Overview

text/event-stream is the media type for Server-Sent Events (SSE), a one-way real-time stream in which the server keeps one HTTP connection open and continuously pushes text events to the client. The browser's EventSource API consumes it.

Details

The event stream is a simple text protocol using line-based `data:`, `event:`, `id:`, and `retry:` fields, with a single blank line ending one event: e.g. `data: hello\n\n`. The encoding is always UTF-8. The server keeps the stream open (not closing the response) with `Cache-Control: no-cache` and settings to prevent proxy buffering. On disconnect the browser auto-reconnects, sending the last `id:` as a `Last-Event-ID` header, making resumption easy.

Unlike WebSocket, SSE is one-way (server to client) and runs over plain HTTP, making it firewall/proxy friendly and simple to implement. However, under HTTP/1.1 it can hit the per-domain concurrent-connection limit (about 6), which HTTP/2 multiplexing alleviates. It suits real-time notifications, progress updates, and LLM token streaming.

Syntax

e.g. Content-Type: text/event-stream

Notes

• If a reverse proxy (e.g. nginx) buffers the response, events do not arrive in real time — disable buffering and flush immediately.
• Under HTTP/1.1 the browser's concurrent-connection limit (~6) can block multiple SSE tabs, so HTTP/2 is recommended.
• SSE is one-way; use WebSocket for bidirectional real time. It carries text only, so wrap binary data (e.g. as base64).

Related types

Related headers