Unknown methods return -32601 Method not found

When a client sends a request for an unrecognized method, the JSON-RPC 2.0 spec requires a -32601 Method not found error response. A custom server without a fallback case silently drops the request — the client hangs indefinitely waiting for a response that never arrives. This is CWE-703: the unhandled case is a real input condition in production (a newer client calling a method the server hasn't implemented yet). Returning the wrong error code, like -32603 Internal error, misleads the client into treating a known gap as an unexpected server failure.

Add a default case to every message router that returns -32601. The MCP SDK handles this automatically — only custom implementations need explicit treatment.

// src/transport/router.ts
function handleRequest(request: JsonRpcRequest) {
  switch (request.method) {
    case 'initialize': return handleInitialize(request)
    case 'tools/list':  return handleToolsList(request)
    case 'tools/call':  return handleToolsCall(request)
    default:
      return {
        jsonrpc: '2.0',
        id: request.id,
        error: { code: -32601, message: `Method not found: ${request.method}` }
      }
  }
}

• ID: mcp-server.error-resilience.method-not-found

• Severity: high

• What to look for: Enumerate the server's handling of unknown method names. Count whether unrecognized methods return -32601 MethodNotFound error. Check how the server handles requests for methods it does not implement. When a client sends a request with an unknown method field, the server should respond with JSON-RPC error code -32601 (Method not found). For SDK-based servers, this is handled automatically. For custom implementations, check for a default/fallback case in the message router.

• Pass criteria: Unknown method requests receive a -32601 error response with a descriptive message. The server does not crash, hang, or silently ignore unknown methods. 100% of unknown method calls must return a -32601 error code.

• Fail criteria: Unknown methods cause a crash, are silently ignored (no response), or return an incorrect error code.

• Skip (N/A) when: All checks skip when no MCP server is detected.

• Cross-reference: For structured error codes, see structured-errors.

• Detail on fail: "Custom message router has no default case — unknown methods are silently ignored and no response is sent. The client will hang waiting for a response" or "Unknown methods return error code -32603 (Internal Error) instead of -32601 (Method not found)"

• Remediation: Always have a fallback for unknown methods:

  // The MCP SDK handles MethodNotFound automatically in src/index.ts
  // Verify: calling a non-existent method returns { error: { code: -32601, message: "Method not found" } }
  

  function handleRequest(request: JsonRpcRequest) {
    switch (request.method) {
      case 'initialize': return handleInitialize(request)
      case 'tools/list': return handleToolsList(request)
      case 'tools/call': return handleToolsCall(request)
      // ... other methods
      default:
        return {
          jsonrpc: '2.0',
          id: request.id,
          error: { code: -32601, message: `Method not found: ${request.method}` }
        }
    }
  }