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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Session ID not present in memory store should return 404, not 400","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 session ID is provided by a client, but it is not present in the in-memory session store for the server, the server returns a 400 https://github.com/modelcontextprotocol/python-sdk/blob/d0443a18328a2fc9e87da6feee9130f95c0b37a7/src/mcp/server/streamable_http_manager.py#L250-L256\n\nThis is a problem, because as noted in https://github.com/modelcontextprotocol/python-sdk/issues/880 sessions are not persisted between restarts\n\nThis library should return a 404, as in https://github.com/modelcontextprotocol/python-sdk/blob/d0443a18328a2fc9e87da6feee9130f95c0b37a7/src/mcp/server/streamable_http.py#L371 So that the client can re-initialize and get a new session ID as needed (as in https://github.com/mark3labs/mcp-go/blob/8f5b048218f6d044c3322f16b8cb0b08e10bf5d0/client/transport/streamable_http.go#L257). \n\n400 isn't applicable because the request isn't _bad_, but the server simply doesn't persist sessions between restarts.\n\n### Example Code\n\n```Python\n\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\n3.10.12 and 2.3.4\n```","author":{"url":"https://github.com/sthomson-wyn","@type":"Person","name":"sthomson-wyn"},"datePublished":"2025-06-23T11:44:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/1004/python-sdk/issues/1004"}