The MCP server should not send a response to notifications sent by the client

[mikekistler]

Describe the bug

The MCP Specification says

Notifications are sent from the client to the server or vice versa, as a one-way message. The receiver MUST NOT send a response.

But traces of traffic from the C# MCP server seem to indicate that it does send a response to the "notifications/initialized" message sent by the client.

trce: HttpLogger[0]
      HTTP Request:
      POST http://localhost:3001/
      Accept: application/json, text/event-stream
      Mcp-Session-Id: CkeD4IN4sOyVVBWHR4Ahpg
      MCP-Protocol-Version: 2025-06-18
      X-Trace-Id: 90f2662b-acd0-46a7-a75d-857d4b9b9b39
      Content-Type: application/json; charset=utf-8
      
      {"method":"notifications/initialized","params":{},"jsonrpc":"2.0"}
      
trce: HttpLogger[0]
      HTTP Response: Accepted
      Date: Tue, 24 Jun 2025 18:37:34 GMT
      Server: Kestrel
      Cache-Control: no-store, no-cache
      X-Trace-Id: 90f2662b-acd0-46a7-a75d-857d4b9b9b39
      Mcp-Session-Id: CkeD4IN4sOyVVBWHR4Ahpg
      Content-Length: 0
      Content-Encoding: identity

To Reproduce

Create a client that configures logging of all HTTP traffic and then use that client to connect to a C# MCP Server.

In my repo, I also added logic to include an X-Trace-Id header in the request that is echoed in the response, just to make painfully clear which request is associated with a response. You can see in the output above that the values of X-Trace-Id match.

Expected behavior

The MCP server should not send a response to a notification message.

Additional context

Maybe this is a semantic issue. The server gave an "HTTP response" but maybe this is not considered a "JSON RPC response"?

[stephentoub]

Sending a response in the context of the cited MCP spec means sending a JSON-RPC payload, e.g. a JSON-RPC result or error payload. That's not happening here. As for as I can see from the traces you shared, what you're seeing is just the HTTP response implicit to sending an HTTP request.