Server-Sent Events vs WebSockets
A detailed comparison of features, privacy, and developer experience.
Last reviewed: 2026-05-17
Executive Summary
Use SSE for server→client streams (notifications, AI token streaming, dashboards). Use WebSockets when the client also needs to push frequently (chat, collab, games).
Server-Sent Events
A simple one-way streaming protocol over HTTP. The server pushes events; the browser's EventSource API auto-reconnects. Built on regular HTTP, proxies, caches, and CDNs handle it.
WebSockets
A full-duplex protocol that upgrades an HTTP connection to a persistent TCP-like channel. Both sides can push messages anytime. The default for chat, multiplayer games, and live collab.
Editor's Verdict
These solve different shapes of the same problem. SSE is dead-simple, it's just an HTTP response that never ends, with `text/event-stream` framing. The browser's EventSource handles reconnects automatically. It works through any HTTP proxy and CDN without special config. The catch: it's one-way (server to client). For pure server-push use cases (LLM token streaming, dashboard updates, server-pushed notifications), SSE is the right default. WebSockets win when the client also pushes frequently: chat (typing indicators, sending messages), multiplayer games, collaborative editing, voice/video signaling. They're harder to scale (sticky sessions, connection limits) and need their own auth handling, but the bidirectional channel is genuinely necessary for those use cases.
What we ran
SSE Tester is one-way EventSource (`data:` lines). WebSocket Tester is a bidirectional socket. A curl -N stream belongs in SSE Tester; a chat echo belongs in WebSocket Tester. If the server only speaks WebSocket, EventSource will fail the handshake.
📡When to use Server-Sent Events
- Streaming LLM tokens from your server to the browser
- Live dashboards, stock tickers, status feeds
- Server-pushed notifications when the client rarely talks back
🔌When to use WebSockets
- Chat with typing indicators and real-time message send
- Collaborative editing (cursor positions, document changes)
- Multiplayer games or anything sub-50ms bidirectional
| Feature | Server-Sent Events | WebSockets |
|---|---|---|
| Direction | Server → client only | Bidirectional |
| Underlying protocol | HTTP | Upgraded HTTP (then TCP-like) |
| Auto-reconnect | Built into EventSource | DIY |
| Proxies / CDNs | Just works | Need WebSocket-aware infra |
| Auth | HTTP cookies / headers | First message or query param |
| Browser API | EventSource | WebSocket |
| Server scale shape | Easy (HTTP) | Sticky sessions |
| Binary frames |
Key Takeaways
- SSE: server-to-client only, runs over HTTP/1.1+, auto-reconnects, works through proxies.
- WebSockets: bidirectional, after HTTP upgrade, more complex, but supports binary frames natively.
- For AI token streaming, notifications, dashboards, SSE is almost always the right choice.
- For chat, multiplayer, collaborative editing, WebSockets are the right tool.
- SSE is criminally underused, adopt by default for one-way streams unless you need client → server messaging.
Common Mistakes
- Using WebSockets when SSE would do, adds complexity (heartbeats, reconnects, scaling) for no benefit.
- Sending huge SSE messages without flushing, the browser buffers until the connection idles.
- Forgetting that SSE has a per-domain connection limit (6 in HTTP/1.1), use HTTP/2 for multiplexed SSE.
- Choosing WebSockets without considering connection-cost, each WS is a stateful server resource.
Frequently Asked Questions
Why do LLM streaming APIs use SSE?+
Streaming is purely server-to-client (you send a prompt, then receive a stream of tokens). SSE is simpler, works through every proxy, and the auto-reconnect is useful when the connection drops mid-stream. WebSockets would be overkill.
Can I use SSE for chat?+
Half of it. SSE handles 'receive messages from server' fine. But you need a separate HTTP POST for 'send a message'. That works, but if you want sub-100ms latency on send (typing indicators), WebSockets are nicer.
What's the connection-limit gotcha with SSE?+
Over HTTP/1.1 a browser allows only ~6 concurrent connections per origin, and each open EventSource burns one for its entire lifetime. Open the same dashboard in a few tabs and the later tabs hang with no error. HTTP/2 multiplexes many streams over one connection and removes the limit, so serve SSE over HTTP/2 (or consolidate to a single stream and fan out client-side).
How do reconnects differ between the two?+
EventSource reconnects automatically and replays from the `Last-Event-ID` header, so a well-designed SSE endpoint resumes with no gap. A WebSocket gives you an `onclose` event and nothing else — reconnect, backoff, and any missed-message recovery are yours to build. For pure server-push telemetry that alone often decides it in SSE's favour.
How we tested this
We evaluated both Server-Sent Events and WebSockets in real developer workflows to build this comparison. Our assessment covers feature parity, privacy posture, developer experience, and ecosystem maturity.
Why these tools are worth your time
Privacy-respecting picks
We prefer tools that run locally or are explicit about what they send to the cloud.
Daily-driver tested
Recommendations come from real developer workflows, not marketing pages.
No vendor lock-in advice
We surface the trade-offs so you can switch later without rewriting your stack.