.NET: [BREAKING]Delay AIContext Materialization until the end of the pipeline is reached. (#3883)

* Delay AIContext Materialization until the end of the pipeline is reached.

* Address PR comments.

* Address PR comments

dotnet/samples/GettingStarted/Agents/Agent_Step20_AdditionalAIContext/Program.cs

@@ -119,14 +119,13 @@ namespace SampleApp
                 {
                     AIFunctionFactory.Create((string item) => AddTodoItem(context.Session, item), "AddTodoItem", "Adds an item to the todo list."),
                     AIFunctionFactory.Create((int index) => RemoveTodoItem(context.Session, index), "RemoveTodoItem", "Removes an item from the todo list. Index is zero based.")
-                }).ToList(),
+                }),
                 Messages =
                     (inputContext.Messages ?? [])
                     .Concat(
                     [
                         new MEAI.ChatMessage(ChatRole.User, outputMessageBuilder.ToString()).WithAgentRequestMessageSource(AgentRequestMessageSourceType.AIContextProvider, this.GetType().FullName!)
                     ])
-                    .ToList()
             });
         }
 

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

@@ -56,41 +56,44 @@ public sealed class AIContext
     public string? Instructions { get; set; }
 
     /// <summary>
-    /// Gets or sets a collection of messages to add to the conversation history.
+    /// Gets or sets the sequence of messages to use for the current invocation.
     /// </summary>
     /// <value>
-    /// A list of <see cref="ChatMessage"/> instances to be permanently added to the conversation history,
-    /// or <see langword="null"/> if no messages should be added.
+    /// A sequence of <see cref="ChatMessage"/> instances to be used for the current invocation,
+    /// or <see langword="null"/> if no messages should be used.
     /// </value>
     /// <remarks>
     /// <para>
-    /// Unlike <see cref="Instructions"/> and <see cref="Tools"/>, messages added through this property become
-    /// permanent additions to the conversation history. They will persist beyond the current invocation and
-    /// will be available in future interactions within the same conversation thread.
+    /// Unlike <see cref="Instructions"/> and <see cref="Tools"/>, messages added through this property may become
+    /// permanent additions to the conversation history.
+    /// If chat history is managed by the underlying AI service, these messages will become part of chat history.
+    /// If chat history is managed using a <see cref="ChatHistoryProvider"/>, these messages will be passed to the
+    /// <see cref="ChatHistoryProvider.InvokedCoreAsync(ChatHistoryProvider.InvokedContext, System.Threading.CancellationToken)"/> method,
+    /// and the provider can choose which of these messages to permanently add to the conversation history.
     /// </para>
     /// <para>
     /// This property is useful for:
     /// <list type="bullet">
-    /// <item><description>Injecting relevant historical context or background information</description></item>
+    /// <item><description>Injecting relevant historical context e.g. memories</description></item>
+    /// <item><description>Injecting relevant background information e.g. via Retrieval Augmented Generation</description></item>
     /// <item><description>Adding system messages that provide ongoing context</description></item>
-    /// <item><description>Including retrieved information that should be part of the conversation record</description></item>
-    /// <item><description>Inserting contextual exchanges that inform the current conversation</description></item>
     /// </list>
     /// </para>
     /// </remarks>
-    public IList<ChatMessage>? Messages { get; set; }
+    public IEnumerable<ChatMessage>? Messages { get; set; }
 
     /// <summary>
-    /// Gets or sets a collection of tools or functions to make available to the AI model for the current invocation.
+    /// Gets or sets a sequence of tools or functions to make available to the AI model for the current invocation.
     /// </summary>
     /// <value>
-    /// A list of <see cref="AITool"/> instances that will be available to the AI model during the current invocation,
+    /// A sequence of <see cref="AITool"/> instances that will be available to the AI model during the current invocation,
     /// or <see langword="null"/> if no additional tools should be provided.
     /// </value>
     /// <remarks>
     /// <para>
-    /// These tools are transient and apply only to the current AI model invocation. They are combined with any
-    /// tools already configured for the agent to provide an expanded set of capabilities for the specific interaction.
+    /// These tools are transient and apply only to the current AI model invocation. Any existing tools
+    /// are provided as input to the <see cref="AIContextProvider"/> instances, so context providers can choose to modify or replace the existing tools
+    /// as needed based on the current context. The resulting set of tools is then passed to the underlying AI model, which may choose to utilize them when generating responses.
     /// </para>
     /// <para>
     /// Context-specific tools enable:
@@ -102,5 +105,5 @@ public sealed class AIContext
     /// </list>
     /// </para>
     /// </remarks>
-    public IList<AITool>? Tools { get; set; }
+    public IEnumerable<AITool>? Tools { get; set; }
 }

