MCP-ERR-006
Missing required _meta field returns HTTP 400 + JSON-RPC -32602
“A request missing any required field is malformed; the server MUST reject it with JSON-RPC error code -32602 (Invalid params). On HTTP, the response status MUST be 400 Bad Request.” — spec anchor
How to fix this
Reject a request missing a required `_meta` field with HTTP 400 and `-32602`. A missing required field makes the request malformed, not merely incomplete.
How the validator checks this
Probe P4.1, against a streamable-http server on the modern protocol. What it reports:
- pass
- Missing required _meta rejected with 400 and -32602
- warn
- Returned -32602 for missing _meta but HTTP … rather than 400
- fail
- Accepted a request with no _meta — required fields are missing, so it must be rejected with 400 and -32602
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-ERR-007.
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-002Unknown method returns HTTP 404 and JSON-RPC -32601
- 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-007Missing client capability returns -32021 with data.requiredCapabilities
- MCP-ERR-008New implementation-defined codes avoid the legacy -32000…-32019 range