Unknown method returns HTTP 404 and JSON-RPC -32601

How to fix this

Answer an unknown method with HTTP 404 *and* JSON-RPC `-32601`. Both matter: every intermediary reads the status code, none parses the body.

How the validator checks this

Probe P4.3, against a streamable-http server on the modern protocol. What it reports:

pass

Unknown method returned 404 and -32601

warn

Returned -32601 but HTTP … rather than 404. The spec names both — every intermediary reads the status, none parses the body.

fail

Unknown method must return 404 with -32601 — got HTTP …, code

Quoted from the probe that runs this check, so it cannot drift from what the validator actually reports.

This rule is checked deterministically: a fail here is a certain violation, not an inference.

Checked in the same request as MCP-DEP-001, MCP-DEP-002, MCP-DEP-003.

Check your own server against this rule

The validator makes real protocol requests and reports this rule as pass, warn or fail alongside the other 78. Validate a server or read how the check works.

Other Error codes rules

• MCP-ERR-001Unsupported version returns -32022 with data.supported[]
• MCP-ERR-003Resource-not-found uses -32602, not the retired -32002
• MCP-ERR-004The retired -32042 code is never emitted
• MCP-ERR-005No undefined code is emitted in the reserved -32020…-32099 range
• MCP-ERR-006Missing required _meta field returns HTTP 400 + JSON-RPC -32602
• MCP-ERR-007Missing client capability returns -32021 with data.requiredCapabilities
• MCP-ERR-008New implementation-defined codes avoid the legacy -32000…-32019 range