diff --git a/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageIndex.cs b/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageIndex.cs
index 003a70f2b3a..3edfb4591aa 100644
--- a/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageIndex.cs
+++ b/dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageIndex.cs
@@ -5,6 +5,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
+using System.Text.Json;
 using Microsoft.Extensions.AI;
 using Microsoft.ML.Tokenizers;
 using Microsoft.Shared.DiagnosticIds;
@@ -525,5 +526,10 @@ private static bool HasOnlyReasoning(ChatMessage message) =>
 
     private static bool IsSummaryMessage(ChatMessage message) =>
         message.AdditionalProperties?.TryGetValue(CompactionMessageGroup.SummaryPropertyKey, out object? value) is true
-            && value is true;
+            && value switch
+            {
+                bool b => b,
+                JsonElement j => j.ValueKind == JsonValueKind.True,
+                _ => false,
+            };
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionMessageIndexTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionMessageIndexTests.cs
index ea0ecd0d449..4613479d2a5 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionMessageIndexTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionMessageIndexTests.cs
@@ -3,6 +3,7 @@
 using System;
 using System.Buffers;
 using System.Collections.Generic;
+using System.Text.Json;
 using Microsoft.Agents.AI.Compaction;
 using Microsoft.Extensions.AI;
 using Microsoft.ML.Tokenizers;
@@ -1474,4 +1475,34 @@ public void UpdateIncrementallyAppendsReasoningToolCallGroup()
         Assert.Equal(CompactionGroupKind.ToolCall, index.Groups[2].Kind);
         Assert.Equal(3, index.Groups[2].MessageCount); // reasoning + toolCall + toolResult
     }
+
+    [Theory]
+    [InlineData(null, CompactionGroupKind.AssistantText)]
+    [InlineData(false, CompactionGroupKind.AssistantText)]
+    [InlineData("false", CompactionGroupKind.AssistantText)]
+    [InlineData("null", CompactionGroupKind.AssistantText)]
+    [InlineData("\"Unexpected string\"", CompactionGroupKind.AssistantText)]
+    [InlineData(true, CompactionGroupKind.Summary)]
+    [InlineData("true", CompactionGroupKind.Summary)]
+    public void SummaryPropertyKeyIsRespected(object? summaryPropertyValue, CompactionGroupKind expectedCompactionGroupKind)
+    {
+        ChatMessage message = new(ChatRole.Assistant, "Hello");
+        message.AdditionalProperties ??= [];
+
+        // Convert string to JsonElement for the deserialized case
+        if (summaryPropertyValue != null)
+        {
+            message.AdditionalProperties[CompactionMessageGroup.SummaryPropertyKey] = summaryPropertyValue switch
+            {
+                string s => JsonDocument.Parse(s).RootElement,
+                _ => summaryPropertyValue
+            };
+        }
+
+        CompactionMessageIndex index = CompactionMessageIndex.Create([message]);
+
+        // Assert — the message is recognized as a summary message
+        Assert.Single(index.Groups);
+        Assert.Equal(expectedCompactionGroupKind, index.Groups[0].Kind);
+    }
 }