+        Assert.Contains("Key facts from earlier.", included[0].Text);
+        Assert.Equal("Second question", included[1].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesSystemMessagesAsync()
+    {
+        // Arrange
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient(),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "Old question"),
+            new ChatMessage(ChatRole.Assistant, "Old answer"),
+            new ChatMessage(ChatRole.User, "Recent question"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert
+        List<ChatMessage> included = [.. index.GetIncludedMessages()];
+
+        Assert.Equal("You are helpful.", included[0].Text);
+        Assert.Equal(ChatRole.System, included[0].Role);
+    }
+
+    [Fact]
+    public async Task CompactAsyncInsertsSummaryGroupAtCorrectPositionAsync()
+    {
+        // Arrange
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient("Summary text."),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "System prompt."),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — summary should be inserted after system, before preserved group
+        CompactionMessageGroup summaryGroup = index.Groups.First(g => g.Kind == CompactionGroupKind.Summary);
+        Assert.NotNull(summaryGroup);
+        Assert.Contains("[Summary]", summaryGroup.Messages[0].Text);
+        Assert.True(summaryGroup.Messages[0].AdditionalProperties!.ContainsKey(CompactionMessageGroup.SummaryPropertyKey));
+    }
+
+    [Fact]
+    public async Task CompactAsyncHandlesEmptyLlmResponseAsync()
+    {
+        // Arrange — LLM returns whitespace
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient("   "),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — should use fallback text
+        List<ChatMessage> included = [.. index.GetIncludedMessages()];
+        Assert.Contains("[Summary unavailable]", included[0].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncNothingToSummarizeReturnsFalseAsync()
+    {
+        // Arrange — preserve 5 but only 2 non-system groups
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient(),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 5);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncUsesCustomPromptAsync()
+    {
+        // Arrange — capture the messages sent to the chat client
+        List<ChatMessage>? capturedMessages = null;
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .Callback<IEnumerable<ChatMessage>, ChatOptions?, CancellationToken>((msgs, _, _) =>
+                capturedMessages = [.. msgs])
+            .ReturnsAsync(new ChatResponse([new ChatMessage(ChatRole.Assistant, "Custom summary.")]));
+
+        const string CustomPrompt = "Summarize in bullet points only.";
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1,
+            summarizationPrompt: CustomPrompt);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — the custom prompt should be the system message, followed by the original messages
+        Assert.NotNull(capturedMessages);
+        Assert.Equal(2, capturedMessages.Count);
+        Assert.Equal(ChatRole.System, capturedMessages![0].Role);
+        Assert.Equal(CustomPrompt, capturedMessages[0].Text);
+        Assert.Equal(ChatRole.User, capturedMessages[1].Role);
+        Assert.Equal("Q1", capturedMessages[1].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSetsExcludeReasonAsync()
+    {
+        // Arrange
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient(),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Old"),
+            new ChatMessage(ChatRole.User, "New"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert
+        CompactionMessageGroup excluded = index.Groups.First(g => g.IsExcluded);
+        Assert.NotNull(excluded.ExcludeReason);
+        Assert.Contains("SummarizationCompactionStrategy", excluded.ExcludeReason);
+    }
+
+    [Fact]
+    public async Task CompactAsyncTargetStopsMarkingEarlyAsync()
+    {
+        // Arrange — 4 non-system groups, preserve 1, target met after 1 exclusion
+        int exclusionCount = 0;
+        bool TargetAfterOne(CompactionMessageIndex _) => ++exclusionCount >= 1;
+
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient("Partial summary."),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1,
+            target: TargetAfterOne);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — only 1 group should have been summarized (target met after first exclusion)
+        int excludedCount = index.Groups.Count(g => g.IsExcluded);
+        Assert.Equal(1, excludedCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesMultipleRecentGroupsAsync()
+    {
+        // Arrange — preserve 2
+        SummarizationCompactionStrategy strategy = new(
+            CreateMockChatClient("Summary."),
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 2);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — 2 oldest excluded, 2 newest preserved + 1 summary inserted
+        List<ChatMessage> included = [.. index.GetIncludedMessages()];
+        Assert.Equal(3, included.Count); // summary + Q2 + A2
+        Assert.Contains("[Summary]", included[0].Text);
+        Assert.Equal("Q2", included[1].Text);
+        Assert.Equal("A2", included[2].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncWithSystemBetweenSummarizableGroupsAsync()
+    {
+        // Arrange — system group between user/assistant groups to exercise skip logic in loop
+        IChatClient mockClient = CreateMockChatClient("[Summary]");
+        SummarizationCompactionStrategy strategy = new(
+            mockClient,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.System, "System note"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — summary inserted at 0, system group shifted to index 2
+        Assert.True(result);
+        Assert.Equal(CompactionGroupKind.Summary, index.Groups[0].Kind);
+        Assert.Equal(CompactionGroupKind.System, index.Groups[2].Kind);
+        Assert.False(index.Groups[2].IsExcluded); // System never excluded
+    }
+
+    [Fact]
+    public async Task CompactAsyncMaxSummarizableBoundsLoopExitAsync()
