SSE Server-Sent Events
Standard for server-to-client push over HTTP. Simpler than WebSockets — unidirectional (server → client), text-based, automatic reconnect.
Part of HTML5 (WHATWG), EventSource API. Uses Content-Type: text/event-stream. Major users: ChatGPT/Claude streaming (token-by-token), GitHub Codespaces, OpenAI API. Limitation: 6 connections per domain in HTTP/1.1 (HTTP/2 solves it); text only. Competitor: WebSocket (bidirectional, binary), HTTP/2 server push (deprecated in Chrome 2022), WebTransport (more modern, UDP-based).