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

@@ -170,8 +170,7 @@ public sealed class Mem0Provider : AIContextProvider
                 Instructions = inputContext.Instructions,
                 Messages =
                     (inputContext.Messages ?? [])
-                    .Concat(outputMessage is not null ? [outputMessage] : [])
-                    .ToList(),
+                    .Concat(outputMessage is not null ? [outputMessage] : []),
                 Tools = inputContext.Tools
             };
         }

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

@@ -677,7 +677,7 @@ public sealed partial class ChatClientAgent : AIAgent
             throw new InvalidOperationException("Input messages are not allowed when continuing a background response using a continuation token.");
         }
 
-        List<ChatMessage> inputMessagesForChatClient = [];
+        IEnumerable<ChatMessage> inputMessagesForChatClient = inputMessages;
 
         // Populate the session messages only if we are not continuing an existing response as it's not allowed
         if (chatOptions?.ContinuationToken is null)
@@ -688,13 +688,8 @@ public sealed partial class ChatClientAgent : AIAgent
             // The ChatHistoryProvider returns the merged result (history + input messages).
             if (chatHistoryProvider is not null)
             {
-                var invokingContext = new ChatHistoryProvider.InvokingContext(this, typedSession, inputMessages);
-                var providerMessages = await chatHistoryProvider.InvokingAsync(invokingContext, cancellationToken).ConfigureAwait(false);
-                inputMessagesForChatClient.AddRange(providerMessages);
-            }
-            else
-            {
-                inputMessagesForChatClient.AddRange(inputMessages);
+                var invokingContext = new ChatHistoryProvider.InvokingContext(this, typedSession, inputMessagesForChatClient);
+                inputMessagesForChatClient = await chatHistoryProvider.InvokingAsync(invokingContext, cancellationToken).ConfigureAwait(false);
             }
 
             // If we have an AIContextProvider, we should get context from it, and update our
@@ -705,8 +700,8 @@ public sealed partial class ChatClientAgent : AIAgent
                 var aiContext = new AIContext
                 {
                     Instructions = chatOptions?.Instructions,
-                    Messages = inputMessagesForChatClient.ToList(),
-                    Tools = chatOptions?.Tools as List<AITool> ?? chatOptions?.Tools?.ToList()
+                    Messages = inputMessagesForChatClient,
+                    Tools = chatOptions?.Tools
                 };
 
                 foreach (var aiContextProvider in aiContextProviders)
@@ -715,13 +710,14 @@ public sealed partial class ChatClientAgent : AIAgent
                     aiContext = await aiContextProvider.InvokingAsync(invokingContext, cancellationToken).ConfigureAwait(false);
                 }
 
-                // Use the returned messages, tools and instructions directly since the provider accumulated them.
-                inputMessagesForChatClient = aiContext.Messages as List<ChatMessage> ?? aiContext.Messages?.ToList() ?? [];
+                // Materialize the accumulated messages and tools once at the end of the provider pipeline.
+                inputMessagesForChatClient = aiContext.Messages ?? [];
 
-                if (chatOptions?.Tools is { Count: > 0 } || aiContext.Tools is { Count: > 0 })
+                var tools = aiContext.Tools as IList<AITool> ?? aiContext.Tools?.ToList();
+                if (chatOptions?.Tools is { Count: > 0 } || tools is { Count: > 0 })
                 {
                     chatOptions ??= new();
-                    chatOptions.Tools = aiContext.Tools;
+                    chatOptions.Tools = tools;
                 }
 
                 if (chatOptions?.Instructions is not null || aiContext.Instructions is not null)
@@ -750,7 +746,10 @@ public sealed partial class ChatClientAgent : AIAgent
             chatOptions.ConversationId = typedSession.ConversationId;
         }
 
-        return (typedSession, chatOptions, inputMessagesForChatClient, continuationToken);
+        // Materialize the accumulated messages once at the end of the provider pipeline, reusing the existing list if possible.
+        List<ChatMessage> messagesList = inputMessagesForChatClient as List<ChatMessage> ?? inputMessagesForChatClient.ToList();
+
+        return (typedSession, chatOptions, messagesList, continuationToken);
     }
 
     private void UpdateSessionConversationId(ChatClientAgentSession session, string? responseConversationId, CancellationToken cancellationToken)

