diff --git a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionCallContent.cs b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionCallContent.cs
index 836d5a4110b..7c506a7845b 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionCallContent.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionCallContent.cs
@@ -14,7 +14,7 @@ namespace Microsoft.Extensions.AI;
 /// Represents a function call request.
 /// </summary>
 [DebuggerDisplay("{DebuggerDisplay,nq}")]
-public sealed class FunctionCallContent : AIContent
+public class FunctionCallContent : AIContent
 {
     /// <summary>
     /// Initializes a new instance of the <see cref="FunctionCallContent"/> class.
diff --git a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionResultContent.cs b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionResultContent.cs
index 46401347b40..d5eb4884709 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionResultContent.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/FunctionResultContent.cs
@@ -13,7 +13,7 @@ namespace Microsoft.Extensions.AI;
 /// Represents the result of a function call.
 /// </summary>
 [DebuggerDisplay("{DebuggerDisplay,nq}")]
-public sealed class FunctionResultContent : AIContent
+public class FunctionResultContent : AIContent
 {
     /// <summary>
     /// Initializes a new instance of the <see cref="FunctionResultContent"/> class.
diff --git a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json
index 6362fcaa00e..ef1f2ebb496 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json
+++ b/src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json
@@ -1792,7 +1792,7 @@
       ]
     },
     {
-      "Type": "sealed class Microsoft.Extensions.AI.FunctionCallContent : Microsoft.Extensions.AI.AIContent",
+      "Type": "class Microsoft.Extensions.AI.FunctionCallContent : Microsoft.Extensions.AI.AIContent",
       "Stage": "Stable",
       "Methods": [
         {
@@ -1824,7 +1824,7 @@
       ]
     },
     {
-      "Type": "sealed class Microsoft.Extensions.AI.FunctionResultContent : Microsoft.Extensions.AI.AIContent",
+      "Type": "class Microsoft.Extensions.AI.FunctionResultContent : Microsoft.Extensions.AI.AIContent",
       "Stage": "Stable",
       "Methods": [
         {
diff --git a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
index fdc7d57b1ea..c88fe91fc17 100644
--- a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
@@ -1208,6 +1208,13 @@ FunctionResultContent CreateFunctionResultContent(FunctionInvocationResult resul
             object? functionResult;
             if (result.Status == FunctionInvocationStatus.RanToCompletion)
             {
+                // If the result is already a FunctionResultContent with a matching CallId, use it directly.
+                if (result.Result is FunctionResultContent frc &&
+                    frc.CallId == result.CallContent.CallId)
+                {
+                    return frc;
+                }
+
                 functionResult = result.Result ?? "Success: Function completed.";
             }
             else
diff --git a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
index e9672275871..066fb5e3427 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
@@ -354,6 +354,264 @@ public async Task FunctionInvokerDelegateOverridesHandlingAsync()
         await InvokeAndAssertStreamingAsync(options, plan, configurePipeline: configure);
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task FunctionReturningFunctionResultContentWithMatchingCallId_UsesItDirectly(bool streaming)
+    {
+        FunctionResultContent? returnedFrc = null;
+
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                AIFunctionFactory.Create(() => "Result 1", "Func1"),
+            ]
+        };
+
+        using var innerClient = new TestChatClient
+        {
+            GetResponseAsyncCallback = (msgs, opts, ct) =>
+            {
+                var toolMessage = msgs.FirstOrDefault(m => m.Role == ChatRole.Tool);
+                if (toolMessage is null)
+                {
+                    return Task.FromResult(new ChatResponse(
+                        new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")])));
+                }
+                else
+                {
+                    return Task.FromResult(new ChatResponse(new ChatMessage(ChatRole.Assistant, "done")));
+                }
+            },
