Fix JsonSerializerOptions without TypeInfoResolver

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -76,7 +76,7 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Name = options?.Name ?? method.GetCustomAttribute<McpServerToolAttribute>()?.Name ?? DeriveName(method),
             Description = options?.Description,
             MarshalResult = static (result, _, cancellationToken) => new ValueTask<object?>(result),
-            SerializerOptions = options?.SerializerOptions ?? McpJsonUtilities.DefaultOptions,
+            SerializerOptions = GetSerializerOptions(options?.SerializerOptions),
             JsonSchemaCreateOptions = options?.SchemaCreateOptions,
             ConfigureParameterBinding = pi =>
             {
@@ -580,4 +580,29 @@ private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumer
             IsError = allErrorContent && hasAny
         };
     }
+
+    /// <summary>
+    /// Gets the appropriate <see cref="JsonSerializerOptions"/> for tool serialization.
+    /// </summary>
+    /// <remarks>
+    /// If no options are provided, returns the default MCP options. If options are provided but
+    /// don't have a TypeInfoResolver, this method silently mutates the options to add one,
+    /// approximating the behavior of the non-AOT friendly JsonSerializer.Serialize methods.
+    /// </remarks>
+    private static JsonSerializerOptions GetSerializerOptions(JsonSerializerOptions? customOptions)
+    {
+        if (customOptions is null)
+        {
+            return McpJsonUtilities.DefaultOptions;
+        }
+
+        if (customOptions.TypeInfoResolver is null)
+        {
+            Debug.Assert(!customOptions.IsReadOnly, "If no resolver is present then the options must still be editable.");
+            customOptions.TypeInfoResolver = McpJsonUtilities.DefaultOptions.TypeInfoResolver;
+            customOptions.MakeReadOnly();
+        }
+
+        return customOptions;
+    }
 }

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -554,6 +554,44 @@ public async Task ToolWithNullableParameters_ReturnsExpectedSchema(JsonNumberHan
         Assert.True(JsonElement.DeepEquals(expectedSchema, tool.ProtocolTool.InputSchema));
     }
 
+    [Fact]
+    public async Task Create_WithJsonSerializerOptionsWithoutTypeInfoResolver_Works()
+    {
+        // Arrange - Create options without a TypeInfoResolver (simulates issue #1150)
+        JsonSerializerOptions options = new()
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+            WriteIndented = true
+        };
+
+        // Act - Should not throw when creating a tool with these options
+        McpServerTool tool = McpServerTool.Create((string name) => $"Hello, {name}!", new() { SerializerOptions = options });
+
+        // Assert - Verify the tool works correctly
+        var mockServer = new Mock<McpServer>();
+        var request = new RequestContext<CallToolRequestParams>(mockServer.Object, CreateTestJsonRpcRequest())
+        {
+            Params = new CallToolRequestParams
+            {
+                Name = "tool",
+                Arguments = new Dictionary<string, JsonElement>
+                {
+                    ["name"] = JsonDocument.Parse("\"World\"").RootElement.Clone()
+                }
+            },
+        };
+
+        var result = await tool.InvokeAsync(request, TestContext.Current.CancellationToken);
+
+        Assert.NotNull(result);
+        Assert.Single(result.Content);
+        Assert.Equal("Hello, World!", Assert.IsType<TextContentBlock>(result.Content[0]).Text);
+
+        // Verify that the options now have a TypeInfoResolver and are read-only
+        Assert.NotNull(options.TypeInfoResolver);
+        Assert.True(options.IsReadOnly);
+    }
+
     public static IEnumerable<object[]> StructuredOutput_ReturnsExpectedSchema_Inputs()
     {
         yield return new object[] { "string" };