Disable STJ reflection for testing.

src/ModelContextProtocol/Configuration/McpServerBuilderExtensions.cs

@@ -8,6 +8,7 @@
 using ModelContextProtocol.Utils;
 using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
+using System.Text.Json;
 
 namespace Microsoft.Extensions.DependencyInjection;
 
@@ -24,6 +25,7 @@ public static partial class McpServerBuilderExtensions
     /// <summary>Adds <see cref="McpServerTool"/> instances to the service collection backing <paramref name="builder"/>.</summary>
     /// <typeparam name="TToolType">The tool type.</typeparam>
     /// <param name="builder">The builder instance.</param>
+    /// <param name="serializerOptions">The serializer options governing tool parameter marshalling.</param>
     /// <returns>The builder provided in <paramref name="builder"/>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="builder"/> is <see langword="null"/>.</exception>
     /// <remarks>
@@ -35,7 +37,8 @@ public static partial class McpServerBuilderExtensions
         DynamicallyAccessedMemberTypes.PublicMethods |
         DynamicallyAccessedMemberTypes.NonPublicMethods |
         DynamicallyAccessedMemberTypes.PublicConstructors)] TToolType>(
-        this IMcpServerBuilder builder)
+        this IMcpServerBuilder builder,
+        JsonSerializerOptions? serializerOptions = null)
     {
         Throw.IfNull(builder);
 
@@ -44,8 +47,8 @@ public static partial class McpServerBuilderExtensions
             if (toolMethod.GetCustomAttribute<McpServerToolAttribute>() is not null)
             {
                 builder.Services.AddSingleton((Func<IServiceProvider, McpServerTool>)(toolMethod.IsStatic ?
-                    services => McpServerTool.Create(toolMethod, options: new() { Services = services }) :
-                    services => McpServerTool.Create(toolMethod, typeof(TToolType), new() { Services = services })));
+                    services => McpServerTool.Create(toolMethod, options: new() { Services = services, SerializerOptions = serializerOptions }) :
+                    services => McpServerTool.Create(toolMethod, typeof(TToolType), new() { Services = services, SerializerOptions = serializerOptions })));
             }
         }
 
@@ -55,6 +58,7 @@ public static partial class McpServerBuilderExtensions
     /// <summary>Adds <see cref="McpServerTool"/> instances to the service collection backing <paramref name="builder"/>.</summary>
     /// <param name="builder">The builder instance.</param>
     /// <param name="toolTypes">Types with marked methods to add as tools to the server.</param>
+    /// <param name="serializerOptions">The serializer options governing tool parameter marshalling.</param>
     /// <returns>The builder provided in <paramref name="builder"/>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="builder"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentNullException"><paramref name="toolTypes"/> is <see langword="null"/>.</exception>
@@ -64,7 +68,7 @@ public static partial class McpServerBuilderExtensions
     /// instance for each. For instance methods, an instance will be constructed for each invocation of the tool.
     /// </remarks>
     [RequiresUnreferencedCode(WithToolsRequiresUnreferencedCodeMessage)]
-    public static IMcpServerBuilder WithTools(this IMcpServerBuilder builder, params IEnumerable<Type> toolTypes)
+    public static IMcpServerBuilder WithTools(this IMcpServerBuilder builder, IEnumerable<Type> toolTypes, JsonSerializerOptions? serializerOptions = null)
     {
         Throw.IfNull(builder);
         Throw.IfNull(toolTypes);
@@ -78,8 +82,8 @@ public static IMcpServerBuilder WithTools(this IMcpServerBuilder builder, params
                     if (toolMethod.GetCustomAttribute<McpServerToolAttribute>() is not null)
                     {
                         builder.Services.AddSingleton((Func<IServiceProvider, McpServerTool>)(toolMethod.IsStatic ?
-                            services => McpServerTool.Create(toolMethod, options: new() { Services = services }) :
-                            services => McpServerTool.Create(toolMethod, toolType, new() { Services = services })));
+                            services => McpServerTool.Create(toolMethod, options: new() { Services = services , SerializerOptions = serializerOptions }) :
+                            services => McpServerTool.Create(toolMethod, toolType, new() { Services = services , SerializerOptions = serializerOptions })));
                     }
                 }
             }
@@ -92,6 +96,7 @@ public static IMcpServerBuilder WithTools(this IMcpServerBuilder builder, params
     /// Adds types marked with the <see cref="McpServerToolTypeAttribute"/> attribute from the given assembly as tools to the server.
     /// </summary>
     /// <param name="builder">The builder instance.</param>
+    /// <param name="serializerOptions">The serializer options governing tool parameter marshalling.</param>
     /// <param name="toolAssembly">The assembly to load the types from. If <see langword="null"/>, the calling assembly will be used.</param>
     /// <returns>The builder provided in <paramref name="builder"/>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="builder"/> is <see langword="null"/>.</exception>
@@ -116,7 +121,7 @@ public static IMcpServerBuilder WithTools(this IMcpServerBuilder builder, params
     /// </para>
     /// </remarks>
     [RequiresUnreferencedCode(WithToolsRequiresUnreferencedCodeMessage)]
-    public static IMcpServerBuilder WithToolsFromAssembly(this IMcpServerBuilder builder, Assembly? toolAssembly = null)
+    public static IMcpServerBuilder WithToolsFromAssembly(this IMcpServerBuilder builder, Assembly? toolAssembly = null, JsonSerializerOptions? serializerOptions = null)
     {
         Throw.IfNull(builder);
 
@@ -125,7 +130,8 @@ public static IMcpServerBuilder WithToolsFromAssembly(this IMcpServerBuilder bui
         return builder.WithTools(
             from t in toolAssembly.GetTypes()
             where t.GetCustomAttribute<McpServerToolTypeAttribute>() is not null
-            select t);
+            select t,
+            serializerOptions);
     }
     #endregion
 

src/ModelContextProtocol/Server/AIFunctionMcpServerTool.cs

@@ -67,6 +67,7 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Name = options?.Name ?? method.GetCustomAttribute<McpServerToolAttribute>()?.Name,
             Description = options?.Description,
             MarshalResult = static (result, _, cancellationToken) => new ValueTask<object?>(result),
+            SerializerOptions = options?.SerializerOptions ?? McpJsonUtilities.DefaultOptions,
             ConfigureParameterBinding = pi =>
             {
                 if (pi.ParameterType == typeof(RequestContext<CallToolRequestParams>))
@@ -314,7 +315,7 @@ public override async Task<CallToolResponse> InvokeAsync(
             {
                 Content = [new()
                 {
-                    Text = JsonSerializer.Serialize(result, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object))),
+                    Text = JsonSerializer.Serialize(result, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object))),
                     Type = "text"
                 }]
             },

src/ModelContextProtocol/Server/McpServerToolCreateOptions.cs

@@ -1,4 +1,6 @@
+using ModelContextProtocol.Utils.Json;
 using System.ComponentModel;
+using System.Text.Json;
 
 namespace ModelContextProtocol.Server;
 
@@ -122,11 +124,19 @@ public sealed class McpServerToolCreateOptions
     /// </remarks>
     public bool? ReadOnly { get; set; }
 
