Title: Ignore pre-2026 protocol_version pins at the StreamableHTTP transport by maxisbey · Pull Request #2923 · modelcontextprotocol/python-sdk · GitHub
Open Graph Title: Ignore pre-2026 protocol_version pins at the StreamableHTTP transport by maxisbey · Pull Request #2923 · modelcontextprotocol/python-sdk
X Title: Ignore pre-2026 protocol_version pins at the StreamableHTTP transport by maxisbey · Pull Request #2923 · modelcontextprotocol/python-sdk
Description: Follow-up to #2910 / #2917 addressing post-merge review feedback. Motivation and Context #2910 threaded the new protocol_version pin into StreamableHTTPTransport unconditionally and made the constructor pin win over the InitializeResult snoop. That's correct for a 2026-07-28 pin (no initialize is ever sent), but for a pre-2026 stateful pin it meant: the MCP-Protocol-Version header was stamped on the initialize POST itself (the spec only defines it for subsequent requests; a strict server that doesn't support the pinned version may 400 the handshake), and after the server negotiated a different version, the transport kept stamping the original pin on every later request — session.protocol_version and the wire header disagreed. A stateful pin is a session-layer concern (it picks the protocolVersion field in the InitializeRequest body); the transport doesn't need it. This change drops pre-2026 pins at the transport constructor so the header is populated from the negotiated InitializeResult exactly as before #2910, while modern pins continue to be honoured from the first request. Also adds a one-line caveat to the Client.protocol_version docstring noting that a 2026-07-28 pin currently requires a URL/HTTP transport (the in-memory Server/MCPServer path doesn't have a modern entry yet). Review threads not addressed here _streamable_http_modern.py returning -32700 for a valid-JSON non-request body — the body-parse block is replaced by the inbound classifier in the upcoming server-stateless work, which returns INVALID_REQUEST for that case. (At 2026-07-28 there are no client→server notifications over Streamable HTTP, so 202 is not the target shape — a deliberate rejection is.) _related_request_id # type: ignore[call-arg] in server/session.py — already tracked; falls out of the ServerRunner driver split. How Has This Been Tested? New unit test test_stateful_constructor_pin_is_ignored_and_the_negotiated_version_wins covers both the header-omitted-on-initialize and negotiated-version-wins behaviours. Existing test_modern_constructor_pin_is_not_overwritten_by_an_initialize_result (renamed for clarity) still pins the 2026 case. tests/interaction -k 'streamable-http and 2025' (230 tests, which pass a stateful 2025-11-25 pin on every connection) passes unchanged. Breaking Changes None. Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update Checklist I have read the MCP Documentation My code follows the repository's style guidelines New and existing tests pass locally I have added appropriate error handling I have added or updated documentation as needed Additional context The early-return guard in _maybe_extract_protocol_version_from_message is now reachable only via direct invocation in the unit test (a modern-pinned transport never sends initialize in production). Left as-is to keep this PR scoped; the single-owner refactor for the client pin will sweep it. AI Disclaimer
Open Graph Description: Follow-up to #2910 / #2917 addressing post-merge review feedback. Motivation and Context #2910 threaded the new protocol_version pin into StreamableHTTPTransport unconditionally and made the constr...
X Description: Follow-up to #2910 / #2917 addressing post-merge review feedback. Motivation and Context #2910 threaded the new protocol_version pin into StreamableHTTPTransport unconditionally and made the constr...
Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2923
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