POST /mcp with a missing or unparseable Content-Type returns HTTP 500 instead of a 4xx #1966

#:sdk Microsoft.NET.Sdk.Web
#:package ModelContextProtocol.AspNetCore@2.3.0

// Reproduction: a Streamable HTTP POST with a missing or unparseable Content-Type returns HTTP 500
// (unhandled InvalidOperationException) instead of a 4xx.
//
// Run with: dotnet run app.cs   (.NET 10 SDK)

using System.ComponentModel;
using System.Text;
using ModelContextProtocol.Server;

var builder = WebApplication.CreateBuilder(args);
builder.Logging.SetMinimumLevel(LogLevel.None);
builder.WebHost.UseUrls("http://127.0.0.1:0");
builder.Services
    .AddMcpServer()
    .WithHttpTransport(options => options.Stateless = true)
    .WithTools<EchoTool>();

var app = builder.Build();

// Observe only: print the exception that escapes the SDK handler, then rethrow so the host
// still produces its normal response.
app.Use(async (context, next) =>
{
    try
    {
        await next();
    }
    catch (Exception exception)
    {
        Console.WriteLine($"    unhandled {exception.GetType().Name}: {exception.Message}");
        throw;
    }
});
app.MapMcp("/mcp");
await app.StartAsync();

const string initialize =
    """{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"repro","version":"1.0"}}}""";

string?[] contentTypes =
[
    "application/json",
    "text/plain",
    null,
    "",
    "/json"
];

using var client = new HttpClient { BaseAddress = new Uri(app.Urls.First()) };

Console.WriteLine($"{"Content-Type",-30} Status");
foreach (var contentType in contentTypes)
{
    using var request = new HttpRequestMessage(HttpMethod.Post, "/mcp");
    request.Headers.Accept.ParseAdd("application/json");
    request.Headers.Accept.ParseAdd("text/event-stream");
    request.Content = new StringContent(initialize, Encoding.UTF8);
    request.Content.Headers.Remove("Content-Type");

    if (contentType is not null)
        request.Content.Headers.TryAddWithoutValidation("Content-Type", contentType);

    using var response = await client.SendAsync(request);
    Console.WriteLine($"{contentType switch { null => "<absent>", "" => "<empty>", _ => contentType },-30} {(int)response.StatusCode}");
}

await app.StopAsync();

[McpServerToolType]
public sealed class EchoTool
{
    [McpServerTool, Description("Echoes the message.")]
    public static string Echo(string message) => message;
}