diff --git a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
index f5fb103bd4..5adff3452d 100644
--- a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
@@ -216,6 +216,9 @@ private static ChatResponseUpdate ValidateAndEmitRunFinished(string? conversatio
         {
             ConversationId = conversationId,
             ResponseId = responseId,
+            FinishReason = !string.IsNullOrEmpty(runFinished.FinishReason)
+                ? new ChatFinishReason(runFinished.FinishReason)
+                : null,
             CreatedAt = DateTimeOffset.UtcNow
         };
     }
@@ -341,8 +344,13 @@ public static async IAsyncEnumerable<BaseEvent> AsAGUIEventStreamAsync(
         };
 
         string? currentMessageId = null;
+        ChatFinishReason? lastFinishReason = null;
         await foreach (var chatResponse in updates.WithCancellation(cancellationToken).ConfigureAwait(false))
         {
+            if (chatResponse.FinishReason is not null)
+            {
+                lastFinishReason = chatResponse.FinishReason;
+            }
             if (chatResponse is { Contents.Count: > 0 } &&
                 chatResponse.Contents[0] is TextContent &&
                 !string.Equals(currentMessageId, chatResponse.MessageId, StringComparison.Ordinal))
@@ -480,6 +488,7 @@ chatResponse.Contents[0] is TextContent &&
         {
             ThreadId = threadId,
             RunId = runId,
+            FinishReason = lastFinishReason?.Value,
         };
     }
 
diff --git a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/RunFinishedEvent.cs b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/RunFinishedEvent.cs
index 54aebaa333..414e90bef9 100644
--- a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/RunFinishedEvent.cs
+++ b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/RunFinishedEvent.cs
@@ -22,6 +22,10 @@ public RunFinishedEvent()
     [JsonPropertyName("runId")]
     public string RunId { get; set; } = string.Empty;
 
+    [JsonPropertyName("finishReason")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? FinishReason { get; set; }
+
     [JsonPropertyName("result")]
     public JsonElement? Result { get; set; }
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs
index 7d40cc014d..831ea1d0ad 100644
--- a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs
@@ -68,6 +68,100 @@ public async Task AsChatResponseUpdatesAsync_ConvertsRunFinishedEvent_ToResponse
         Assert.Equal("thread1", updates[1].ConversationId);
     }
 
+    [Fact]
+    public async Task AsChatResponseUpdatesAsync_RunFinishedEvent_WithFinishReasonStop_MapsToFinishReasonAsync()
+    {
+        // Arrange
+        List<BaseEvent> events =
+        [
+            new RunStartedEvent { ThreadId = "thread1", RunId = "run1" },
+            new RunFinishedEvent { ThreadId = "thread1", RunId = "run1", FinishReason = "stop" }
+        ];
+
+        // Act
+        List<ChatResponseUpdate> updates = [];
+        await foreach (ChatResponseUpdate update in events.ToAsyncEnumerableAsync().AsChatResponseUpdatesAsync(AGUIJsonSerializerContext.Default.Options))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        Assert.Equal(2, updates.Count);
+        ChatResponseUpdate finishUpdate = updates[1];
+        Assert.NotNull(finishUpdate.FinishReason);
+        Assert.Equal(ChatFinishReason.Stop.Value, finishUpdate.FinishReason.Value.Value);
+    }
+
+    [Fact]
+    public async Task AsChatResponseUpdatesAsync_RunFinishedEvent_WithFinishReasonToolCalls_MapsToFinishReasonAsync()
+    {
+        // Arrange
+        List<BaseEvent> events =
+        [
+            new RunStartedEvent { ThreadId = "thread1", RunId = "run1" },
+            new RunFinishedEvent { ThreadId = "thread1", RunId = "run1", FinishReason = "tool_calls" }
+        ];
+
+        // Act
+        List<ChatResponseUpdate> updates = [];
+        await foreach (ChatResponseUpdate update in events.ToAsyncEnumerableAsync().AsChatResponseUpdatesAsync(AGUIJsonSerializerContext.Default.Options))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        Assert.Equal(2, updates.Count);
+        ChatResponseUpdate finishUpdate = updates[1];
+        Assert.NotNull(finishUpdate.FinishReason);
+        Assert.Equal(ChatFinishReason.ToolCalls.Value, finishUpdate.FinishReason.Value.Value);
+    }
+
+    [Fact]
+    public async Task AsChatResponseUpdatesAsync_RunFinishedEvent_WithNullFinishReason_MapsToNullFinishReasonAsync()
+    {
+        // Arrange
+        List<BaseEvent> events =
+        [
+            new RunStartedEvent { ThreadId = "thread1", RunId = "run1" },
+            new RunFinishedEvent { ThreadId = "thread1", RunId = "run1", FinishReason = null }
+        ];
+
+        // Act
+        List<ChatResponseUpdate> updates = [];
