Missing client capability returns -32021 with data.requiredCapabilities

How to fix this

When a request needs a capability the client did not declare, return `-32021` with `data.requiredCapabilities` naming exactly what was missing.

How the validator checks this

Probe P4.1, against a streamable-http server on the modern protocol. What it reports:

skip

Cannot construct a request that requires a specific client capability without invoking a real tool — out of scope for a read-only probe

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

This rule is checked heuristically: a fail is inferred rather than certain, so it bumps the overall grade up one level instead of being treated as certain non-compliance.

Checked in the same request as MCP-ERR-006.

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-006Missing required _meta field returns HTTP 400 + JSON-RPC -32602
• MCP-ERR-008New implementation-defined codes avoid the legacy -32000…-32019 range