René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Return 405 on GET when stateless_http=True","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\nWhen a Streamable HTTP server is configured with `stateless_http=True`, `GET /mcp` is still accepted and opens an SSE stream that has no session context and can never receive server-initiated messages — a dead-end.\n\nThe MCP Streamable HTTP spec explicitly permits returning `405 Method Not Allowed` when the server does not offer an SSE stream at the endpoint\n([spec](https://modelcontextprotocol.io/specification/2025-11-25/basic/transports)):\n\n\u003e The server **MUST** return HTTP 405 Method Not Allowed if an SSE stream is\n\u003e not offered at the endpoint.\n\nA stateless server cannot offer one — there is no session to push to — so GET should 405. The TypeScript SDK already behaves this way in its stateless example.\n\nA prior PR (#2262) proposed essentially this change but was closed for lack of a corresponding issue per CONTRIBUTING.md. \nOpening this issue to agree on scope so the fix can be re-submitted.\n\n### Example Code\n\n```Python\nfrom mcp.server.fastmcp import FastMCP\n\nmcp = FastMCP(\"my-server\", stateless_http=True, json_response=True)\n\n@mcp.tool()\ndef ping() -\u003e str:\n    return \"pong\"\n\napp = mcp.streamable_http_app()\n```\n\nA couple of clarifications for triage:\n\n### Current vs expected behavior\n\nToday (v1.27.0):\n```\n$ curl -i -X GET http://localhost:8000/mcp\nHTTP/1.1 200 OK\ncontent-type: text/event-stream\n... (stream idles until platform timeout)\n```\n\nExpected:\n```\n$ curl -i -X GET http://localhost:8000/mcp\nHTTP/1.1 405 Method Not Allowed\nAllow: POST\n```\n\n### Concrete impact\nThe idle GET SSE stream holds a long-lived connection per client with no useful payload, exhausting concurrent connection limits on serverless platforms like Cloud Run. Background previously raised in #2232 / #1941.\n\n### Proposed resolution\n\nPR #2262 already has a working implementation. Once this issue is labeled `ready for work`, reopening it (narrowed to GET per this issue's scope) should be sufficient — no new PR needed.\n\n### Python \u0026 MCP Python SDK\n\n```Text\n1.27.0\n```","author":{"url":"https://github.com/itomise","@type":"Person","name":"itomise"},"datePublished":"2026-04-19T09:15:05.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/2474/python-sdk/issues/2474"}