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

The React UseEventSource hook from the @reactuses/core library provides a declarative interface for Server-Sent Events with automatic reconnection, addressing the limitations of the native EventSource API. The hook exposes data, status, and error as state, simplifying live-updating components for use cases like deployment logs and AI streaming. It also includes useFetchEventSource for requests with custom headers or POST bodies, which is essential for AI completions endpoints.

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 https://reactuse.com/browser/useeventsource/ from @reactuses/core 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 Authorization header or a POST body, which in 2026 means every AI-completions endpoint. npm install @reactuses/core js 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 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: 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. 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. event: trade line requires its own addEventListener "trade", … 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 event: string | null status "CONNECTING" | "CONNECTED" | "DISCONNECTED" . Render it; that's your live-indicator. error: Event | null lastEventId: string | null id: field of the last message, i.e. your resume cursor. open / close close is open reconnects and resets the retry counter. eventSourceRef 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 https://reactuse.com/browser/usefetcheventsource/ solves this by speaking SSE over fetch built on Microsoft's battle-tested fetch-event-source js 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 EventSource fetch + stream parser Last-Event-ID resumeSimple rule: start with useEventSource ; the moment you type the word Authorization , switch. 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. useDocumentVisibility close when hidden and open on return — the Last-Event-ID handshake makes resume cheap. useBroadcastChannel useNetwork useOnline fetch . SSE earns its keep only when the stream outlives the request. fetch instead of base64-ing it through a text stream. Last-Event-ID — right for every server-to-client feed. useEventSource 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 . EventSource can't send an Authorization header or a POST body. useFetchEventSource close means 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 https://reactuse.com — one install, tree-shakeable, no dependencies to babysit. npm install @reactuses/core