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Title: fix: return HTTP 404 for unknown session IDs instead of 400 by max-rousseau · Pull Request #1808 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: fix: return HTTP 404 for unknown session IDs instead of 400 by max-rousseau · Pull Request #1808 · modelcontextprotocol/python-sdk

X Title: fix: return HTTP 404 for unknown session IDs instead of 400 by max-rousseau · Pull Request #1808 · modelcontextprotocol/python-sdk

Description: Summary Fixes incorrect HTTP status code returned by StreamableHTTPSessionManager when receiving requests with unknown or expired session IDs. Per the MCP Streamable HTTP transport specification, servers MUST respond with HTTP 404 (Not Found) when the session ID is not recognized. The previous implementation returned HTTP 400 (Bad Request), which incorrectly signals a malformed request rather than a missing resource. This change aligns the Python SDK behavior with: The MCP specification requirements The TypeScript SDK implementation Changes Return HTTP 404 instead of 400 for unknown session IDs Use JSON-RPC error format in the response body, matching TypeScript SDK behavior Fixes #1727

Open Graph Description: Summary Fixes incorrect HTTP status code returned by StreamableHTTPSessionManager when receiving requests with unknown or expired session IDs. Per the MCP Streamable HTTP transport specification, s...

X Description: Summary Fixes incorrect HTTP status code returned by StreamableHTTPSessionManager when receiving requests with unknown or expired session IDs. Per the MCP Streamable HTTP transport specification, s...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/1808

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4aaf4ad fix: return HTTP 404 for unknown session IDs instead of 400 max-rousseau Dec 20, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/1808/commits/4aaf4ad9335633317a11cd4c5a52e55feaeb45fd
75b142b refactor: use typed models and add test for session 404 response maxisbey Dec 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/1808/commits/75b142bcc325b4fcff5c10befa8c31ac5918e6f0
f86ea93 test: add pragma no cover to mock_receive function maxisbey Dec 31, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/1808/commits/f86ea932d00b47359b933ee216806810101aea68
06ecc2b chore: add TODO comment for error code alignment maxisbey Dec 31, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/1808/commits/06ecc2b95729016c0a1672134fce49123e15262a
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