+    {
+        // Arrange — large MinimumPreserved so maxSummarizable is small, target never stops
+        IChatClient mockClient = CreateMockChatClient("[Summary]");
+        SummarizationCompactionStrategy strategy = new(
+            mockClient,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 3,
+            target: _ => false);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+            new ChatMessage(ChatRole.Assistant, "A3"),
+        ]);
+
+        // Act — should only summarize 6-3 = 3 groups (not all 6)
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — 3 preserved + 1 summary = 4 included
+        Assert.True(result);
+        Assert.Equal(4, index.IncludedGroupCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncWithPreExcludedGroupAsync()
+    {
+        // Arrange — pre-exclude a group so the count and loop both must skip it
+        IChatClient mockClient = CreateMockChatClient("[Summary]");
+        SummarizationCompactionStrategy strategy = new(
+            mockClient,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+        index.Groups[0].IsExcluded = true; // Pre-exclude Q1
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert
+        Assert.True(result);
+        Assert.True(index.Groups[0].IsExcluded); // Still excluded
+    }
+
+    [Fact]
+    public async Task CompactAsyncWithEmptyTextMessageInGroupAsync()
+    {
+        // Arrange — a message with null text (FunctionCallContent) in a summarized group
+        IChatClient mockClient = CreateMockChatClient("[Summary]");
+        SummarizationCompactionStrategy strategy = new(
+            mockClient,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+        ];
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(messages);
+
+        // Act — the tool-call group's message has null text
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — compaction succeeded despite null text
+        Assert.True(result);
+    }
+
+    #region Error resilience
+
+    [Fact]
+    public async Task CompactAsyncLlmFailureRestoresGroupsAsync()
+    {
+        // Arrange — chat client throws a non-cancellation exception
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new InvalidOperationException("Service unavailable"));
+
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        int originalGroupCount = index.Groups.Count;
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — returns false, all groups restored to non-excluded
+        Assert.False(result);
+        Assert.Equal(originalGroupCount, index.Groups.Count);
+        Assert.All(index.Groups, g => Assert.False(g.IsExcluded));
+        Assert.All(index.Groups, g => Assert.Null(g.ExcludeReason));
+    }
+
+    [Fact]
+    public async Task CompactAsyncLlmFailurePreservesAllOriginalMessagesAsync()
+    {
+        // Arrange — verify that after failure, GetIncludedMessages returns all original messages
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new HttpRequestException("Timeout"));
+
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        List<ChatMessage> originalIncluded = [.. index.GetIncludedMessages()];
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — all original messages still included
+        List<ChatMessage> afterIncluded = [.. index.GetIncludedMessages()];
+        Assert.Equal(originalIncluded.Count, afterIncluded.Count);
+        for (int i = 0; i < originalIncluded.Count; i++)
+        {
+            Assert.Same(originalIncluded[i], afterIncluded[i]);
+        }
+    }
+
+    [Fact]
+    public async Task CompactAsyncLlmFailureDoesNotInsertSummaryGroupAsync()
+    {
+        // Arrange
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new InvalidOperationException("API error"));
+
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(index);
+
+        // Assert — no Summary group was inserted
+        Assert.DoesNotContain(index.Groups, g => g.Kind == CompactionGroupKind.Summary);
+    }
+
+    [Fact]
+    public async Task CompactAsyncCancellationPropagatesAsync()
+    {
+        // Arrange — OperationCanceledException should NOT be caught
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new OperationCanceledException("Cancelled"));
+
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act & Assert — OperationCanceledException propagates
+        await Assert.ThrowsAsync<OperationCanceledException>(
+            () => strategy.CompactAsync(index).AsTask());
+    }
+
+    [Fact]
+    public async Task CompactAsyncTaskCancellationPropagatesAsync()
+    {
+        // Arrange — TaskCanceledException (subclass of OperationCanceledException) should also propagate
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new TaskCanceledException("Task cancelled"));
+
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act & Assert — TaskCanceledException propagates (inherits from OperationCanceledException)
+        await Assert.ThrowsAsync<TaskCanceledException>(
+            () => strategy.CompactAsync(index).AsTask());
+    }
+
+    [Fact]
+    public async Task CompactAsyncLlmFailureWithMultipleExcludedGroupsRestoresAllAsync()
+    {
+        // Arrange — multiple groups excluded before failure, all must be restored
+        Mock<IChatClient> mockClient = new();
+        mockClient.Setup(c => c.GetResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new InvalidOperationException("Rate limited"));
+
+        SummarizationCompactionStrategy strategy = new(
+            mockClient.Object,
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1,
+            target: _ => false); // Never stop — exclude as many as possible
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "System prompt"),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — all non-system groups restored
+        Assert.False(result);
+        Assert.All(index.Groups, g => Assert.False(g.IsExcluded));
+        Assert.All(index.Groups, g => Assert.Null(g.ExcludeReason));
+        Assert.Equal(6, index.IncludedGroupCount);
+    }
+
+    #endregion
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ToolResultCompactionStrategyTests.cs

