What is the text/event-stream format?

Each SSE message is a block of colon-prefixed fields (event:, data:, id:, retry:) followed by a blank line. Multi-line data uses multiple data: lines. The browser's EventSource API parses this automatically.

Is the event field required?

No. Omitting the event field makes it a generic "message" event, which EventSource handles via the onmessage callback. Named events are dispatched only to addEventListener listeners for that specific name.

What does the retry field do?

retry sets the reconnection delay in milliseconds. If the connection drops, the browser waits this long before reconnecting. The default is browser-defined (usually 3000 ms).

Can SSE send binary data?

No. SSE is text-only. Base64-encode binary payloads and decode them in the client if needed.

How does SSE differ from WebSockets?

SSE is one-directional (server to client) over plain HTTP, so it works through proxies and needs no upgrade handshake. WebSockets are bidirectional but require a protocol upgrade. Use SSE for feeds, notifications, and log streaming; use WebSockets for interactive chat or gaming.

Server-Sent Events Builder

Configure SSE event fields and instantly preview the raw text/event-stream wire format plus a ready-to-use JavaScript EventSource client snippet.

Configure SSE Event

Event Name (optional — omit for generic "message")

Data (required — multi-line supported)

Event ID (optional — used for Last-Event-ID reconnect)

Retry Interval (ms) (optional — reconnect delay)

Generate N events in sequence

text/event-stream

Fill in fields and click Generate...

JavaScript (EventSource)

Generate first to see client code...

Server-side Examples

Generate first to see server code...

SSE Field Reference

Field	Required	Description
data:	Yes	The event payload. Repeat for multi-line.
event:	No	Named event type; defaults to "message".
id:	No	Sets Last-Event-ID for reconnect resumption.
retry:	No	Reconnect delay in milliseconds.

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Summary

Server-Sent Events (SSE) is a lightweight HTTP push technology that lets a server stream events to a browser over a single persistent connection. The wire format is plain text with each field on its own line, terminated by a blank line. This tool lets you configure all four SSE fields — event, data, id, and retry — and instantly previews the correct wire format along with a copy-paste JavaScript EventSource client that connects to your endpoint.

How it works

Fill in the Event Name, Data payload, optional ID, and optional Retry interval.
The raw text/event-stream output updates live as you type.
Copy the raw SSE block to paste into your server-side handler.
Switch to the Client Code tab to get a ready-to-use JavaScript EventSource snippet.
Copy either block with one click and paste directly into your project.

Use cases

Prototype SSE payloads before wiring up a real server.
Learn the text/event-stream wire format visually.
Generate boilerplate EventSource client code for Node.js, Python, or Go backends.
Debug SSE streams by comparing expected vs. actual wire output.
Teach colleagues how SSE differs from WebSockets.
Quickly produce multi-line data events with correct continuation formatting.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu