<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup><TargetFramework>net10.0</TargetFramework><Nullable>enable</Nullable><ImplicitUsings>enable</ImplicitUsings></PropertyGroup>
  <ItemGroup><PackageReference Include="ModelContextProtocol.AspNetCore" Version="$(McpVer)" /></ItemGroup>
</Project>

// Minimal MCP server, public API only. Two tools that differ only in Annotations.
using System.ComponentModel;
using System.Text;
using ModelContextProtocol.AspNetCore;
using ModelContextProtocol.Protocol;
using ModelContextProtocol.Server;

var stateless = args.Contains("--stateless");
var builder = WebApplication.CreateBuilder(args);
builder.WebHost.UseUrls("http://127.0.0.1:5599");
builder.Logging.SetMinimumLevel(LogLevel.Warning);
builder.Services.AddMcpServer()
    .WithHttpTransport(o => { if (stateless) o.SessionMode = HttpServerSessionMode.Stateless; })
    .WithTools<DemoTools>();
var app = builder.Build();
app.MapMcp("/mcp");
await app.StartAsync();

using var http = new HttpClient { BaseAddress = new Uri("http://127.0.0.1:5599") };
string? session = null;
async Task<string> Post(string json)
{
    var req = new HttpRequestMessage(HttpMethod.Post, "/mcp") { Content = new StringContent(json, Encoding.UTF8, "application/json") };
    req.Headers.TryAddWithoutValidation("Accept", "application/json, text/event-stream");
    req.Headers.TryAddWithoutValidation("MCP-Protocol-Version", "2025-11-25");
    if (session is not null) req.Headers.TryAddWithoutValidation("Mcp-Session-Id", session);
    var res = await http.SendAsync(req);
    if (res.Headers.TryGetValues("Mcp-Session-Id", out var s)) session = s.First();
    var body = (await res.Content.ReadAsStringAsync()).Replace("\r", "");
    var data = body.Split('\n').FirstOrDefault(l => l.StartsWith("data: "))?[6..] ?? body;
    return $"{(int)res.StatusCode} {data}";
}

Console.WriteLine($"mode: {(stateless ? "stateless" : "stateful (default)")}, SDK {typeof(McpServerToolAttribute).Assembly.GetName().Version}");
if (!stateless)
{
    Console.WriteLine("initialize -> " + (await Post("""{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"repro","version":"1"}}}"""))[..3]);
    await Post("""{"jsonrpc":"2.0","method":"notifications/initialized"}""");
}
foreach (var tool in new[] { "plain", "annotated", "annotated_link" })
{
    Console.WriteLine($"{tool} -> " + await Post("{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"tools/call\",\"params\":{\"name\":\"" + tool + "\",\"arguments\":{}}}"));
}
await app.StopAsync();

[McpServerToolType]
public sealed class DemoTools
{
    [McpServerTool(Name = "plain"), Description("Text block without annotations.")]
    public static CallToolResult Plain() => new() { Content = [new TextContentBlock { Text = "x" }] };

    [McpServerTool(Name = "annotated"), Description("Same text block with annotations.")]
    public static CallToolResult Annotated() => new()
    {
        Content = [new TextContentBlock { Text = "x", Annotations = new Annotations { Audience = [Role.User] } }],
    };

    [McpServerTool(Name = "annotated_link"), Description("Resource link with annotations.")]
    public static CallToolResult AnnotatedLink() => new()
    {
        Content = [new ResourceLinkBlock { Uri = "https://example.com/x", Name = "x", Annotations = new Annotations { Audience = [Role.User] } }],
    };
}

dotnet run -p:McpVer=2.3.1
dotnet run -p:McpVer=2.4.0

plain -> 200 {"result":{"content":[{"type":"text","text":"x"}]},"id":1,"jsonrpc":"2.0"}
annotated -> 200 {"result":{"content":[{"type":"text","text":"x","annotations":{"audience":["user"]}}]},"id":1,"jsonrpc":"2.0"}
annotated_link -> 200 {"result":{"content":[{"type":"resource_link","uri":"https://example.com/x","name":"x","annotations":{"audience":["user"]}}]},"id":1,"jsonrpc":"2.0"}

plain -> 200 {"result":{"content":[{"type":"text","text":"x"}]},"id":1,"jsonrpc":"2.0"}
annotated -> 200 {"error":{"code":-32603,"message":"An error occurred."},"id":1,"jsonrpc":"2.0"}
annotated_link -> 200 {"error":{"code":-32603,"message":"An error occurred."},"id":1,"jsonrpc":"2.0"}

warn: ModelContextProtocol.Server.McpServer
      Server (repro 1.0.0.0) method 'tools/call' request handler failed.
      System.InvalidOperationException: Cannot insert the values of 'TypeInfoResolver' or 'TypeInfoResolverChain' to 'TypeInfoResolverChain'.
         at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_InvalidChainedResolver()
         at System.Text.Json.JsonSerializerOptions.OptionsBoundJsonTypeInfoResolverChain.ValidateAddedValue(IJsonTypeInfoResolver item)
         at System.Text.Json.Serialization.ConfigurationList`1.Add(TItem item)
         at ModelContextProtocol.Protocol.ContentBlock.Converter.CreateNestedOptions(JsonSerializerOptions callerOptions)
         at System.Runtime.CompilerServices.ConditionalWeakTable`2.GetValue(TKey key, CreateValueCallback createValueCallback)
         at ModelContextProtocol.Protocol.ContentBlock.Converter.GetNestedTypeInfo[T](JsonSerializerOptions options)
         at ModelContextProtocol.Protocol.ContentBlock.Converter.Write(Utf8JsonWriter writer, ContentBlock value, JsonSerializerOptions options)

JsonSerializerOptions nestedOptions = new(callerOptions);
nestedOptions.TypeInfoResolverChain.Add(McpJsonUtilities.JsonContext.Default);

Note

This issue was prepared with AI assistance (Claude Code). I reviewed the repro and its results before filing.