feat(server): add MarshalResult to McpServerToolCreateOptions

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -74,7 +74,7 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
         {
             Name = options?.Name ?? method.GetCustomAttribute<McpServerToolAttribute>()?.Name ?? DeriveName(method),
             Description = options?.Description,
-            MarshalResult = static (result, _, cancellationToken) => new ValueTask<object?>(result),
+            MarshalResult = options?.MarshalResult ?? (static (result, _, cancellationToken) => new ValueTask<object?>(result)),
             SerializerOptions = options?.SerializerOptions ?? McpJsonUtilities.DefaultOptions,
             JsonSchemaCreateOptions = options?.SchemaCreateOptions,
             ConfigureParameterBinding = pi =>

src/ModelContextProtocol.Core/Server/McpServerToolCreateOptions.cs

@@ -157,6 +157,16 @@ public sealed class McpServerToolCreateOptions
     /// </value>
     public JsonSerializerOptions? SerializerOptions { get; set; }
 
+    /// <summary>
+    /// Gets or sets a delegate used to determine the object returned from invoking the tool,
+    /// based on the result of the underlying method.
+    /// </summary>
+    /// <value>
+    /// The default is <see langword="null"/>, which means the underlying method's result is
+    /// returned as-is.
+    /// </value>
+    public Func<object?, Type?, CancellationToken, ValueTask<object?>>? MarshalResult { get; set; }
+
     /// <summary>
     /// Gets or sets the JSON schema options when creating an <see cref="AIFunction"/> from a method.
     /// </summary>
@@ -214,6 +224,7 @@ internal McpServerToolCreateOptions Clone() =>
             UseStructuredContent = UseStructuredContent,
             OutputSchema = OutputSchema,
             SerializerOptions = SerializerOptions,
+            MarshalResult = MarshalResult,
             SchemaCreateOptions = SchemaCreateOptions,
             Metadata = Metadata,
             Icons = Icons,

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -457,6 +457,45 @@ public async Task SupportsSchemaCreateOptions()
         );
     }
 
+    [Fact]
+    public async Task SupportsCustomResultMarshaling()
+    {
+        Type? seenDeclaredResultType = null;
+        Mock<McpServer> mockServer = new();
+        McpServerTool tool = McpServerTool.Create(() => 21 * 2, new()
+        {
+            Name = "marshaller",
+            MarshalResult = (result, declaredResultType, cancellationToken) =>
+            {
+                seenDeclaredResultType = declaredResultType;
+                Assert.False(cancellationToken.IsCancellationRequested);
+                return new ValueTask<object?>($"doubled:{result}");
+            },
+        });
+
+        var result = await tool.InvokeAsync(
+            new RequestContext<CallToolRequestParams>(mockServer.Object, CreateTestJsonRpcRequest(), new() { Name = "marshaller" }),
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal(typeof(int), seenDeclaredResultType);
+        var text = Assert.IsType<TextContentBlock>(Assert.Single(result.Content));
+        Assert.Equal("doubled:42", text.Text);
+    }
+
+    [Fact]
+    public async Task MarshalResult_Unset_ReturnsUnderlyingResultAsIs()
+    {
+        Mock<McpServer> mockServer = new();
+        McpServerTool tool = McpServerTool.Create(() => "42", new() { Name = "passthrough" });
+
+        var result = await tool.InvokeAsync(
+            new RequestContext<CallToolRequestParams>(mockServer.Object, CreateTestJsonRpcRequest(), new() { Name = "passthrough" }),
+            TestContext.Current.CancellationToken);
+
+        var text = Assert.IsType<TextContentBlock>(Assert.Single(result.Content));
+        Assert.Equal("42", text.Text);
+    }
+
     [Theory]
     [MemberData(nameof(StructuredOutput_ReturnsExpectedSchema_Inputs))]
     public async Task StructuredOutput_Enabled_ReturnsExpectedSchema<T>(T value)