React useEventSource Hook: Server-Sent Events with Auto-Reconnect (2026)

Live notifications, deployment logs, stock tickers, AI responses streaming in token by token — none of these need a WebSocket. They're all one-directional: the server talks, the client listens. The browser has had a native protocol for exactly this since 2011 — Server-Sent Events (SSE) — and it runs over plain HTTP, passes through proxies and load balancers untouched, and reconnects automatically when the connection drops. What SSE doesn't have is a good React story. The native EventSource API is imperative: you construct it, attach listeners, and must tear it down at exactly the right moment — the classic effect-lifecycle minefield. useEventSource from @reactuses/core turns the whole thing into declarative state: data, status, and error your component just renders. This post covers the hook's full API, the reconnect behavior that native EventSource gets subtly wrong, and useFetchEventSource — the fetch-based variant you'll need the moment your stream requires an Authorization header or a POST body, which in 2026 means every AI-completions endpoint. npm install @reactuses/core import { useEventSource } from "@reactuses/core"; function DeploymentLog() { const { data, status } = useEventSource("/api/deploy/stream"); return ( <div> <span>{status === "CONNECTED" ? "🟢 live" : "🟡 connecting…"}</span> <pre>{data}</pre> </div> ); } That's a complete live-updating component. The hook opens the connection on mount, updates data on every message, exposes the connection lifecycle as status ("CONNECTING" | "CONNECTED" | "DISCONNECTED"), and closes the stream when the component unmounts. No refs, no listeners, no cleanup function to forget. Server-Sent Events is just an HTTP response that never finishes. The server replies with Content-Type: text/event-stream and writes messages as plain text, separated by blank lines: data: {"price": 101.42} id: 7 event: trade data: {"symbol": "ACME", "qty": 200} id: 8 Three field types matter: data: — the payload (always a string; JSON-encode structured data yourself). event: — an optional event name, so one stream can carry multiple channels. id: — an optional event ID. The browser remembers the last one and sends it back as a Last-Event-ID header when it reconnects, so a well-built server can resume where the client left off. Because it's plain HTTP, SSE works through corporate proxies, CDNs, and HTTP/2 multiplexing without the upgrade-handshake drama WebSockets sometimes hit. The trade-off: it's server → client only, and the native browser API can only send GET requests with no custom headers. Keep that limitation in mind — it's the reason the second hook in this post exists. Wiring EventSource by hand looks manageable: // ⚠️ hand-rolled — three bugs waiting to happen function Ticker() { const [price, setPrice] = useState<string | null>(null); useEffect(() => { const es = new EventSource("/api/prices"); es.onmessage = e => setPrice(e.data); return () => es.close(); }, []); return <span>{price}</span>; } The problems show up in production, not in the demo: Reconnect is infinite and invisible. When the server drops the connection, EventSource retries forever, silently. If your API is down, every open tab hammers it every few seconds until the heat death of the universe — and you have no state telling the UI "we're offline" so you can show a banner or give up. Errors are opaque. onerror gives you a bare Event — no status code, no reason. If you don't track connection state yourself, your UI happily shows stale data as if it were live. Named events need manual bookkeeping. Every event: trade line requires its own addEventListener("trade", …) and a matching removeEventListener in cleanup. Miss one and you leak listeners across React 18 StrictMode's mount-unmount-mount cycle. None of this is hard, exactly. It's just easy to get 90% right — which is the worst kind of wrong. Everything the manual version does badly, as returned state: const { data, event, status, error, lastEventId, open, close, eventSourceRef } = useEventSource(url, events?, options?); data: string | null — payload of the most recent message. event: string | null — the name of the last named event received (see below). status — "CONNECTING" | "CONNECTED" | "DISCONNECTED". Render it; that's your live-indicator. error: Event | null — the last connection error, cleared on successful reconnect. lastEventId: string | null — the id: field of the last message, i.e. your resume cursor. open() / close() — manual control. close() is explicit: it also disables auto-reconnect, so "user clicked pause" stays paused. open() reconnects and resets the retry counter. eventSourceRef — escape hatch to the raw EventSource instance if you need it. Pass the event names you care about as the second argument, and the hook registers — and cleans up — every listener for you: const { data, event } = useEventSource("/api/stream", ["trade", "quote"]); // event === "trade" | "quote" | null — which channel data came from useEffect(() => { if (event === "trade") appendTrade(JSON.parse(data!)); }, [data, event]); The autoReconnect option replaces EventSource's silent infinite retry with a policy you choose: const { status } = useEventSource("/api/notifications", [], { autoReconnect: { retries: 5, // give up after 5 attempts (or pass a () => boolean) delay: 2000, // wait 2s between attempts onFailed: () => toast.error("Live updates unavailable — refresh to retry"), }, }); retries defaults to -1 (retry forever, matching native behavior), but now it's a decision rather than a surprise, and