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

@@ -171,7 +171,7 @@ public sealed class ChatHistoryMemoryProvider : AIContextProvider, IDisposable
             {
                 Instructions = inputContext.Instructions,
                 Messages = inputContext.Messages,
-                Tools = (inputContext.Tools ?? []).Concat(tools).ToList()
+                Tools = (inputContext.Tools ?? []).Concat(tools)
             };
         }
 
@@ -204,8 +204,7 @@ public sealed class ChatHistoryMemoryProvider : AIContextProvider, IDisposable
                     .Concat(
                     [
                         new ChatMessage(ChatRole.User, contextText).WithAgentRequestMessageSource(AgentRequestMessageSourceType.AIContextProvider, this.GetType().FullName!)
-                    ])
-                    .ToList(),
+                    ]),
                 Tools = inputContext.Tools
             };
         }

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

@@ -105,7 +105,7 @@ public sealed class TextSearchProvider : AIContextProvider
             {
                 Instructions = inputContext.Instructions,
                 Messages = inputContext.Messages,
-                Tools = (inputContext.Tools ?? []).Concat(this._tools).ToList()
+                Tools = (inputContext.Tools ?? []).Concat(this._tools)
             };
         }
 
@@ -161,8 +161,7 @@ public sealed class TextSearchProvider : AIContextProvider
                     .Concat(
                     [
                         new ChatMessage(ChatRole.User, formatted).WithAgentRequestMessageSource(AgentRequestMessageSourceType.AIContextProvider, this.GetType().FullName!)
-                    ])
-                    .ToList(),
+                    ]),
                 Tools = inputContext.Tools
             };
         }

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

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System.Linq;
 using Microsoft.Extensions.AI;
 
 namespace Microsoft.Agents.AI.Abstractions.UnitTests;
@@ -33,9 +34,10 @@ public class AIContextTests
         };
 
         Assert.NotNull(context.Messages);
-        Assert.Equal(2, context.Messages.Count);
-        Assert.Equal("Hello", context.Messages[0].Text);
-        Assert.Equal("Hi there!", context.Messages[1].Text);
+        var messages = context.Messages.ToList();
+        Assert.Equal(2, messages.Count);
+        Assert.Equal("Hello", messages[0].Text);
+        Assert.Equal("Hi there!", messages[1].Text);
     }
 
     [Fact]
@@ -51,8 +53,9 @@ public class AIContextTests
         };
 
         Assert.NotNull(context.Tools);
-        Assert.Equal(2, context.Tools.Count);
-        Assert.Equal("Function1", context.Tools[0].Name);
-        Assert.Equal("Function2", context.Tools[1].Name);
+        var tools = context.Tools.ToList();
+        Assert.Equal(2, tools.Count);
+        Assert.Equal("Function1", tools[0].Name);
+        Assert.Equal("Function2", tools[1].Name);
     }
 }

dotnet/tests/Microsoft.Agents.AI.Mem0.IntegrationTests/Mem0ProviderTests.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Headers;
 using System.Threading;
@@ -54,7 +55,7 @@ public sealed class Mem0ProviderTests : IDisposable
 
         await sut.ClearStoredMemoriesAsync(mockSession);
         var ctxBefore = await sut.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, mockSession, new AIContext { Messages = new List<ChatMessage> { question } }));
-        Assert.DoesNotContain("Caoimhe", ctxBefore.Messages?[0].Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxBefore.Messages?.LastOrDefault()?.Text ?? string.Empty);
 
         // Act
         await sut.InvokedAsync(new AIContextProvider.InvokedContext(s_mockAgent, mockSession, [input]));
@@ -63,8 +64,8 @@ public sealed class Mem0ProviderTests : IDisposable
         var ctxAfterClearing = await sut.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, mockSession, new AIContext { Messages = new List<ChatMessage> { question } }));
 
         // Assert
-        Assert.Contains("Caoimhe", ctxAfterAdding.Messages?[0].Text ?? string.Empty);
-        Assert.DoesNotContain("Caoimhe", ctxAfterClearing.Messages?[0].Text ?? string.Empty);
+        Assert.Contains("Caoimhe", ctxAfterAdding.Messages?.LastOrDefault()?.Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxAfterClearing.Messages?.LastOrDefault()?.Text ?? string.Empty);
     }
 
