fix: return HTTP 404 for unknown session IDs instead of 400 #1808
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Per the MCP Streamable HTTP transport spec, servers MUST respond with HTTP 404 when the session ID is not found. This aligns behavior with the TypeScript SDK implementation. Github-Issue: modelcontextprotocol#1727
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Summary
Fixes incorrect HTTP status code returned by
StreamableHTTPSessionManagerwhen 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:
Changes
Fixes #1727