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Title: Fix unknown-method error code and add a protocol version registry by maxisbey · Pull Request #2836 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: Fix unknown-method error code and add a protocol version registry by maxisbey · Pull Request #2836 · modelcontextprotocol/python-sdk

X Title: Fix unknown-method error code and add a protocol version registry by maxisbey · Pull Request #2836 · modelcontextprotocol/python-sdk

Description: Bundled fixes around the same version/dispatch seams: unknown methods now get the JSON-RPC-correct error code on both sides, and protocol version comparisons go through a small registry instead of raw string compares. Motivation and Context Unknown methods answered -32602 instead of -32601. A request with an unknown method failed inside the request-union validation and fell into the generic invalid-params handler, so peers received -32602 (Invalid params) instead of -32601 (Method not found). This affected the server runner for pre-initialization requests and the client-side receive loop. JSON-RPC 2.0 reserves -32601 for exactly this case, and clients that probe for optional/newer methods need to distinguish "method doesn't exist" from "bad params" to fall back correctly. Fixes #1561 (its repro — an unknown method sent before any handshake — is the exact pre-initialization path fixed here). Lexicographic version compares on wire input. Three call sites compared protocol versions with raw string ordering (e.g. >= "2025-11-25"), including against the client-supplied MCP-Protocol-Version header. Any non-date string sorts somewhere (letters sort after digits, so e.g. "draft" compares newer than every release) and can silently flip version-gated behavior. Comparisons now go through mcp.shared.version: an ordered registry of released protocol versions plus is_version_at_least(), which treats unknown wire strings conservatively (no version-gated behavior enabled) and raises on an unknown threshold, since that's a typo in our own code. Also included: ServerSession.protocol_version: read access to the negotiated version (the field was previously write-only internal state). docs/migration.md correction: it claimed extra fields on MCP types raise a ValidationError; they are silently ignored. The doc now describes actual behavior. How Has This Been Tested? Regression tests for both error-code fixes were written first and verified to fail on main with the old behavior (-32602 with empty data) before the fix. Full suite green locally at 100% line+branch coverage; pyright and ruff clean. The registry refactor of the three compare sites is behavior-preserving for all released versions (covered by equivalence tests); only unknown-string inputs change outcome, conservatively. Breaking Changes No API changes. One wire-visible behavior change: requests with unknown methods now receive -32601 (Method not found) instead of -32602 — the JSON-RPC-specified code. Anything matching on the old, incorrect code for this case would need updating. 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 Commits are stacked for review (registry first, then the two error-code fixes, then the property and doc fix) and are independently revertable. A v1.x backport of the -32601 fix is prepared separately. AI Disclaimer

Open Graph Description: Bundled fixes around the same version/dispatch seams: unknown methods now get the JSON-RPC-correct error code on both sides, and protocol version comparisons go through a small registry instead of ...

X Description: Bundled fixes around the same version/dispatch seams: unknown methods now get the JSON-RPC-correct error code on both sides, and protocol version comparisons go through a small registry instead of ...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2836

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