+    /// <summary>
+    /// Gets or sets the JSON serializer options to use when marshalling data to/from JSON.
+    /// </summary>
+    /// <remarks>
+    /// Defaults to <see cref="McpJsonUtilities.DefaultOptions"/> if left unspecified.
+    /// </remarks>
+    public JsonSerializerOptions? SerializerOptions { get; set; }
+
     /// <summary>
     /// Creates a shallow clone of the current <see cref="McpServerToolCreateOptions"/> instance.
     /// </summary>
     internal McpServerToolCreateOptions Clone() =>
-        new McpServerToolCreateOptions()
+        new McpServerToolCreateOptions
         {
             Services = Services,
             Name = Name,
@@ -135,6 +145,7 @@ internal McpServerToolCreateOptions Clone() =>
             Destructive = Destructive,
             Idempotent = Idempotent,
             OpenWorld = OpenWorld,
-            ReadOnly = ReadOnly
+            ReadOnly = ReadOnly,
+            SerializerOptions = SerializerOptions,
         };
 }

tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj

@@ -10,6 +10,11 @@
     <RootNamespace>ModelContextProtocol.AspNetCore.Tests</RootNamespace>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(TargetFramework)' == 'net9.0'">
+    <!-- For better test coverage, only disable reflection in one of the targets -->
+    <JsonSerializerIsReflectionEnabledByDefault>false</JsonSerializerIsReflectionEnabledByDefault>
+  </PropertyGroup>
+
   <PropertyGroup>
     <!-- Without this, tests are currently not showing results until all tests complete
          https://xunit.net/docs/getting-started/v3/microsoft-testing-platform

tests/ModelContextProtocol.AspNetCore.Tests/SseIntegrationTests.cs

@@ -10,11 +10,12 @@
 using ModelContextProtocol.Server;
 using ModelContextProtocol.Tests.Utils;
 using ModelContextProtocol.Utils.Json;
+using System.Text.Json.Serialization;
 using TestServerWithHosting.Tools;
 
 namespace ModelContextProtocol.Tests;
 
-public class SseIntegrationTests(ITestOutputHelper outputHelper) : KestrelInMemoryTest(outputHelper)
+public partial class SseIntegrationTests(ITestOutputHelper outputHelper) : KestrelInMemoryTest(outputHelper)
 {
     private SseClientTransportOptions DefaultTransportOptions = new()
     {
@@ -41,7 +42,7 @@ public async Task ConnectAndReceiveMessage_InMemoryServer()
         await using var mcpClient = await ConnectMcpClient(httpClient);
 
         // Send a test message through POST endpoint
-        await mcpClient.SendNotificationAsync("test/message", new { message = "Hello, SSE!" }, cancellationToken: TestContext.Current.CancellationToken);
+        await mcpClient.SendNotificationAsync("test/message", new Envelope { Message = "Hello, SSE!" }, serializerOptions: JsonContext.Default.Options, cancellationToken: TestContext.Current.CancellationToken);
 
         Assert.True(true);
     }
@@ -57,7 +58,7 @@ public async Task ConnectAndReceiveMessage_InMemoryServer_WithFullEndpointEventU
         await using var mcpClient = await ConnectMcpClient(httpClient);
 
         // Send a test message through POST endpoint
-        await mcpClient.SendNotificationAsync("test/message", new { message = "Hello, SSE!" }, cancellationToken: TestContext.Current.CancellationToken);
+        await mcpClient.SendNotificationAsync("test/message", new Envelope { Message = "Hello, SSE!" }, serializerOptions: JsonContext.Default.Options, cancellationToken: TestContext.Current.CancellationToken);
 
         Assert.True(true);
     }
@@ -73,7 +74,7 @@ public async Task ConnectAndReceiveNotification_InMemoryServer()
             mcpServer.RegisterNotificationHandler("test/notification", async (notification, cancellationToken) =>
             {
                 Assert.Equal("Hello from client!", notification.Params?["message"]?.GetValue<string>());
-                await mcpServer.SendNotificationAsync("test/notification", new { message = "Hello from server!" }, cancellationToken: cancellationToken);
+                await mcpServer.SendNotificationAsync("test/notification", new Envelope { Message = "Hello from server!" }, serializerOptions: JsonContext.Default.Options, cancellationToken: cancellationToken);
             });
             return mcpServer.RunAsync(cancellationToken);
         });
@@ -90,7 +91,7 @@ public async Task ConnectAndReceiveNotification_InMemoryServer()
         });
 
