A server built on this SDK answers every unknown JSON-RPC method with -32602 Invalid request parameters, where JSON-RPC 2.0 requires -32601 Method not found.

Why it matters

Catalogue scanners probe for optional capabilities and read the error code to decide whether a method is unsupported or genuinely malformed. Smithery, scanning our production server, logged:

Warning: Failed to list triggers: MCP error -32602: Invalid request parameters

-32602 reads as "you called it wrong", so the sensible client behaviour is to retry with different parameters — but no parameters will ever work, because the method does not exist. -32601 is the answer that lets a client move on.

Reproduction

Against any server on this SDK, after initialize + notifications/initialized:

{"jsonrpc":"2.0","id":9,"method":"totally/bogus"}

Response:

{"jsonrpc":"2.0","id":9,"error":{"code":-32602,"message":"Invalid request parameters","data":""}}

Same code for triggers/list, completions/complete and logging/setLevel on a server that declares none of those capabilities — the four are indistinguishable from a genuine schema violation.

Cause

In mcp/shared/session.py, self._receive_request_type.model_validate(...) raises for a method outside the request union, and the surrounding except Exception maps everything to INVALID_PARAMS:

except Exception as e:
    # For request validation errors, send a proper JSON-RPC error
    # response instead of crashing the server
    logging.warning(f"Failed to validate request: {e}")
    error_response = JSONRPCError(
        ...
        error=ErrorData(code=INVALID_PARAMS, message="Invalid request parameters", data=""),
    )

The comment is accurate for its original purpose — the union is just doing double duty as a method registry, so "method I do not have" and "parameters I cannot parse" arrive at the same handler.

Suggested fix

Distinguish the two before building the error: if message.message.root.method is not among the known request methods, answer METHOD_NOT_FOUND and keep INVALID_PARAMS for a known method that fails validation. Populating data with the validation error would also help — it is currently an empty string, so the response carries no diagnostic at all.

Happy to open a PR if the approach looks right.

Observed on mcp 1.27.1, Python 3.14.