@@ -0,0 +1,351 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="ToolResultCompactionStrategy"/> class.
+/// </summary>
+public class ToolResultCompactionStrategyTests
+{
+    [Fact]
+    public async Task CompactAsyncTriggerNotMetReturnsFalseAsync()
+    {
+        // Arrange — trigger requires > 1000 tokens
+        ToolResultCompactionStrategy strategy = new(CompactionTriggers.TokensExceed(1000));
+
+        ChatMessage toolCall = new(ChatRole.Assistant, [new FunctionCallContent("call1", "get_weather")]);
+        ChatMessage toolResult = new(ChatRole.Tool, "Sunny");
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "What's the weather?"),
+            toolCall,
+            toolResult,
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncCollapsesOldToolGroupsAsync()
+    {
+        // Arrange — always trigger
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("call1", "get_weather")]),
+            new ChatMessage(ChatRole.Tool, "Sunny and 72°F"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        // Q1 + collapsed tool summary + Q2
+        Assert.Equal(3, included.Count);
+        Assert.Equal("Q1", included[0].Text);
+        Assert.Equal("[Tool Calls]\nget_weather:\n  - Sunny and 72°F", included[1].Text);
+        Assert.Equal("Q2", included[2].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesRecentToolGroupsAsync()
+    {
+        // Arrange — protect 2 recent non-system groups (the tool group + Q2)
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 3);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("call1", "search")]),
+            new ChatMessage(ChatRole.Tool, "Results"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert — all groups are in the protected window, nothing to collapse
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesSystemMessagesAsync()
+    {
+        // Arrange
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("call1", "fn")]),
+            new ChatMessage(ChatRole.Tool, "result"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        Assert.Equal("You are helpful.", included[0].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncExtractsMultipleToolNamesAsync()
+    {
+        // Arrange — assistant calls two tools
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1);
+
+        ChatMessage multiToolCall = new(ChatRole.Assistant,
+        [
+            new FunctionCallContent("c1", "get_weather"),
+            new FunctionCallContent("c2", "search_docs"),
+        ]);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            multiToolCall,
+            new ChatMessage(ChatRole.Tool, "Sunny"),
+            new ChatMessage(ChatRole.Tool, "Found 3 docs"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        string collapsed = included[1].Text!;
+        Assert.Equal("[Tool Calls]\nget_weather:\n  - Sunny\nsearch_docs:\n  - Found 3 docs", collapsed);
+    }
+
+    [Fact]
+    public async Task CompactAsyncNoToolGroupsReturnsFalseAsync()
+    {
+        // Arrange — trigger fires but no tool groups to collapse
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 0);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncCompoundTriggerRequiresTokensAndToolCallsAsync()
+    {
+        // Arrange — compound: tokens > 0 AND has tool calls
+        ToolResultCompactionStrategy strategy = new(
+            CompactionTriggers.All(
+                CompactionTriggers.TokensExceed(0),
+                CompactionTriggers.HasToolCalls()),
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+            new ChatMessage(ChatRole.Tool, "result"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncTargetStopsCollapsingEarlyAsync()
+    {
+        // Arrange — 2 tool groups, target met after first collapse
+        int collapseCount = 0;
+        bool TargetAfterOne(CompactionMessageIndex _) => ++collapseCount >= 1;
+
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1,
+            target: TargetAfterOne);
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn1")]),
+            new ChatMessage(ChatRole.Tool, "result1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c2", "fn2")]),
+            new ChatMessage(ChatRole.Tool, "result2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — only first tool group collapsed, second left intact
+        Assert.True(result);
+
+        // Count collapsed tool groups (excluded with ToolCall kind)
+        int collapsedToolGroups = 0;
+        foreach (CompactionMessageGroup group in index.Groups)
+        {
+            if (group.IsExcluded && group.Kind == CompactionGroupKind.ToolCall)
+            {
+                collapsedToolGroups++;
+            }
+        }
+
+        Assert.Equal(1, collapsedToolGroups);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSkipsPreExcludedAndSystemGroupsAsync()
+    {
+        // Arrange — pre-excluded and system groups in the enumeration
+        ToolResultCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 0);
+
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.System, "System prompt"),
+            new ChatMessage(ChatRole.User, "Q0"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+            new ChatMessage(ChatRole.Tool, "Result 1"),
+            new ChatMessage(ChatRole.User, "Q1"),
+        ];
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create(messages);
+        // Pre-exclude the last user group
+        index.Groups[index.Groups.Count - 1].IsExcluded = true;
+
+        // Act
+        bool result = await strategy.CompactAsync(index);
+
+        // Assert — system never excluded, pre-excluded skipped
+        Assert.True(result);
+        Assert.False(index.Groups[0].IsExcluded); // System stays
+    }
+
+    [Fact]
+    public async Task CompactAsyncDeduplicatesDuplicateToolNamesAsync()
+    {
+        // Arrange — same tool called multiple times
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionCallContent("c1", "get_weather"),
+                new FunctionCallContent("c2", "get_weather"),
+            ]),
+            new ChatMessage(ChatRole.Tool, "Sunny"),
+            new ChatMessage(ChatRole.Tool, "Rainy"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert — duplicate names listed once with all results
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        Assert.Equal("[Tool Calls]\nget_weather:\n  - Sunny\n  - Rainy", included[1].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncIncludesResultsFromFunctionResultContentAsync()
+    {
+        // Arrange — tool results provided as FunctionResultContent (matched by CallId)
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionCallContent("c1", "get_weather"),
+                new FunctionCallContent("c2", "search_docs"),
+            ]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("c1", "Sunny and 72°F")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("c2", "Found 3 docs")]),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert — results matched by CallId and included in summary
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        Assert.Equal("[Tool Calls]\nget_weather:\n  - Sunny and 72°F\nsearch_docs:\n  - Found 3 docs", included[1].Text);
+    }
+
+    [Fact]
+    public async Task CompactAsyncDeduplicatesWithFunctionResultContentAsync()
+    {
+        // Arrange — same tool called multiple times with FunctionResultContent
+        ToolResultCompactionStrategy strategy = new(
+            trigger: _ => true,
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionCallContent("c1", "get_weather"),
+                new FunctionCallContent("c2", "get_weather"),
+                new FunctionCallContent("c3", "search_docs"),
+            ]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("c1", "Sunny")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("c2", "Rainy")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("c3", "Found 3 docs")]),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert — duplicate tool name results listed under same key
+        List<ChatMessage> included = [.. groups.GetIncludedMessages()];
+        Assert.Equal("[Tool Calls]\nget_weather:\n  - Sunny\n  - Rainy\nsearch_docs:\n  - Found 3 docs", included[1].Text);
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/TruncationCompactionStrategyTests.cs