+        await foreach (ChatResponseUpdate update in events.ToAsyncEnumerableAsync().AsChatResponseUpdatesAsync(AGUIJsonSerializerContext.Default.Options))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        Assert.Equal(2, updates.Count);
+        ChatResponseUpdate finishUpdate = updates[1];
+        Assert.Null(finishUpdate.FinishReason);
+    }
+
+    [Fact]
+    public async Task AsChatResponseUpdatesAsync_RunFinishedEvent_WithEmptyFinishReason_MapsToNullFinishReasonAsync()
+    {
+        // Arrange
+        List<BaseEvent> events =
+        [
+            new RunStartedEvent { ThreadId = "thread1", RunId = "run1" },
+            new RunFinishedEvent { ThreadId = "thread1", RunId = "run1", FinishReason = string.Empty }
+        ];
+
+        // Act
+        List<ChatResponseUpdate> updates = [];
+        await foreach (ChatResponseUpdate update in events.ToAsyncEnumerableAsync().AsChatResponseUpdatesAsync(AGUIJsonSerializerContext.Default.Options))
+        {
+            updates.Add(update);
+        }
+
+        // Assert
+        Assert.Equal(2, updates.Count);
+        ChatResponseUpdate finishUpdate = updates[1];
+        Assert.Null(finishUpdate.FinishReason);
+    }
+
     [Fact]
     public async Task AsChatResponseUpdatesAsync_ConvertsRunErrorEvent_ToErrorContentAsync()
     {
diff --git a/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs
index bf2aa6fb0b..353bb96a1e 100644
--- a/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs
@@ -283,4 +283,148 @@ public async Task AsAGUIEventStreamAsync_WithMixedContentTypes_EmitsAllEventType
         Assert.Contains(events, e => e is ToolCallEndEvent);
         Assert.Contains(events, e => e is RunFinishedEvent);
     }
+
+    [Fact]
+    public async Task AsAGUIEventStreamAsync_RunFinishedEvent_ContainsFinishReason_StopAsync()
+    {
+        // Arrange — Simulate an agent run that completes normally with FinishReason.Stop
+        const string ThreadId = "thread1";
+        const string RunId = "run1";
+        List<ChatResponseUpdate> updates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Hello world") { MessageId = "msg1" },
+            new ChatResponseUpdate(ChatRole.Assistant, "!") { MessageId = "msg1", FinishReason = ChatFinishReason.Stop }
+        ];
+
+        // Act
+        List<BaseEvent> events = [];
+        await foreach (BaseEvent evt in updates.ToAsyncEnumerableAsync().AsAGUIEventStreamAsync(ThreadId, RunId, AGUIJsonSerializerContext.Default.Options, CancellationToken.None))
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        RunFinishedEvent finishEvent = Assert.IsType<RunFinishedEvent>(events.Last());
+        Assert.Equal(ChatFinishReason.Stop.Value, finishEvent.FinishReason);
+    }
+
+    [Fact]
+    public async Task AsAGUIEventStreamAsync_RunFinishedEvent_ContainsFinishReason_ToolCallsAsync()
+    {
+        // Arrange — Simulate an agent run that ends with FinishReason.ToolCalls (client-side tool call)
+        const string ThreadId = "thread1";
+        const string RunId = "run1";
+        FunctionCallContent functionCall = new("call_1", "GetWeather", new Dictionary<string, object?> { ["city"] = "Seattle" });
+        List<ChatResponseUpdate> updates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, [functionCall])
+            {
+                MessageId = "msg1",
+                FinishReason = ChatFinishReason.ToolCalls
+            }
+        ];
+
+        // Act
+        List<BaseEvent> events = [];
+        await foreach (BaseEvent evt in updates.ToAsyncEnumerableAsync().AsAGUIEventStreamAsync(ThreadId, RunId, AGUIJsonSerializerContext.Default.Options, CancellationToken.None))
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        RunFinishedEvent finishEvent = Assert.IsType<RunFinishedEvent>(events.Last());
+        Assert.Equal(ChatFinishReason.ToolCalls.Value, finishEvent.FinishReason);
+    }
+
+    [Fact]
+    public async Task AsAGUIEventStreamAsync_RunFinishedEvent_FinishReasonNull_WhenNoUpdatesAsync()
+    {
+        // Arrange — No ChatResponseUpdates emitted (empty stream)
+        const string ThreadId = "thread1";
+        const string RunId = "run1";
+        List<ChatResponseUpdate> updates = [];
+
+        // Act
+        List<BaseEvent> events = [];
+        await foreach (BaseEvent evt in updates.ToAsyncEnumerableAsync().AsAGUIEventStreamAsync(ThreadId, RunId, AGUIJsonSerializerContext.Default.Options, CancellationToken.None))
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        RunFinishedEvent finishEvent = Assert.IsType<RunFinishedEvent>(events.Last());
+        Assert.Null(finishEvent.FinishReason);
+    }
+
+    [Fact]
+    public async Task AsAGUIEventStreamAsync_RunFinishedEvent_CapturesLastFinishReasonAsync()
+    {
+        // Arrange — Multiple updates, only the last one has FinishReason set.