+            GetStreamingResponseAsyncCallback = (msgs, opts, ct) =>
+            {
+                var toolMessage = msgs.FirstOrDefault(m => m.Role == ChatRole.Tool);
+                if (toolMessage is null)
+                {
+                    return YieldAsync(new ChatResponse(
+                        new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")])).ToChatResponseUpdates());
+                }
+                else
+                {
+                    return YieldAsync(new ChatResponse(new ChatMessage(ChatRole.Assistant, "done")).ToChatResponseUpdates());
+                }
+            }
+        };
+
+        using var client = new FunctionInvokingChatClient(innerClient)
+        {
+            FunctionInvoker = (ctx, cancellationToken) =>
+            {
+                returnedFrc = new FunctionResultContent(ctx.CallContent.CallId, "Custom result from function")
+                {
+                    RawRepresentation = "CustomRaw"
+                };
+                return new ValueTask<object?>(returnedFrc);
+            }
+        };
+
+        var messages = new List<ChatMessage>
+        {
+            new ChatMessage(ChatRole.User, "hello"),
+        };
+
+        ChatResponse response;
+        if (streaming)
+        {
+            response = await client.GetStreamingResponseAsync(messages, options).ToChatResponseAsync();
+        }
+        else
+        {
+            response = await client.GetResponseAsync(messages, options);
+        }
+
+        // Verify that the FunctionResultContent was used directly (same reference)
+        var toolMessage = response.Messages.First(m => m.Role == ChatRole.Tool);
+        var capturedFrc = Assert.Single(toolMessage.Contents.OfType<FunctionResultContent>());
+        Assert.Same(returnedFrc, capturedFrc);
+        Assert.Equal("Custom result from function", capturedFrc.Result);
+        Assert.Equal("CustomRaw", capturedFrc.RawRepresentation);
+        Assert.Equal("callId1", capturedFrc.CallId);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task FunctionReturningFunctionResultContentWithMismatchedCallId_WrapsIt(bool streaming)
+    {
+        FunctionResultContent? returnedFrc = null;
+
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                AIFunctionFactory.Create(() => "Result 1", "Func1"),
+            ]
+        };
+
+        using var innerClient = new TestChatClient
+        {
+            GetResponseAsyncCallback = (msgs, opts, ct) =>
+            {
+                var toolMessage = msgs.FirstOrDefault(m => m.Role == ChatRole.Tool);
+                if (toolMessage is null)
+                {
+                    return Task.FromResult(new ChatResponse(
+                        new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")])));
+                }
+                else
+                {
+                    return Task.FromResult(new ChatResponse(new ChatMessage(ChatRole.Assistant, "done")));
+                }
+            },
+            GetStreamingResponseAsyncCallback = (msgs, opts, ct) =>
+            {
+                var toolMessage = msgs.FirstOrDefault(m => m.Role == ChatRole.Tool);
+                if (toolMessage is null)
+                {
+                    return YieldAsync(new ChatResponse(
+                        new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")])).ToChatResponseUpdates());
+                }
+                else
+                {
+                    return YieldAsync(new ChatResponse(new ChatMessage(ChatRole.Assistant, "done")).ToChatResponseUpdates());
+                }
+            }
+        };
+
+        using var client = new FunctionInvokingChatClient(innerClient)
+        {
+            FunctionInvoker = (ctx, cancellationToken) =>
+            {
+                // Return a FunctionResultContent with a different CallId
+                returnedFrc = new FunctionResultContent("differentCallId", "Result from function");
+                return new ValueTask<object?>(returnedFrc);
+            }
+        };
+
+        var messages = new List<ChatMessage>
+        {
+            new ChatMessage(ChatRole.User, "hello"),
+        };
+
+        ChatResponse response;
+        if (streaming)
+        {
+            response = await client.GetStreamingResponseAsync(messages, options).ToChatResponseAsync();
+        }
+        else
+        {