@@ -0,0 +1,328 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="TruncationCompactionStrategy"/> class.
+/// </summary>
+public class TruncationCompactionStrategyTests
+{
+    [Fact]
+    public async Task CompactAsyncAlwaysTriggerCompactsToPreserveRecentAsync()
+    {
+        // Arrange — always-trigger means always compact
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "First"),
+            new ChatMessage(ChatRole.Assistant, "Response 1"),
+            new ChatMessage(ChatRole.User, "Second"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        Assert.Equal(1, groups.Groups.Count(g => !g.IsExcluded));
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerNotMetReturnsFalseAsync()
+    {
+        // Arrange — trigger requires > 1000 tokens, conversation is tiny
+        TruncationCompactionStrategy strategy = new(
+            minimumPreservedGroups: 1,
+            trigger: CompactionTriggers.TokensExceed(1000));
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+        Assert.Equal(2, groups.IncludedGroupCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncTriggerMetExcludesOldestGroupsAsync()
+    {
+        // Arrange — trigger on groups > 2
+        TruncationCompactionStrategy strategy = new(
+            minimumPreservedGroups: 1,
+            trigger: CompactionTriggers.GroupsExceed(2));
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "First"),
+            new ChatMessage(ChatRole.Assistant, "Response 1"),
+            new ChatMessage(ChatRole.User, "Second"),
+            new ChatMessage(ChatRole.Assistant, "Response 2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert — incremental: excludes until GroupsExceed(2) is no longer met → 2 groups remain
+        Assert.True(result);
+        Assert.Equal(2, groups.IncludedGroupCount);
+        // Oldest 2 excluded, newest 2 kept
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.True(groups.Groups[1].IsExcluded);
+        Assert.False(groups.Groups[2].IsExcluded);
+        Assert.False(groups.Groups[3].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesSystemMessagesAsync()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "You are helpful."),
+            new ChatMessage(ChatRole.User, "First"),
+            new ChatMessage(ChatRole.Assistant, "Response 1"),
+            new ChatMessage(ChatRole.User, "Second"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        // System message should be preserved
+        Assert.False(groups.Groups[0].IsExcluded);
+        Assert.Equal(CompactionGroupKind.System, groups.Groups[0].Kind);
+        // Oldest non-system groups excluded
+        Assert.True(groups.Groups[1].IsExcluded);
+        Assert.True(groups.Groups[2].IsExcluded);
+        // Most recent kept
+        Assert.False(groups.Groups[3].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncPreservesToolCallGroupAtomicityAsync()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+
+        ChatMessage assistantToolCall = new(ChatRole.Assistant, [new FunctionCallContent("call1", "get_weather")]);
+        ChatMessage toolResult = new(ChatRole.Tool, "Sunny");
+        ChatMessage finalResponse = new(ChatRole.User, "Thanks!");
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create([assistantToolCall, toolResult, finalResponse]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        // Tool call group should be excluded as one atomic unit
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.Equal(CompactionGroupKind.ToolCall, groups.Groups[0].Kind);
+        Assert.Equal(2, groups.Groups[0].Messages.Count);
+        Assert.False(groups.Groups[1].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSetsExcludeReasonAsync()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Old"),
+            new ChatMessage(ChatRole.User, "New"),
+        ]);
+
+        // Act
+        await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.NotNull(groups.Groups[0].ExcludeReason);
+        Assert.Contains("TruncationCompactionStrategy", groups.Groups[0].ExcludeReason);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSkipsAlreadyExcludedGroupsAsync()
+    {
+        // Arrange
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Already excluded"),
+            new ChatMessage(ChatRole.User, "Included 1"),
+            new ChatMessage(ChatRole.User, "Included 2"),
+        ]);
+        groups.Groups[0].IsExcluded = true;
