JSON-RPC 2.0 message format is correct

JSON-RPC 2.0 is the wire protocol underpinning every MCP interaction. A response missing "jsonrpc": "2.0" will be rejected by any compliant client. Responses that don't echo the request id leave the client unable to correlate responses to outstanding requests — causing hangs or misrouted results. Non-standard error formats ({'error': 'message'} instead of {'error': {'code': -32601, 'message': '...'}}) break client error handling. These are CWE-116 (improper encoding) failures that make the server non-interoperable.

Use the official MCP SDK transport, which handles JSON-RPC 2.0 formatting automatically. For custom implementations, every response must match this structure exactly.

// src/transport/handler.ts — JSON-RPC 2.0 response
const response = {
  jsonrpc: '2.0',      // required, must be the string '2.0'
  id: request.id,     // must match the incoming request id exactly
  result: { /* ... */ }
}

// Error responses
const error = {
  jsonrpc: '2.0',
  id: request.id,
  error: {
    code: -32601,                   // integer, from standard codes
    message: 'Method not found',    // human-readable string
  }
}

Never construct raw JSON and write it to stdout — use StdioServerTransport from the SDK.