diff --git a/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTriggers.cs b/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTriggers.cs
index 44a426f1e2..d2064c4449 100644
--- a/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTriggers.cs
+++ b/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionTriggers.cs
@@ -20,6 +20,12 @@ public static class CompactionTriggers
     public static readonly CompactionTrigger Always =
         _ => true;
 
+    /// <summary>
+    /// Always trigger compaction, 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>
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactingChatClientTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactingChatClientTests.cs
index 80292c7ed8..c165d8c1e6 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactingChatClientTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactingChatClientTests.cs
@@ -1,8 +1,7 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -348,7 +347,7 @@ public sealed class CompactingChatClientTests : IDisposable
         SetRunContext(session);
 
         // Use an IEnumerable (not a List) to trigger the copy path
-        IEnumerable<ChatMessage> messages = new ChatMessage[] { new(ChatRole.User, "Hello") };
+        IEnumerable<ChatMessage> messages = [new(ChatRole.User, "Hello")];
 
         // Act
         ChatResponse response = await client.GetResponseAsync(messages);
@@ -380,7 +379,7 @@ public sealed class CompactingChatClientTests : IDisposable
         AgentRunContext context = new(
             mockAgent.Object,
             session,
-            new List<ChatMessage> { new(ChatRole.User, "test") },
+            [new(ChatRole.User, "test")],
             null);
         SetCurrentRunContext(context);
     }
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/MessageIndexTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/MessageIndexTests.cs
index b1fa5c59cb..b17b0d1b95 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/MessageIndexTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/MessageIndexTests.cs
@@ -800,6 +800,68 @@ public class MessageIndexTests
         Assert.Equal(4, inserted.TokenCount);
     }
 
+    [Fact]
+    public void CreateWithStandaloneToolMessageGroupsAsAssistantText()
+    {
+        // A Tool message not preceded by an assistant tool-call falls through to the else branch
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.Tool, "Orphaned tool result"),
+        ];
+
+        MessageIndex index = MessageIndex.Create(messages);
+
+        // The Tool message should be grouped as AssistantText (the default fallback)
+        Assert.Single(index.Groups);
+        Assert.Equal(MessageGroupKind.AssistantText, index.Groups[0].Kind);
+    }
+
+    [Fact]
+    public void CreateWithAssistantNonSummaryWithPropertiesFallsToAssistantText()
+    {
+        // Assistant message with AdditionalProperties but NOT a summary
+        ChatMessage assistant = new(ChatRole.Assistant, "Regular response");
+        (assistant.AdditionalProperties ??= [])["someOtherKey"] = "value";
+
+        MessageIndex index = MessageIndex.Create([assistant]);
+
+        Assert.Single(index.Groups);
+        Assert.Equal(MessageGroupKind.AssistantText, index.Groups[0].Kind);
+    }
+
+    [Fact]
+    public void ComputeByteCountHandlesNullAndNonNullText()
+    {
+        // Mix of messages: one with text (non-null), one without (null Text)
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+        ];
+
+        int byteCount = MessageIndex.ComputeByteCount(messages);
+
+        // Only "Hello" contributes bytes (5 bytes UTF-8)
+        Assert.Equal(5, byteCount);
+    }
+
+    [Fact]
+    public void ComputeTokenCountHandlesNullAndNonNullText()
+    {
+        // Mix: one with text, one without
+        SimpleWordTokenizer tokenizer = new();
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.User, "Hello world"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+        ];
+
+        int tokenCount = MessageIndex.ComputeTokenCount(messages, tokenizer);
+
+        // Only "Hello world" contributes tokens (2 words)
+        Assert.Equal(2, tokenCount);
+    }
+
     /// <summary>
     /// A simple tokenizer that counts whitespace-separated words as tokens.
     /// </summary>
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SlidingWindowCompactionStrategyTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SlidingWindowCompactionStrategyTests.cs
index 08c3b50ad3..709a44e0a4 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SlidingWindowCompactionStrategyTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SlidingWindowCompactionStrategyTests.cs
@@ -221,4 +221,30 @@ public class SlidingWindowCompactionStrategyTests
         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),
+            minimumPreserved: 0);
+
+        MessageIndex index = MessageIndex.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
+    }
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SummarizationCompactionStrategyTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SummarizationCompactionStrategyTests.cs
index 9d6bfdb05b..0bf225ae34 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SummarizationCompactionStrategyTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/SummarizationCompactionStrategyTests.cs
@@ -246,7 +246,7 @@ public class SummarizationCompactionStrategyTests
     {
         // Arrange — 4 non-system groups, preserve 1, target met after 1 exclusion
         int exclusionCount = 0;
-        CompactionTrigger TargetAfterOne = _ => ++exclusionCount >= 1;
+        bool TargetAfterOne(MessageIndex _) => ++exclusionCount >= 1;
 
         SummarizationCompactionStrategy strategy = new(
             CreateMockChatClient("Partial summary."),
@@ -297,4 +297,114 @@ public class SummarizationCompactionStrategyTests
         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,
+            minimumPreserved: 1);
+
+        MessageIndex index = MessageIndex.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(MessageGroupKind.Summary, index.Groups[0].Kind);
+        Assert.Equal(MessageGroupKind.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,
+            minimumPreserved: 3,
+            target: _ => false);
+
+        MessageIndex index = MessageIndex.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,
+            minimumPreserved: 1);
+
+        MessageIndex index = MessageIndex.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,
+            minimumPreserved: 1);
+
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+            new ChatMessage(ChatRole.User, "Q1"),
+            new ChatMessage(ChatRole.Assistant, "A1"),
+        ];
+
+        MessageIndex index = MessageIndex.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);
+    }
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ToolResultCompactionStrategyTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ToolResultCompactionStrategyTests.cs
index 66300e69f0..2b925aef76 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ToolResultCompactionStrategyTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ToolResultCompactionStrategyTests.cs
@@ -233,4 +233,30 @@ public class ToolResultCompactionStrategyTests
 
         Assert.Equal(1, collapsedToolGroups);
     }
+
+    [Fact]
+    public async Task CompactAsyncSkipsPreExcludedAndSystemGroupsAsync()
+    {
+        // Arrange — pre-excluded and system groups in the enumeration
+        ToolResultCompactionStrategy strategy = new(CompactionTriggers.Always, minimumPreserved: 0);
+
+        List<ChatMessage> messages =
+        [
+            new ChatMessage(ChatRole.System, "System prompt"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("c1", "fn")]),
+            new ChatMessage(ChatRole.Tool, "Result 1"),
+            new ChatMessage(ChatRole.User, "Q1"),
+        ];
+
+        MessageIndex index = MessageIndex.Create(messages);
+        // Pre-exclude the 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
+    }
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/TruncationCompactionStrategyTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/TruncationCompactionStrategyTests.cs
index ceb40b7495..2783fa029c 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/TruncationCompactionStrategyTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/TruncationCompactionStrategyTests.cs
@@ -274,4 +274,55 @@ public class TruncationCompactionStrategyTests
         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, minimumPreserved: 2, target: CompactionTriggers.Never);
+        MessageIndex groups = MessageIndex.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, minimumPreserved: 1);
+        MessageIndex groups = MessageIndex.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
+    }
 }