SDK 0.4 not working with OpenAI's internal MCP client

[mbuptivo]

Hi,
I created a stateless MCP server and I am trying to connect it with OpenAI, initially via platform's dashboard chat/prompt test interface. I'm facing a problem with openai sending a GET to my /mcp endpoint, which, in stateless mode, returns a 405 method not allowed error. Acording to mcp protocol description, this should be ok.

https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#listening-for-messages-from-the-server

The server MUST either return Content-Type: text/event-stream in response to this HTTP GET, or else return HTTP 405 Method Not Allowed, indicating that the server does not offer an SSE stream at this endpoint.

But this disrupts openai's client, which returns "Error retrieving tool list from MCP server: 'TestMCP'. Http status code: 405 (Method Not Allowed)". This happens any time before or after getting the tools list.

NOTICE: I've also temporarily tried without "stateless = true", but it gives another error. In any case, I want it to work in stateless mode.

Has anyone been able to work around this?
Thanks!

[mikekistler]

Sounds like a bug in the OpenAI client. Have you opened an issue with them?

[mbuptivo]

Not yet, I' still trying to understand what's happening. It seems that the MCP client from openai's playground behaves differently from the one in the node SDK. But this does not make any sense, both use the responses API.

Basically, I have a local MCP server accessible via ngrok (I use it to inspect traffic).
I configured the server with official openai responses' API SDK (acting as client): I only receive POSTs to /mcp (it works)
Same server on openai developer's playground: It sends GETs to /mcp, receives 405 and fails

I'm gonna report this.