New implementation-defined codes avoid the legacy -32000…-32019 range

How to fix this

Move implementation-defined codes out of `-32000`…`-32019`. New codes must not be allocated there, and new implementations should avoid the range entirely.

How the validator checks this

Probe P2.1, against a streamable-http server on the modern protocol. What it reports:

pass

No response used the legacy -32000…-32019 range

warn

Used … from the legacy -32000…-32019 range, which new implementations SHOULD NOT use at all

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-HDR-008, MCP-HDR-009, MCP-RES-003, MCP-RES-006, MCP-RES-007, MCP-ERR-003, MCP-ERR-004, MCP-ERR-005, MCP-CAC-001, MCP-CAC-002, MCP-CAC-003, MCP-CAC-004, MCP-CAC-005, MCP-SCH-001, MCP-SCH-002, MCP-SCH-003, MCP-SCH-004, MCP-SCH-005, MCP-SCH-006, MCP-SCH-007, MCP-SEC-004, MCP-SEC-005, MCP-SEC-006, MCP-DEP-005.

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-007Missing client capability returns -32021 with data.requiredCapabilities