core
Direction
depends
WebSocket
Full-duplex: client and server can send at any time.
SSE
Half-duplex: server→client only; the client uses separate HTTP requests to send.
Realtime · Updated 2026-06-05
WebSocket vs SSE
If data only flows server→client — live feeds, notifications, dashboards, LLM token streams — Server-Sent Events are simpler, ride plain HTTP, and reconnect automatically. If the client also needs to push messages with low latency, or you need binary frames — chat, multiplayer, collaborative editing — use WebSockets. Many apps pair SSE for streaming with normal POSTs for the occasional client message.
WebSockets
Full-duplex, bidirectional messaging over one connection.
www.rfc-editor.org/rfc/rfc6455 ↗Server-Sent Events
One-way server→client streaming over plain HTTP.
html.spec.whatwg.org/multipage/server-sent-events.html ↗Both keep a connection open for server updates, but the contract differs. SSE is a long-lived HTTP response that streams text events and is built into the browser’s EventSource with auto-reconnect. WebSockets upgrade the connection to a separate full-duplex protocol where either side can send text or binary at any time. The question is whether you truly need the client→server channel.
Quick takes
If you're…
- Updates flow only from server to client SSE SSE is purpose-built for one-way streams and is far less machinery than a socket.
- The client must send messages with low latency too WebSocket WebSockets are full-duplex; SSE has no client→server channel.
- You are streaming LLM tokens or live logs to a browser SSE SSE over HTTP is the common, simple choice and reconnects automatically.
- You need to send binary frames WebSocket SSE is text-only (UTF-8); WebSockets carry binary natively.
- You want it to just work through proxies and with auto-reconnect SSE SSE is ordinary HTTP with built-in retry; WebSocket upgrades sometimes trip middleboxes.
- You are building chat, multiplayer, or collaborative editing WebSocket These need frequent, low-latency, two-way messages — exactly what sockets are for.
Decision wizard
A few questions, a verdict.
At a glance
The scorecard.
Dimension
WebSocket
SSE
Edge
In depth
Dimension by dimension.
core
Protocol
edge: SSE
WebSocket
Upgrades HTTP to a distinct ws:// protocol after a handshake.
SSE
Plain HTTP response with content-type text/event-stream — no upgrade.
features
Payload types
edge: WebSocket
WebSocket
Text and binary frames.
SSE
UTF-8 text only (binary must be encoded, e.g. base64).
features
Reconnection
edge: SSE
WebSocket
Manual: you implement reconnect/backoff and resume logic yourself.
SSE
Built in: EventSource auto-reconnects and resumes via Last-Event-ID.
ops
Infra & proxies
edge: SSE
WebSocket
Needs proxies/load balancers that understand WebSocket upgrades and long-lived sockets.
SSE
Works through any HTTP infrastructure; nothing special to configure.
ops
Connection limits
depends
WebSocket
One socket per client; scales to many concurrent connections with the right server.
SSE
Over HTTP/1.1, browsers cap ~6 connections per origin; HTTP/2+ multiplexing removes that.
When to pick neither
A different shape of problem.
- Long pollingYou need a fallback that works on the most restrictive networks
- WebTransportYou want QUIC datagrams/streams in the browser (HTTP/3-based, newer)
- Service-to-service streaming where both ends are yours
Situational picks
For specific cases.
A live dashboard, notification feed, or price ticker
SSE
One-way streaming with auto-reconnect over plain HTTP is exactly what SSE is for.
Streaming LLM responses to a web client
SSE
SSE is the common, low-friction choice; it reconnects and needs no protocol upgrade.
A chat app, game, or collaborative editor
WebSocket
Frequent low-latency two-way messages are the WebSocket sweet spot.
Mostly server push with rare client events
SSE
Use SSE for the stream and a normal POST for the occasional client message — simpler than a socket.
Sources
Primary material.
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