         // Send a test message through POST endpoint
-        await mcpClient.SendNotificationAsync("test/notification", new { message = "Hello from client!" }, cancellationToken: TestContext.Current.CancellationToken);
+        await mcpClient.SendNotificationAsync("test/notification", new Envelope { Message = "Hello from client!" }, serializerOptions: JsonContext.Default.Options, cancellationToken: TestContext.Current.CancellationToken);
 
         var message = await receivedNotification.Task.WaitAsync(TimeSpan.FromSeconds(10), TestContext.Current.CancellationToken);
         Assert.Equal("Hello from server!", message);
@@ -205,4 +206,13 @@ private static void MapAbsoluteEndpointUriMcp(IEndpointRouteBuilder endpoints)
             await context.Response.WriteAsync("Accepted");
         });
     }
+
+    public class Envelope
+    {
+        public required string Message { get; set; }
+    }
+
+    [JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
+    [JsonSerializable(typeof(Envelope))]
+    partial class JsonContext : JsonSerializerContext;
 }

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Connections;
 using ModelContextProtocol.Protocol.Types;
 using ModelContextProtocol.Server;
+using ModelContextProtocol.Utils.Json;
 using Serilog;
 using System.Text;
 using System.Text.Json;
@@ -124,7 +125,7 @@ static CreateMessageRequestParams CreateRequestSamplingParams(string context, st
                                         },
                                         "required": ["message"]
                                     }
-                                    """),
+                                    """, McpJsonUtilities.DefaultOptions),
                             },
                             new Tool()
                             {
@@ -145,7 +146,7 @@ static CreateMessageRequestParams CreateRequestSamplingParams(string context, st
                                         },
                                         "required": ["prompt", "maxTokens"]
                                     }
-                                    """),
+                                    """, McpJsonUtilities.DefaultOptions),
                             }
                         ]
                     });

tests/ModelContextProtocol.Tests/Client/McpClientExtensionsTests.cs

@@ -5,6 +5,7 @@
 using ModelContextProtocol.Protocol.Messages;
 using ModelContextProtocol.Protocol.Types;
 using ModelContextProtocol.Server;
+using ModelContextProtocol.Utils.Json;
 using Moq;
 using System.Text.Json;
 using System.Text.Json.Serialization.Metadata;
@@ -379,7 +380,7 @@ public async Task AsClientLoggerProvider_MessagesSentToClient()
         await using (client.RegisterNotificationHandler(NotificationMethods.LoggingMessageNotification,
             (notification, cancellationToken) =>
             {
-                Assert.True(channel.Writer.TryWrite(JsonSerializer.Deserialize<LoggingMessageNotificationParams>(notification.Params)));
+                Assert.True(channel.Writer.TryWrite(JsonSerializer.Deserialize<LoggingMessageNotificationParams>(notification.Params, McpJsonUtilities.DefaultOptions)));
                 return Task.CompletedTask;
             }))
         {
@@ -398,7 +399,7 @@ public async Task AsClientLoggerProvider_MessagesSentToClient()
 
                 Assert.Equal("TestLogger", m.Logger);
 
-                string ? s = JsonSerializer.Deserialize<string>(m.Data.Value);
+                string ? s = JsonSerializer.Deserialize<string>(m.Data.Value, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(s);
 
                 if (s.Contains("Information"))

tests/ModelContextProtocol.Tests/Client/McpClientFactoryTests.cs

@@ -3,6 +3,7 @@
 using ModelContextProtocol.Protocol.Messages;
 using ModelContextProtocol.Protocol.Transport;
 using ModelContextProtocol.Protocol.Types;
+using ModelContextProtocol.Utils.Json;
 using Moq;
 using System.Text.Json;
 using System.Threading.Channels;
@@ -107,7 +108,7 @@ public virtual Task SendMessageAsync(IJsonRpcMessage message, CancellationToken
                                 Name = "NopTransport",
                                 Version = "1.0.0"
                             },
-                        }),
+                        }, McpJsonUtilities.DefaultOptions),
                     });
                     break;
             }

tests/ModelContextProtocol.Tests/ClientIntegrationTests.cs

@@ -4,12 +4,14 @@
 using ModelContextProtocol.Protocol.Transport;
 using ModelContextProtocol.Protocol.Types;
 using ModelContextProtocol.Tests.Utils;
+using ModelContextProtocol.Utils.Json;
 using OpenAI;
 using System.Text.Json;
+using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Tests;
 