     [Fact(Skip = SkipReason)]
@@ -79,7 +80,7 @@ public sealed class Mem0ProviderTests : IDisposable
 
         await sut.ClearStoredMemoriesAsync(mockSession);
         var ctxBefore = await sut.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, mockSession, new AIContext { Messages = new List<ChatMessage> { question } }));
-        Assert.DoesNotContain("Caoimhe", ctxBefore.Messages?[0].Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxBefore.Messages?.LastOrDefault()?.Text ?? string.Empty);
 
         // Act
         await sut.InvokedAsync(new AIContextProvider.InvokedContext(s_mockAgent, mockSession, [assistantIntro]));
@@ -88,8 +89,8 @@ public sealed class Mem0ProviderTests : IDisposable
         var ctxAfterClearing = await sut.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, mockSession, new AIContext { Messages = new List<ChatMessage> { question } }));
 
         // Assert
-        Assert.Contains("Caoimhe", ctxAfterAdding.Messages?[0].Text ?? string.Empty);
-        Assert.DoesNotContain("Caoimhe", ctxAfterClearing.Messages?[0].Text ?? string.Empty);
+        Assert.Contains("Caoimhe", ctxAfterAdding.Messages?.LastOrDefault()?.Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxAfterClearing.Messages?.LastOrDefault()?.Text ?? string.Empty);
     }
 
     [Fact(Skip = SkipReason)]
@@ -110,8 +111,8 @@ public sealed class Mem0ProviderTests : IDisposable
 
         var ctxBefore1 = await sut1.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, mockSession1, new AIContext { Messages = new List<ChatMessage> { question } }));
         var ctxBefore2 = await sut2.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, mockSession2, new AIContext { Messages = new List<ChatMessage> { question } }));
-        Assert.DoesNotContain("Caoimhe", ctxBefore1.Messages?[0].Text ?? string.Empty);
-        Assert.DoesNotContain("Caoimhe", ctxBefore2.Messages?[0].Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxBefore1.Messages?.LastOrDefault()?.Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxBefore2.Messages?.LastOrDefault()?.Text ?? string.Empty);
 
         // Act
         await sut1.InvokedAsync(new AIContextProvider.InvokedContext(s_mockAgent, mockSession1, [assistantIntro]));
@@ -119,8 +120,8 @@ public sealed class Mem0ProviderTests : IDisposable
         var ctxAfterAdding2 = await GetContextWithRetryAsync(sut2, mockSession2, question);
 
         // Assert
-        Assert.Contains("Caoimhe", ctxAfterAdding1.Messages?[0].Text ?? string.Empty);
-        Assert.DoesNotContain("Caoimhe", ctxAfterAdding2.Messages?[0].Text ?? string.Empty);
+        Assert.Contains("Caoimhe", ctxAfterAdding1.Messages?.LastOrDefault()?.Text ?? string.Empty);
+        Assert.DoesNotContain("Caoimhe", ctxAfterAdding2.Messages?.LastOrDefault()?.Text ?? string.Empty);
 
         // Cleanup
         await sut1.ClearStoredMemoriesAsync(mockSession1);
@@ -133,7 +134,7 @@ public sealed class Mem0ProviderTests : IDisposable
         for (int i = 0; i < attempts; i++)
         {
             ctx = await provider.InvokingAsync(new AIContextProvider.InvokingContext(s_mockAgent, session, new AIContext { Messages = new List<ChatMessage> { question } }), CancellationToken.None);
-            var text = ctx.Messages?[0].Text;
+            var text = ctx.Messages?.LastOrDefault()?.Text;
             if (!string.IsNullOrEmpty(text) && text.IndexOf("Caoimhe", StringComparison.OrdinalIgnoreCase) >= 0)
             {
                 break;

dotnet/tests/Microsoft.Agents.AI.Mem0.UnitTests/Mem0ProviderTests.cs

@@ -118,9 +118,10 @@ public sealed class Mem0ProviderTests : IDisposable
         Assert.Equal("What is my name?", doc.RootElement.GetProperty("query").GetString());
 
