How to use SSE / EventSource Tester
SSE Tester gives you a focused EventSource client for validating server-sent event streams such as token streaming, notification feeds, and progress channels. Connect to any HTTP or HTTPS endpoint, monitor message and named events, inspect lastEventId values, and summarize traffic by event type with live counters. You can enable withCredentials, subscribe to custom event names, and quickly test against the built-in /api/debug/sse-demo stream while keeping request behavior and event logs inside your browser session.
- Enter an http:// or https:// SSE endpoint, or click Try our demo stream to use /api/debug/sse-demo.
- Set withCredentials if the endpoint expects cookies or credentialed cross-origin requests.
- Add any custom event names (comma-separated) to attach dedicated listeners before connecting.
- Connect and monitor event name, payload data, lastEventId, and timestamps in the rolling log.
- Use counter chips and clear/pause controls to isolate stream patterns during troubleshooting.
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CORS requirements for cross-origin SSE
EventSource requests are normal HTTP GET calls, so cross-origin SSE depends on standard CORS response headers from the stream server. If Access-Control-Allow-Origin does not include your origin (or * for non-credentialed cases), browsers terminate the stream and trigger onerror quickly, often before any event arrives. Credentialed mode adds stricter rules: the server must return Access-Control-Allow-Credentials: true and cannot use a wildcard origin. When diagnosing immediate failures, inspect response headers first, then verify content type is text/event-stream and that proxy layers are not buffering chunks.
Retry behavior and lastEventId semantics
SSE is one-way but resilient: when a connection drops, browsers attempt reconnects automatically using retry timing from the stream or browser defaults. Servers can include id fields so clients send Last-Event-ID on reconnect and resume from the right position, reducing duplicate processing. Named events let one stream carry multiple logical channels while retaining ordering guarantees per connection. In practice, effective SSE diagnostics require checking event naming conventions, id continuity, and reconnect behavior together to ensure consumers can recover smoothly from transient network interruptions.
Worked examples
Demo tick stream smoke test
Inputs: Click Try demo stream and listen for tick events
Result: tick counter increases every ~2s with timestamp payloads
Named event listener check
Inputs: Custom names: token,progress on /events endpoint
Result: Counter chips split token and progress events separately
Glossary
- EventSource
- A browser API for consuming one-way text/event-stream responses from an HTTP endpoint.
- SSE event name
- The optional event field that labels an SSE message so clients can route it to named listeners.
- lastEventId
- An identifier attached to events that helps clients resume streams after reconnects.
- CORS header
- HTTP response metadata controlling whether a browser may read cross-origin stream responses.
- Retry interval
- The delay before a client reconnect attempt after an SSE stream disconnect.
Related reading
Frequently Asked Questions
Why use SSE / EventSource Tester?
- Connect to EventSource endpoints quickly and inspect each event with timestamp and lastEventId
- Subscribe to custom named events through comma-separated listeners without writing code
- Group incoming traffic by event name to spot dominant stream patterns or missing events
- Use one-click demo streaming to verify UI behavior before pointing at production endpoints
Common use cases
- Validate LLM token streaming endpoints that push incremental message chunks over SSE
- Debug notification streams where named events represent different product event classes
- Confirm CORS and credential settings for cross-origin streams from browser clients
- Reproduce flaky stream disconnects and document event ordering during QA sessions
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