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fix(session): return METHOD_NOT_FOUND for unknown request methods - #3210

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fix(session): return METHOD_NOT_FOUND for unknown request methods#3210
Ranga-Prasath-22 wants to merge 4 commits into
modelcontextprotocol:v1.xfrom
Ranga-Prasath-22:fix/v1-unknown-method-error-code

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@Ranga-Prasath-22 Ranga-Prasath-22 commented Jul 29, 2026 •
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Summary

Return JSON-RPC -32601 METHOD_NOT_FOUND for request methods that are not part of the session's request schema, while preserving -32602 INVALID_PARAMS for known methods with invalid parameters. Error responses echo the request id unchanged.

Why

BaseSession previously mapped every request-model validation failure to -32602. That made an unsupported method indistinguishable from a malformed call and caused clients to retry requests that could never succeed.

Implementation

  • Derive the supported request-method set from the configured Pydantic request type once per session.
  • Apply the distinction centrally in BaseSession, covering both client and server sessions and custom request unions.
  • Fail closed if schema introspection cannot produce a method set.

Behavior note

Validation-error responses no longer include the empty data="" field (-32602 and the new -32601 paths both omit it). This is spec-legal — the JSON-RPC data member is optional and its type is implementation-defined — and matches the TypeScript SDK, but clients inspecting error.data may notice the change.

Tests

  • Symmetric regression coverage for unknown methods and malformed known methods on both client and server sessions.
  • Unknown notifications are verified to produce no response, per JSON-RPC requirements.
  • Direct assertion that _extract_known_request_methods extracts the full expected method set for ClientRequest/ServerRequest, plus fallback behavior for non-string defaults and introspection failures.

Validation

  • Focused session tests pass (tests/shared/test_session.py, tests/server/test_session.py).
  • Ruff format and Ruff lint pass.
  • Behavior verified against the earlier review comments on this PR.

Fixes #3193
Resolves #1561

@Ranga-Prasath-22 Ranga-Prasath-22 changed the title fix(session): return -32601 METHOD_NOT_FOUND on unknown request methods fix(session): return METHOD_NOT_FOUND for unknown request methods Jul 29, 2026
@Ranga-Prasath-22

Ranga-Prasath-22 commented Jul 31, 2026

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Just a quiet follow-up with some verification evidence, no rush -- I know the queue is deep.

Status: this PR makes the session return JSON-RPC -32601 (METHOD_NOT_FOUND) for unknown request methods instead of -32602 (INVALID_PARAMS).

Repro (stdlib + SDK only, anyio memory streams against ServerSession):

# send an unknown-method request to a ServerSession
await client_to_server_send.send(SessionMessage(types.JSONRPCMessage(
    types.JSONRPCRequest(jsonrpc="2.0", id=1, method="totally/bogus", params={}))))
resp = await server_to_client_receive.receive()
print(resp.message.root.error.code, resp.message.root.error.message)

Observed (same script, pip install -e ., one checkout per branch):

checkout error code message
v1.x (98b7159) -32602 Invalid request parameters
PR head (9936777) -32601 Method not found

Also checked for regressions: a known method with malformed params (initialize with protocolVersion: 12345) still returns -32602 on the PR head, so classification only affects unknown methods.

Happy to add docs or adjust the error message wording if that's the blocker -- could I get a review pass?

@Ranga-Prasath-22
test(session): cover unknown-notification no-response and method-set …
42d5702 
…extraction
@Ranga-Prasath-22
fix(session): remove redundant Any cast flagged by pyright
540b484 
t is typed Any, so t.model_fields["method"].default is already Any; the
cast(Any, ...) was redundant and tripped pyright's reportUnnecessaryCast, which
fails the pre-commit hook. Drop the cast and the now-unused import.
@Ranga-Prasath-22

Ranga-Prasath-22 commented Aug 1, 2026

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Follow-up: fix the pre-commit (pyright) failure

Root cause. checks / pre-commit failed because pyright reported reportUnnecessaryCast at src/mcp/shared/session.py:178: cast(Any, t.model_fields["method"].default) is redundant — t is typed Any, so .default is already Any, making the cast(Any, …) a no-op that pyright flags.

Fix. Drop the cast(Any, …) and the now-unused cast import.

Verification.

  • pre-commit run --all-files (local): end-of-file-fixer, prettier, markdownlint, ruff-format, and ruff all pass; pyright now reports 0 errors on the PR's files. (The remaining local pyright noise is Windows-only analysis of the posix/win32 mcp.os modules in untouched files, which does not occur on the ubuntu CI runner.)
  • ruff format/check on session.py: clean.
  • pytest tests/server/test_session.py tests/shared/test_session.py -q: 19 passed.

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