         Assert.NotNull(aiContext.Messages);
-        Assert.Equal(2, aiContext.Messages.Count);
-        Assert.Equal(AgentRequestMessageSourceType.External, aiContext.Messages[0].GetAgentRequestMessageSourceType());
-        var contextMessage = aiContext.Messages[1];
+        var messages = aiContext.Messages.ToList();
+        Assert.Equal(2, messages.Count);
+        Assert.Equal(AgentRequestMessageSourceType.External, messages[0].GetAgentRequestMessageSourceType());
+        var contextMessage = messages[1];
         Assert.Equal(ChatRole.User, contextMessage.Role);
         Assert.Contains("Name is Caoimhe", contextMessage.Text);
         Assert.Equal(AgentRequestMessageSourceType.AIContextProvider, contextMessage.GetAgentRequestMessageSourceType());

dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs

@@ -496,9 +496,9 @@ public partial class ChatClientAgentTests
             .Returns((AIContextProvider.InvokingContext ctx, CancellationToken _) =>
                 new ValueTask<AIContext>(new AIContext
                 {
-                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages).ToList(),
+                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages),
                     Instructions = ctx.AIContext.Instructions + "\ncontext provider instructions",
-                    Tools = (ctx.AIContext.Tools ?? []).Concat(new[] { AIFunctionFactory.Create(() => { }, "context provider function") }).ToList()
+                    Tools = (ctx.AIContext.Tools ?? []).Concat(new[] { AIFunctionFactory.Create(() => { }, "context provider function") })
                 }));
         mockProvider
             .Protected()
@@ -567,7 +567,7 @@ public partial class ChatClientAgentTests
             .Returns((AIContextProvider.InvokingContext ctx, CancellationToken _) =>
                 new ValueTask<AIContext>(new AIContext
                 {
-                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages).ToList(),
+                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages),
                 }));
         mockProvider
             .Protected()
@@ -626,7 +626,7 @@ public partial class ChatClientAgentTests
                 new ValueTask<AIContext>(new AIContext
                 {
                     Instructions = ctx.AIContext.Instructions,
-                    Messages = ctx.AIContext.Messages?.ToList(),
+                    Messages = ctx.AIContext.Messages,
                     Tools = ctx.AIContext.Tools
                 }));
 
@@ -1875,9 +1875,9 @@ public partial class ChatClientAgentTests
             .Returns((AIContextProvider.InvokingContext ctx, CancellationToken _) =>
                 new ValueTask<AIContext>(new AIContext
                 {
-                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages).ToList(),
+                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages),
                     Instructions = ctx.AIContext.Instructions + "\ncontext provider instructions",
-                    Tools = (ctx.AIContext.Tools ?? []).Concat(new[] { AIFunctionFactory.Create(() => { }, "context provider function") }).ToList()
+                    Tools = (ctx.AIContext.Tools ?? []).Concat(new[] { AIFunctionFactory.Create(() => { }, "context provider function") })
                 }));
         mockProvider
             .Protected()
@@ -1954,7 +1954,7 @@ public partial class ChatClientAgentTests
             .Returns((AIContextProvider.InvokingContext ctx, CancellationToken _) =>
                 new ValueTask<AIContext>(new AIContext
                 {
-                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages).ToList(),
+                    Messages = (ctx.AIContext.Messages ?? []).Concat(aiContextProviderMessages),
                 }));
         mockProvider
             .Protected()

dotnet/tests/Microsoft.Agents.AI.UnitTests/Data/TextSearchProviderTests.cs

@@ -106,12 +106,13 @@ public sealed class TextSearchProviderTests
         Assert.Equal("Sample user question?\nAdditional part", capturedInput);
         Assert.Null(aiContext.Instructions); // TextSearchProvider uses a user message for context injection.
         Assert.NotNull(aiContext.Messages);
-        Assert.Equal(3, aiContext.Messages!.Count); // 2 input messages + 1 search result message
-        Assert.Equal("Sample user question?", aiContext.Messages![0].Text);
-        Assert.Equal("Additional part", aiContext.Messages![1].Text);
-        Assert.Equal(AgentRequestMessageSourceType.External, aiContext.Messages![0].GetAgentRequestMessageSourceType());
-        Assert.Equal(AgentRequestMessageSourceType.External, aiContext.Messages![1].GetAgentRequestMessageSourceType());
-        var message = aiContext.Messages!.Last();
+        var messages = aiContext.Messages!.ToList();
+        Assert.Equal(3, messages.Count); // 2 input messages + 1 search result message
+        Assert.Equal("Sample user question?", messages[0].Text);
+        Assert.Equal("Additional part", messages[1].Text);
+        Assert.Equal(AgentRequestMessageSourceType.External, messages[0].GetAgentRequestMessageSourceType());
+        Assert.Equal(AgentRequestMessageSourceType.External, messages[1].GetAgentRequestMessageSourceType());
+        var message = messages.Last();
         Assert.Equal(ChatRole.User, message.Role);
         Assert.Equal(AgentRequestMessageSourceType.AIContextProvider, message.GetAgentRequestMessageSourceType());
         string text = message.Text!;
@@ -181,11 +182,13 @@ public sealed class TextSearchProviderTests
 