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        Assert.True(groups.Groups[0].IsExcluded); // was already excluded
+        Assert.True(groups.Groups[1].IsExcluded); // newly excluded
+        Assert.False(groups.Groups[2].IsExcluded); // kept
+    }
+
+    [Fact]
+    public async Task CompactAsyncMinimumPreservedKeepsMultipleAsync()
+    {
+        // Arrange — keep 2 most recent
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 2);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.True(result);
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.True(groups.Groups[1].IsExcluded);
+        Assert.False(groups.Groups[2].IsExcluded);
+        Assert.False(groups.Groups[3].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncNothingToRemoveReturnsFalseAsync()
+    {
+        // Arrange — preserve 5 but only 2 groups
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 5);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, "Hi!"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public async Task CompactAsyncCustomTargetStopsEarlyAsync()
+    {
+        // Arrange — always trigger, custom target stops after 1 exclusion
+        int targetChecks = 0;
+        bool TargetAfterOne(CompactionMessageIndex _) => ++targetChecks >= 1;
+
+        TruncationCompactionStrategy strategy = new(
+            CompactionTriggers.Always,
+            minimumPreservedGroups: 1,
+            target: TargetAfterOne);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert — only 1 group excluded (target met after first)
+        Assert.True(result);
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.False(groups.Groups[1].IsExcluded);
+        Assert.False(groups.Groups[2].IsExcluded);
+        Assert.False(groups.Groups[3].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncIncrementalStopsAtTargetAsync()
+    {
+        // Arrange — trigger on groups > 2, target is default (inverse of trigger: groups <= 2)
+        TruncationCompactionStrategy strategy = new(
+            CompactionTriggers.GroupsExceed(2),
+            minimumPreservedGroups: 1);
+
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+            new ChatMessage(ChatRole.User, "Q3"),
+        ]);
+
+        // Act — 5 groups, trigger fires (5 > 2), compacts until groups <= 2
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert — should stop at 2 included groups (not go all the way to 1)
+        Assert.True(result);
+        Assert.Equal(2, groups.IncludedGroupCount);
+    }
+
+    [Fact]
+    public async Task CompactAsyncLoopExitsWhenMaxRemovableReachedAsync()
+    {
+        // Arrange — target never stops (always false), so the loop must exit via removed >= maxRemovable
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 2, target: CompactionTriggers.Never);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+            new ChatMessage(ChatRole.Assistant, "A2"),
+        ]);
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert — only 2 removed (maxRemovable = 4 - 2 = 2), 2 preserved
+        Assert.True(result);
+        Assert.Equal(2, groups.IncludedGroupCount);
+        Assert.True(groups.Groups[0].IsExcluded);
+        Assert.True(groups.Groups[1].IsExcluded);
+        Assert.False(groups.Groups[2].IsExcluded);
+        Assert.False(groups.Groups[3].IsExcluded);
+    }
+
+    [Fact]
+    public async Task CompactAsyncSkipsPreExcludedAndSystemGroupsAsync()
+    {
+        // Arrange — has excluded + system groups that the loop must skip
+        TruncationCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreservedGroups: 1);
+        CompactionMessageIndex groups = CompactionMessageIndex.Create(
+        [
+            new ChatMessage(ChatRole.System, "System"),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+            new ChatMessage(ChatRole.User, "Q2"),
+        ]);
+        // Pre-exclude one group
+        groups.Groups[1].IsExcluded = true;
+
+        // Act
+        bool result = await strategy.CompactAsync(groups);
+
+        // Assert — system preserved, pre-excluded skipped, A1 removed, Q2 preserved
+        Assert.True(result);
+        Assert.False(groups.Groups[0].IsExcluded); // System
+        Assert.True(groups.Groups[1].IsExcluded);  // Pre-excluded Q1
+        Assert.True(groups.Groups[2].IsExcluded);  // Newly excluded A1
+        Assert.False(groups.Groups[3].IsExcluded); // Preserved Q2
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Microsoft.Agents.AI.UnitTests.csproj

