Declares protocolVersion and handles version mismatch

The protocolVersion field in the initialize response is how MCP clients determine whether this server is compatible with features they intend to use. An invented version string (e.g., "1.0") does not match any valid MCP release and breaks version-gated features in strict clients. A missing protocolVersion entirely leaves clients unable to negotiate compatibility. The MCP spec uses date-based versions (e.g., "2024-11-05") as the canonical format — non-date strings signal that the server was not built against the actual specification.

The SDK sets the correct protocolVersion automatically. For custom implementations, use the exact date-format string from the MCP specification.

// src/handlers/initialize.ts — correct version string
return {
  jsonrpc: '2.0',
  id: request.id,
  result: {
    protocolVersion: '2024-11-05',  // date-based, not semver
    capabilities: {
      tools: { listChanged: true },
    },
    serverInfo: { name: 'my-server', version: '1.0.0' },
  },
}

Check the MCP changelog for the latest valid protocol version string and update when you adopt new spec features.