         // Assert
         Assert.NotNull(aiContext.Messages); // Input messages are preserved.
-        Assert.Single(aiContext.Messages!);
-        Assert.Equal("Q?", aiContext.Messages![0].Text);
+        var messages = aiContext.Messages!.ToList();
+        Assert.Single(messages);
+        Assert.Equal("Q?", messages[0].Text);
         Assert.NotNull(aiContext.Tools);
-        Assert.Single(aiContext.Tools);
-        var tool = aiContext.Tools.Single();
+        var tools = aiContext.Tools!.ToList();
+        Assert.Single(tools);
+        var tool = tools[0];
         Assert.Equal(expectedName, tool.Name);
         Assert.Equal(expectedDescription, tool.Description);
     }
@@ -202,8 +205,9 @@ public sealed class TextSearchProviderTests
 
         // Assert
         Assert.NotNull(aiContext.Messages); // Input messages are preserved on error.
-        Assert.Single(aiContext.Messages!);
-        Assert.Equal("Q?", aiContext.Messages![0].Text);
+        var messages = aiContext.Messages!.ToList();
+        Assert.Single(messages);
+        Assert.Equal("Q?", messages[0].Text);
         Assert.Null(aiContext.Tools);
         this._loggerMock.Verify(
             l => l.Log(
@@ -297,9 +301,10 @@ public sealed class TextSearchProviderTests
 
         // Assert
         Assert.NotNull(aiContext.Messages);
-        Assert.Equal(2, aiContext.Messages!.Count); // 1 input message + 1 formatted result message
-        Assert.Equal("Q?", aiContext.Messages![0].Text);
-        Assert.Equal("Custom formatted context with 2 results.", aiContext.Messages![1].Text);
+        var messages = aiContext.Messages!.ToList();
+        Assert.Equal(2, messages.Count); // 1 input message + 1 formatted result message
+        Assert.Equal("Q?", messages[0].Text);
+        Assert.Equal("Custom formatted context with 2 results.", messages[1].Text);
     }
 
     [Fact]
@@ -332,9 +337,10 @@ public sealed class TextSearchProviderTests
 
         // Assert
         Assert.NotNull(aiContext.Messages);
-        Assert.Equal(2, aiContext.Messages!.Count); // 1 input message + 1 formatted result message
-        Assert.Equal("Q?", aiContext.Messages![0].Text);
-        Assert.Equal("R1,R2", aiContext.Messages![1].Text);
+        var messages = aiContext.Messages!.ToList();
+        Assert.Equal(2, messages.Count); // 1 input message + 1 formatted result message
+        Assert.Equal("Q?", messages[0].Text);
+        Assert.Equal("R1,R2", messages[1].Text);
     }
 
     [Fact]
@@ -350,8 +356,9 @@ public sealed class TextSearchProviderTests
 
         // Assert
         Assert.NotNull(aiContext.Messages); // Input messages are preserved when no results found.
-        Assert.Single(aiContext.Messages!);
-        Assert.Equal("Q?", aiContext.Messages![0].Text);
+        var messages = aiContext.Messages!.ToList();
+        Assert.Single(messages);
+        Assert.Equal("Q?", messages[0].Text);
         Assert.Null(aiContext.Instructions);
         Assert.Null(aiContext.Tools);
     }

dotnet/tests/Microsoft.Agents.AI.UnitTests/Memory/ChatHistoryMemoryProviderTests.cs

@@ -396,9 +396,10 @@ public class ChatHistoryMemoryProviderTests
             Times.Once);
 
         Assert.NotNull(aiContext.Messages);
-        Assert.Equal(2, aiContext.Messages.Count);
-        Assert.Equal(AgentRequestMessageSourceType.External, aiContext.Messages[0].GetAgentRequestMessageSourceType());
-        Assert.Equal(AgentRequestMessageSourceType.AIContextProvider, aiContext.Messages[1].GetAgentRequestMessageSourceType());
+        var messages = aiContext.Messages.ToList();
+        Assert.Equal(2, messages.Count);
+        Assert.Equal(AgentRequestMessageSourceType.External, messages[0].GetAgentRequestMessageSourceType());
+        Assert.Equal(AgentRequestMessageSourceType.AIContextProvider, messages[1].GetAgentRequestMessageSourceType());
     }
 
     [Fact]