@@ -16,6 +16,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" />
     <PackageReference Include="Microsoft.Extensions.Logging" />
+    <PackageReference Include="Microsoft.ML.Tokenizers" />
     <PackageReference Include="OpenTelemetry" />
     <PackageReference Include="OpenTelemetry.Exporter.InMemory" />
     <PackageReference Include="System.Linq.AsyncEnumerable" />

dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/MessageMergerTests.cs

@@ -37,5 +37,36 @@ public class MessageMergerTests
         response.CreatedAt.Should().NotBe(creationTime);
         response.Messages[0].CreatedAt.Should().Be(creationTime);
         response.Messages[0].Contents.Should().HaveCount(1);
+        response.FinishReason.Should().BeNull();
+    }
+
+    [Fact]
+    public void Test_MessageMerger_PropagatesFinishReasonFromUpdates()
+    {
+        // Arrange
+        string responseId = Guid.NewGuid().ToString("N");
+        string messageId = Guid.NewGuid().ToString("N");
+
+        MessageMerger merger = new();
+
+        foreach (AgentResponseUpdate update in "Hello".ToAgentRunStream(agentId: TestAgentId1, messageId: messageId, responseId: responseId))
+        {
+            merger.AddUpdate(update);
+        }
+
+        // Add a final update with FinishReason set
+        merger.AddUpdate(new AgentResponseUpdate
+        {
+            ResponseId = responseId,
+            MessageId = messageId,
+            FinishReason = ChatFinishReason.ContentFilter,
+            Role = ChatRole.Assistant,
+        });
+
+        // Act
+        AgentResponse response = merger.ComputeMerged(responseId);
+
+        // Assert - FinishReason from the update should propagate through
+        response.FinishReason.Should().Be(ChatFinishReason.ContentFilter);
     }
 }

dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/PolymorphicOutputTests.cs

@@ -0,0 +1,276 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using FluentAssertions;
+
+namespace Microsoft.Agents.AI.Workflows.UnitTests;
+
+/// <summary>
+/// Regression tests for polymorphic output type handling in workflows.
+/// Verifies that executors can return derived types when the declared output type is a base class.
+/// </summary>
+/// <remarks>
+/// This addresses GitHub issue #4134: InvalidOperationException when returning derived type as workflow output.
+/// </remarks>
+public partial class PolymorphicOutputTests
+{
+    #region Test Type Hierarchy
+
+    /// <summary>
+    /// Base class used as declared output type.
+    /// </summary>
+    public class BaseOutput
+    {
+        public virtual string Name => "BaseOutput";
+    }
+
+    /// <summary>
+    /// Derived class returned at runtime.
+    /// </summary>
+    public class DerivedOutput : BaseOutput
+    {
+        public override string Name => "DerivedOutput";
+    }
+
+    /// <summary>
+    /// Second-level derived class for testing multiple inheritance levels.
+    /// </summary>
+    public class GrandchildOutput : DerivedOutput
+    {
+        public override string Name => "GrandchildOutput";
+    }
+
+    /// <summary>
+    /// Unrelated class that should NOT be accepted as output.
+    /// </summary>
+    public class UnrelatedOutput
+    {
+        public string Name => "UnrelatedOutput";
+    }
+
+    #endregion
+
+    #region Test Executors
+
+    /// <summary>
+    /// Executor that declares BaseOutput as yield type but returns DerivedOutput.
+    /// </summary>
+    internal sealed class DerivedOutputExecutor() : Executor(nameof(DerivedOutputExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private async ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            await Task.Delay(10, cancellationToken);
+
+            // Arrange: Return a derived type where the method signature declares the base type
+            return new DerivedOutput();
+        }
+    }
+
+    /// <summary>
+    /// Executor that declares BaseOutput as yield type but returns GrandchildOutput (two levels deep).
+    /// </summary>
+    internal sealed class GrandchildOutputExecutor() : Executor(nameof(GrandchildOutputExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private async ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            await Task.Delay(10, cancellationToken);
+
+            // Arrange: Return a grandchild type (two inheritance levels)
+            return new GrandchildOutput();
+        }
+    }
+
+    /// <summary>
+    /// Executor that attempts to return an unrelated type - should fail validation.
+    /// This executor intentionally bypasses type safety to test runtime validation.
+    /// </summary>
+    internal sealed class UnrelatedOutputExecutor() : Executor(nameof(UnrelatedOutputExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private async ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            // Arrange: Attempt to yield an unrelated type - should throw
+            UnrelatedOutput unrelated = new();
+            await context.YieldOutputAsync(unrelated, cancellationToken).ConfigureAwait(false);
+
+            // This line should not be reached
+            return new BaseOutput();
+        }
+    }
+
+    /// <summary>
+    /// Executor that returns the exact declared type (baseline test).
+    /// </summary>
+    internal sealed class ExactTypeExecutor() : Executor(nameof(ExactTypeExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            BaseOutput result = new();
+            return new ValueTask<BaseOutput>(result);
+        }
+    }
+
+    #endregion
+
+    #region Tests
+
+    /// <summary>
+    /// Verifies that returning a derived type when the declared output type is a base class succeeds.
+    /// This is the main regression test for GitHub issue #4134.
+    /// </summary>
+    [Fact]
+    public async Task ReturningDerivedType_WhenBaseTypeIsDeclared_ShouldSucceedAsync()
+    {
+        // Arrange
+        DerivedOutputExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        events.Should().NotBeEmpty("workflow should produce events");
+
+        List<WorkflowOutputEvent> outputEvents = events.OfType<WorkflowOutputEvent>().ToList();
+        outputEvents.Should().ContainSingle("workflow should produce exactly one output event");
+
+        WorkflowOutputEvent outputEvent = outputEvents.Single();
+        outputEvent.Data.Should().BeOfType<DerivedOutput>("output should be the derived type");
+        ((DerivedOutput)outputEvent.Data!).Name.Should().Be("DerivedOutput");
+
+        // Verify no error events
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().BeEmpty("workflow should not produce error events");
+    }
+
+    /// <summary>
+    /// Verifies that returning a grandchild type (multiple inheritance levels) succeeds.
+    /// </summary>
+    [Fact]
+    public async Task ReturningGrandchildType_WhenBaseTypeIsDeclared_ShouldSucceedAsync()
+    {
+        // Arrange
+        GrandchildOutputExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        events.Should().NotBeEmpty("workflow should produce events");
+
+        List<WorkflowOutputEvent> outputEvents = events.OfType<WorkflowOutputEvent>().ToList();
+        outputEvents.Should().ContainSingle("workflow should produce exactly one output event");
+
+        WorkflowOutputEvent outputEvent = outputEvents.Single();
+        outputEvent.Data.Should().BeOfType<GrandchildOutput>("output should be the grandchild type");
+        ((GrandchildOutput)outputEvent.Data!).Name.Should().Be("GrandchildOutput");
+
+        // Verify no error events
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().BeEmpty("workflow should not produce error events");
+    }
+
+    /// <summary>
+    /// Verifies that returning an unrelated type still throws InvalidOperationException.
+    /// This ensures the fix doesn't break the existing validation for truly incompatible types.
+    /// </summary>
+    [Fact]
+    public async Task ReturningUnrelatedType_WhenBaseTypeIsDeclared_ShouldFailAsync()
+    {
+        // Arrange
+        UnrelatedOutputExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert: Should have an error event with InvalidOperationException message
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().ContainSingle("workflow should produce exactly one error event");
+
+        WorkflowErrorEvent errorEvent = errorEvents.Single();
+        string errorMessage = errorEvent.Data?.ToString() ?? string.Empty;
+        errorMessage.Should().Contain("Cannot output object of type UnrelatedOutput");
+        errorMessage.Should().Contain("BaseOutput");
+    }
+
+    /// <summary>
+    /// Verifies that returning the exact declared type still works (baseline test).
+    /// </summary>
+    [Fact]
+    public async Task ReturningExactType_WhenSameTypeIsDeclared_ShouldSucceedAsync()
+    {
+        // Arrange: Create an executor that returns the exact declared type
+        ExactTypeExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        events.Should().NotBeEmpty("workflow should produce events");
+
+        List<WorkflowOutputEvent> outputEvents = events.OfType<WorkflowOutputEvent>().ToList();
+        outputEvents.Should().ContainSingle("workflow should produce exactly one output event");
+
+        WorkflowOutputEvent outputEvent = outputEvents.Single();
+        outputEvent.Data.Should().BeOfType<BaseOutput>("output should be the exact base type");
+
+        // Verify no error events
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().BeEmpty("workflow should not produce error events");
+    }
+
+    #endregion
+}