+        // Verifies that FinishReason is captured from the last update that has it.
+        const string ThreadId = "thread1";
+        const string RunId = "run1";
+        List<ChatResponseUpdate> updates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Part 1") { MessageId = "msg1" },
+            new ChatResponseUpdate(ChatRole.Assistant, "Part 2") { MessageId = "msg1" },
+            new ChatResponseUpdate(ChatRole.Assistant, "Part 3") { MessageId = "msg1", FinishReason = ChatFinishReason.Stop }
+        ];
+
+        // Act
+        List<BaseEvent> events = [];
+        await foreach (BaseEvent evt in updates.ToAsyncEnumerableAsync().AsAGUIEventStreamAsync(ThreadId, RunId, AGUIJsonSerializerContext.Default.Options, CancellationToken.None))
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        RunFinishedEvent finishEvent = Assert.IsType<RunFinishedEvent>(events.Last());
+        Assert.Equal(ChatFinishReason.Stop.Value, finishEvent.FinishReason);
+    }
+
+    [Fact]
+    public async Task AsAGUIEventStreamAsync_RunFinishedEvent_FinishReason_SerializedCorrectlyAsync()
+    {
+        // Arrange — Verify the FinishReason is serialized in the JSON output per AG-UI spec
+        const string ThreadId = "thread1";
+        const string RunId = "run1";
+        List<ChatResponseUpdate> updates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Done") { MessageId = "msg1", FinishReason = ChatFinishReason.Stop }
+        ];
+
+        // Act
+        List<BaseEvent> events = [];
+        await foreach (BaseEvent evt in updates.ToAsyncEnumerableAsync().AsAGUIEventStreamAsync(ThreadId, RunId, AGUIJsonSerializerContext.Default.Options, CancellationToken.None))
+        {
+            events.Add(evt);
+        }
+
+        // Assert — Serialize the RunFinishedEvent and verify finishReason is present in JSON
+        RunFinishedEvent finishEvent = Assert.IsType<RunFinishedEvent>(events.Last());
+        string json = System.Text.Json.JsonSerializer.Serialize(finishEvent, AGUIJsonSerializerContext.Default.Options.GetTypeInfo(typeof(RunFinishedEvent)));
+        Assert.Contains("\"finishReason\"", json);
+        Assert.Contains("\"stop\"", json);
+    }
+
+    [Fact]
+    public async Task AsAGUIEventStreamAsync_RunFinishedEvent_FinishReasonNull_OmittedFromSerializationAsync()
+    {
+        // Arrange — When FinishReason is null, it should be omitted from JSON serialization
+        const string ThreadId = "thread1";
+        const string RunId = "run1";
+        List<ChatResponseUpdate> updates = [];
+
+        // Act
+        List<BaseEvent> events = [];
+        await foreach (BaseEvent evt in updates.ToAsyncEnumerableAsync().AsAGUIEventStreamAsync(ThreadId, RunId, AGUIJsonSerializerContext.Default.Options, CancellationToken.None))
+        {
+            events.Add(evt);
+        }
+
+        // Assert — Serialize the RunFinishedEvent and verify finishReason is NOT present in JSON
+        RunFinishedEvent finishEvent = Assert.IsType<RunFinishedEvent>(events.Last());
+        string json = System.Text.Json.JsonSerializer.Serialize(finishEvent, AGUIJsonSerializerContext.Default.Options.GetTypeInfo(typeof(RunFinishedEvent)));
+        Assert.DoesNotContain("\"finishReason\"", json);
+    }
 }