-public class ClientIntegrationTests : LoggedTest, IClassFixture<ClientIntegrationTestFixture>
+public partial class ClientIntegrationTests : LoggedTest, IClassFixture<ClientIntegrationTestFixture>
 {
     private static readonly string? s_openAIKey = Environment.GetEnvironmentVariable("AI:OpenAI:ApiKey");
 
@@ -261,7 +263,7 @@ public async Task SubscribeResource_Stdio()
                 [
                     new(NotificationMethods.ResourceUpdatedNotification, (notification, cancellationToken) =>
                     {
-                        var notificationParams = JsonSerializer.Deserialize<ResourceUpdatedNotificationParams>(notification.Params);
+                        var notificationParams = JsonSerializer.Deserialize<ResourceUpdatedNotificationParams>(notification.Params, McpJsonUtilities.DefaultOptions);
                         tcs.TrySetResult(true);
                         return Task.CompletedTask;
                     })
@@ -291,7 +293,7 @@ public async Task UnsubscribeResource_Stdio()
                 [
                     new(NotificationMethods.ResourceUpdatedNotification, (notification, cancellationToken) =>
                     {
-                        var notificationParams = JsonSerializer.Deserialize<ResourceUpdatedNotificationParams>(notification.Params);
+                        var notificationParams = JsonSerializer.Deserialize<ResourceUpdatedNotificationParams>(notification.Params, McpJsonUtilities.DefaultOptions);
                         receivedNotification.TrySetResult(true);
                         return Task.CompletedTask;
                     })
@@ -442,13 +444,18 @@ public async Task Notifications_Stdio(string clientId)
 
         // Verify we can send notifications without errors
         await client.SendNotificationAsync(NotificationMethods.RootsUpdatedNotification, cancellationToken: TestContext.Current.CancellationToken);
-        await client.SendNotificationAsync("test/notification", new { test = true }, cancellationToken: TestContext.Current.CancellationToken);
+        await client.SendNotificationAsync("test/notification", new TestNotification { Test = true }, cancellationToken: TestContext.Current.CancellationToken, serializerOptions: JsonContext3.Default.Options);
 
         // assert
         // no response to check, if no exception is thrown, it's a success
         Assert.True(true);
     }
 
+    class TestNotification
+    {
+        public required bool Test { get; set; }
+    }
+
     [Fact]
     public async Task CallTool_Stdio_MemoryServer()
     {
@@ -557,7 +564,7 @@ public async Task SetLoggingLevel_ReceivesLoggingMessages(string clientId)
                 [
                     new(NotificationMethods.LoggingMessageNotification, (notification, cancellationToken) =>
                     {
-                        var loggingMessageNotificationParameters = JsonSerializer.Deserialize<LoggingMessageNotificationParams>(notification.Params);
+                        var loggingMessageNotificationParameters = JsonSerializer.Deserialize<LoggingMessageNotificationParams>(notification.Params, McpJsonUtilities.DefaultOptions);
                         if (loggingMessageNotificationParameters is not null)
                         {
                             receivedNotification.TrySetResult(true);
@@ -574,4 +581,7 @@ public async Task SetLoggingLevel_ReceivesLoggingMessages(string clientId)
         // assert
         await receivedNotification.Task;
     }
+
+    [JsonSerializable(typeof(TestNotification))]
+    partial class JsonContext3 : JsonSerializerContext;
 }