How does StreamableHTTP use Server-Sent Events to enable server-to-client messages?

After initialization, the client makes a GET request that establishes a Server-Sent Events connection, creating a long-lived HTTP response that stays open. The server can then push messages through this persistent channel at any time, working around HTTP's client-only-initiates limitation. This SSE connection is the key mechanism for server-to-client communication. Through it, the server can send sampling requests, progress notifications, logging messages, and other server-initiated communications impossible with standard HTTP request-response.