onFailed gives you the moment to tell the user. Pair it with status === "DISCONNECTED" to render a degraded-mode UI instead of silently stale numbers. By default the hook connects on mount. Pass immediate: false to wait for user intent: const { status, open, close } = useEventSource("/api/live-scores", [], { immediate: false, }); <button onClick={status === "CONNECTED" ? close : open}> {status === "CONNECTED" ? "Pause live scores" : "Go live"} </button> Here is the native API's dirty secret: new EventSource(url) cannot send custom headers. No Authorization: Bearer …, no X-Api-Key, nothing. Your options with the native API are cookies (withCredentials: true) or a token in the query string — one of which doesn't work cross-domain with modern cookie policies, and the other of which lands your token in every access log between the browser and your server. It also can't POST. That matters because the biggest SSE consumers of 2026 — OpenAI-style AI completion endpoints — are all POST /v1/chat/completions with a JSON body and a bearer token, streaming back text/event-stream. The native EventSource API literally cannot call them. useFetchEventSource solves this by speaking SSE over fetch (built on Microsoft's battle-tested fetch-event-source parser), which means the full request is yours to shape: import { useFetchEventSource } from "@reactuses/core"; const { data, status, error } = useFetchEventSource("/api/v1/chat/completions", { method: "POST", headers: { Authorization: `Bearer ${token}` }, body: JSON.stringify({ model: "gpt-5", messages, stream: true }), autoReconnect: { retries: 3, delay: 1000 }, }); Same return shape as useEventSource — data, event, status, error, lastEventId, open, close — so switching between the two is a one-line change, not a rewrite. The onMessage callback is the natural place to accumulate a streamed completion: function Answer({ prompt }: { prompt: string }) { const [text, setText] = useState(""); const { status } = useFetchEventSource("/api/ask", { method: "POST", headers: { Authorization: `Bearer ${token}` }, body: JSON.stringify({ prompt }), onMessage: msg => { if (msg.data === "[DONE]") return; const delta = JSON.parse(msg.data).choices[0]?.delta?.content ?? ""; setText(prev => prev + delta); }, onError: err => { if (isRateLimit(err)) return 5000; // return a number = retry after N ms }, }); return <Markdown>{text}{status === "CONNECTED" && "▌"}</Markdown>; } Two details worth stealing: returning a number from onError overrides the reconnect delay for that attempt (perfect for Retry-After-style backoff), and the functional setText(prev => …) update means token order survives React's batching. useEventSource useFetchEventSource Transport native EventSource fetch + stream parser Custom headers / bearer auth ❌ ✅ POST with body ❌ ✅ Auto Last-Event-ID resume ✅ built-in your server's job Extra bundle weight zero small parser dependency Reach for it when… same-origin or cookie-auth streams AI APIs, token auth, request bodies Simple rule: start with useEventSource; the moment you type the word Authorization, switch. SSR is handled. Both hooks touch EventSource/fetch only inside effects, so they render harmlessly on the server — no typeof window guards in your code. First paint shows status: "DISCONNECTED", then the client connects. Pause hidden tabs. A dashboard left open in a background tab keeps its stream (and your server's connection budget) alive. Combine with useDocumentVisibility to close() when hidden and open() on return — the Last-Event-ID handshake makes resume cheap. One tab streams, the rest listen. Browsers cap concurrent connections per origin over HTTP/1.1 (~6), and every open tab with an SSE stream burns one. The classic fix: hold the stream in one tab and fan messages out with useBroadcastChannel. (Or serve over HTTP/2, where streams multiplex.) Don't reconnect into a dead network. useNetwork or the smaller useOnline tells you the browser is offline — gate your retry UI on it instead of burning the retry budget while the laptop is in a tunnel. The client needs to talk back on the same channel — chat where you send messages, multiplayer cursors, collaborative editing. That's bidirectional; use a WebSocket. Updates are rare. A value that changes a few times an hour doesn't justify a held-open connection — poll it, or refetch on focus with your data library. You're delivering one payload. If the response ends when the data arrives, that's just fetch. SSE earns its keep only when the stream outlives the request. Binary data. SSE is UTF-8 text. Ship binary over WebSocket or chunked fetch instead of base64-ing it through a text stream. SSE is the simplest real-time transport: one long-lived HTTP response, native browser support, automatic resume via Last-Event-ID — right for every server-to-client feed. useEventSource turns the imperative EventSource lifecycle into rendered state (data / status / error), handles named-event listener cleanup, and replaces invisible infinite retry with a reconnect policy you set — retries, delay, onFailed. Native EventSource can't send an Authorization header or a POST body. useFetchEventSource can — same API shape, fetch-based transport — and it's the piece you need for streaming AI completions. close() means stay closed (no auto-reconnect); open() resets the retry budget. Wire them to visibility and network state for streams that behave like a good citizen. useEventSource, useFetchEventSource, and 110+ other SSR-safe, TypeScript-first hooks live in @reactuses/core — one install, tree-shakeable, no dependencies to babysit. npm install @reactuses/core