dotnet/tests/OpenAIAssistant.IntegrationTests/OpenAIAssistantClientExtensionsTests.cs

@@ -19,6 +19,8 @@ namespace OpenAIAssistant.IntegrationTests;
 
 public class OpenAIAssistantClientExtensionsTests
 {
+    private const string SkipCodeInterpreterReason = "OpenAI Assistant Code Interpreter intermittently fails in CI";
+
     private readonly AssistantClient _assistantClient = new OpenAIClient(TestConfiguration.GetRequiredValue(TestSettings.OpenAIApiKey)).GetAssistantClient();
     private readonly OpenAIFileClient _fileClient = new OpenAIClient(TestConfiguration.GetRequiredValue(TestSettings.OpenAIApiKey)).GetOpenAIFileClient();
 
@@ -81,7 +83,7 @@ public class OpenAIAssistantClientExtensionsTests
         }
     }
 
-    [Theory]
+    [Theory(Skip = SkipCodeInterpreterReason)]
     [InlineData("CreateWithChatClientAgentOptionsAsync")]
     [InlineData("CreateWithChatClientAgentOptionsSync")]
     [InlineData("CreateWithParamsAsync")]

python/AGENTS.md

@@ -20,6 +20,13 @@ When making changes to a package, check if the following need updates:
 - The package's `AGENTS.md` file (adding/removing/renaming public APIs, architecture changes, import path changes)
 - The agent skills in `.github/skills/` if conventions, commands, or workflows change
 
+## Pull Request Description Guidance
+
+When preparing a PR description:
+- Follow the repository PR template at `.github/pull_request_template.md` and keep its structure/headings.
+- Describe the net change relative to `main` (this is implied; do not call it out explicitly as "vs main").
+- Do not add ad-hoc validation sections (for example, "Validation" or "Tests run"); CI/CD and the template checklist cover validation status.
+
 ## Quick Reference
 
 Run `uv run poe` from the `python/` directory to see available commands. See [DEV_SETUP.md](DEV_SETUP.md) for detailed usage.