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SSE (EventSource) Tester - Online Stream Listener

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SSE (EventSource) Tester

Connect to any Server-Sent Events endpoint, inspect real‑time stream, add custom headers, and debug effortlessly.

(fetch mode supported)
Disconnected
Messages: 0 Errors: 0 Last ID: –
Event Stream
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Frequently Asked Questions

SSE is a standard that allows servers to push real‑time updates to web clients over a single HTTP connection. Unlike WebSockets, SSE is unidirectional (server to client) and uses plain HTTP, making it simpler and broadly compatible with proxies and load balancers.

Paste your SSE endpoint URL into the input, optionally add custom headers (e.g., Authorization), and click Connect. Received events, data, and IDs will appear in real time. You can enable auto‑reconnect to survive temporary network drops.

Browsers enforce the same‑origin policy. If the SSE server doesn't include the Access-Control-Allow-Origin header, you'll see a CORS error. Ask the server administrator to enable CORS, or use a proxy. Our tool automatically switches to fetch mode when custom headers are provided, but CORS restrictions still apply.

Absolutely. Use the Custom Headers section to add key‑value pairs (e.g., Authorization: Bearer your-token). The tool will automatically use the Fetch API (with streaming) to support headers, since the native EventSource interface does not allow custom request headers.

In native EventSource mode, the browser handles reconnection automatically, respecting the optional retry: field sent by the server. In Fetch mode, we manually reconnect after the same retry interval (default 3 seconds) when the connection is lost, as long as the auto‑reconnect switch is active.

SSE is a one‑way communication channel (server to client) over HTTP, perfect for feeds and notifications. WebSockets provide full‑duplex communication and are better for interactive applications. SSE is simpler, works over standard ports, and offers built‑in reconnection.

Yes, as long as your browser allows connections to localhost and the server includes the necessary CORS headers (if the tool runs on a different origin). You can use URLs like http://localhost:8080/events.
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