Module: MCP::ErrorCodes

Defined in:
lib/mcp/error_codes.rb

Overview

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

All three constants below are introduced by the stateless lifecycle of the MCP 2026-07-28 draft (SEP-2575):

  • HEADER_MISMATCH rejects an HTTP request whose headers do not match the corresponding body values, or whose required headers are missing or malformed (no error.data). It is reserved for the Streamable HTTP transport, since headers do not exist on stdio.
  • MISSING_REQUIRED_CLIENT_CAPABILITY rejects a request that requires a client capability the request did not declare (error.data: { requiredCapabilities: {...} }). Raised via Server::MissingRequiredClientCapabilityError.
  • UNSUPPORTED_PROTOCOL_VERSION rejects a request whose _meta-carried protocol version the server does not support (error.data: { supported: [...], requested: "..." }). Raised via Server::UnsupportedProtocolVersionError.

The values come from the spec's MCP-specific error code block, which is allocated sequentially from -32020 toward -32099.

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

Constant Summary collapse

HEADER_MISMATCH = 
-32020
MISSING_REQUIRED_CLIENT_CAPABILITY = 
-32021
UNSUPPORTED_PROTOCOL_VERSION = 
-32022