mcp-streamable-http-requires-sse-handshake

MCP streamable-HTTP servers require a two-step protocol: first POST to /mcp with Accept: application/json, text/event-stream and a JSON-RPC initialize payload to receive an mcp-session-id in the response headers. Subsequent tool calls must include that session ID header. Response body is SSE format with event: message and data: JSON payload lines.