+            response = await client.GetResponseAsync(messages, options);
+        }
+
+        // Verify the result is wrapped - the outer FunctionResultContent has the correct CallId
+        // and the inner one is reference-equal to what was returned
+        var toolMessage = response.Messages.First(m => m.Role == ChatRole.Tool);
+        var frc = Assert.Single(toolMessage.Contents.OfType<FunctionResultContent>());
+        Assert.Equal("callId1", frc.CallId);
+        Assert.Same(returnedFrc, frc.Result);
+        var innerFrc = (FunctionResultContent)frc.Result!;
+        Assert.Equal("differentCallId", innerFrc.CallId);
+        Assert.Equal("Result from function", innerFrc.Result);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task FunctionReturningDerivedFunctionResultContent_PropagatesInstanceToInnerClient(bool streaming)
+    {
+        DerivedFunctionResultContent? returnedFrc = null;
+
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                AIFunctionFactory.Create(() => "Result 1", "Func1"),
+            ]
+        };
+
+        using var innerClient = new TestChatClient
+        {
+            GetResponseAsyncCallback = (msgs, opts, ct) =>
+            {
+                var toolMessage = msgs.FirstOrDefault(m => m.Role == ChatRole.Tool);
+                if (toolMessage is null)
+                {
+                    return Task.FromResult(new ChatResponse(
+                        new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")])));
+                }
+                else
+                {
+                    return Task.FromResult(new ChatResponse(new ChatMessage(ChatRole.Assistant, "done")));
+                }
+            },
+            GetStreamingResponseAsyncCallback = (msgs, opts, ct) =>
+            {
+                var toolMessage = msgs.FirstOrDefault(m => m.Role == ChatRole.Tool);
+                if (toolMessage is null)
+                {
+                    return YieldAsync(new ChatResponse(
+                        new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")])).ToChatResponseUpdates());
+                }
+                else
+                {
+                    return YieldAsync(new ChatResponse(new ChatMessage(ChatRole.Assistant, "done")).ToChatResponseUpdates());
+                }
+            }
+        };
+
+        using var client = new FunctionInvokingChatClient(innerClient)
+        {
+            FunctionInvoker = (ctx, cancellationToken) =>
+            {
+                // Return a derived FunctionResultContent
+                returnedFrc = new DerivedFunctionResultContent(ctx.CallContent.CallId, "Derived result")
+                {
+                    CustomProperty = "CustomValue"
+                };
+                return new ValueTask<object?>(returnedFrc);
+            }
+        };
+
+        var messages = new List<ChatMessage>
+        {
+            new ChatMessage(ChatRole.User, "hello"),
+        };
+
+        ChatResponse response;
+        if (streaming)
+        {
+            response = await client.GetStreamingResponseAsync(messages, options).ToChatResponseAsync();
+        }
+        else
+        {
+            response = await client.GetResponseAsync(messages, options);
+        }
+
+        // Verify that the derived FunctionResultContent instance was propagated to the inner client
+        // and is reference-equal to what was returned
+        var toolMessage = response.Messages.First(m => m.Role == ChatRole.Tool);
+        var capturedFrc = Assert.Single(toolMessage.Contents.OfType<FunctionResultContent>());
+        Assert.Same(returnedFrc, capturedFrc);
+        Assert.IsType<DerivedFunctionResultContent>(capturedFrc);
+        var derivedFrc = (DerivedFunctionResultContent)capturedFrc;
+        Assert.Equal("callId1", derivedFrc.CallId);
+        Assert.Equal("Derived result", derivedFrc.Result);
+        Assert.Equal("CustomValue", derivedFrc.CustomProperty);
+    }
+
+    /// <summary>A derived FunctionResultContent for testing purposes.</summary>
+    private sealed class DerivedFunctionResultContent : FunctionResultContent
+    {
+        public DerivedFunctionResultContent(string callId, object? result)
+            : base(callId, result)
+        {
+        }
+
+        public string? CustomProperty { get; set; }
+    }
+
     [Fact]
     public async Task ContinuesWithSuccessfulCallsUntilMaximumIterations()
     {