Passing a malformed body throws a 500 error instead of 400. Very hard to figure out what is incorrect in the request being issued by the caller

[aashmangupta]

Description
When sending an InitializeRequest to the MCP C# SDK server with a malformed or incomplete request body (e.g., missing required fields like clientInfo.Version, the server responds with a 500 Internal Server Error.
This behavior is problematic because:

• A 500 error implies a server-side failure, not a client-side input issue.
• According to MCP protocol expectations, the server should return a 400 Bad Request or a structured JSON-RPC error response indicating what was wrong with the request.

Steps to reproduce the behavior:

1. Issue a POST mcp call with this request body:

{ "jsonrpc": "2.0", "id": "7", "method": "initialize", "params": { "protocolVersion": "2025-06-18", "clientInfo": { "name": "Insomnia" } } }

Expected behavior
A 400 error should be returned telling the user that the ClientInfo.Version property is missing

Additional Context
Our controllers are mapped like this in our Startup.cs file:

app.UseEndpoints(endpoints => { endpoints.MapControllers(); endpoints.MapMcp("/mcp/environments/{environmentId}/servers/{serverName}"); });
and controller is defined like this:

[HttpGet] [HttpPost] [HttpDelete] [Route("mcp/environments/{environmentId}/servers/{serverName}")] public IActionResult Invoke() { return this.Ok(); }

[lntutor]

I would like to work on this. I plan to add an HTTP integration test for an invalid initialize payload, then map request deserialization/validation failures to a client error instead of an unhandled 500. I cannot self-assign without repository permissions.