Module: MCP::ErrorCodes

Defined in:

lib/mcp/error_codes.rb

Overview

MCP-specific JSON-RPC error codes, complementing the generic codes in JsonRpcHandler::ErrorCode.

Both constants below are introduced by the stateless lifecycle of the MCP 2026-07-28 draft (SEP-2575): UNSUPPORTED_PROTOCOL_VERSION rejects a request whose _meta-carried protocol version the server does not support (error.data: { supported: [...], requested: "..." }), and MISSING_REQUIRED_CLIENT_CAPABILITY rejects a request that requires a client capability the request did not declare (error.data: { requiredCapabilities: {...} }). The SDK exports the vocabulary; it does not raise these codes itself yet.

The values come from the spec's MCP-specific error code block, which is allocated sequentially from -32020 toward -32099. -32020 (HEADER_MISMATCH, SEP-2243) precedes the two codes defined here.

https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2575

Constant Summary

MISSING_REQUIRED_CLIENT_CAPABILITY =

-32021

UNSUPPORTED_PROTOCOL_VERSION =

-32022