SSE / EventSource Tester

How to use it

1. Capture a transcript: run the curl command below against your own endpoint, or copy a stream's response body out of your browser's network panel.
2. Paste it and press Parse — the timeline lists every event a spec client would dispatch, with its type, joined data, id and source lines.
3. Read the notices: unknown fields, NUL ids, non-digit retry values and the missing-final-blank-line trap are called out with line numbers.
4. Press Replay to watch the events land in order at 0.5x, 1x, 2x or instantly — or switch to the author mode to emit a stream body of your own.

text/event-stream is the format EventSource reads: UTF-8 lines where data:, event:, id: and retry: build a block, and a blank line dispatches it. Paste a captured transcript — curl -N output, an LLM streaming log, a response body copied from your network panel — and every event is parsed by the exact WHATWG grammar, listed on a timeline you can replay at a chosen pace, entirely on this device.

Examples

event: token
data: The quick

event: token
data: brown fox
data: jumps over

event: done
data: [DONE]

#1 · token · id — · data "The quick"
#2 · token · id — · data "brown fox\njumps over"
#3 · done · id — · data "[DONE]"

retry: 3000
id: 41
data: first chunk

data: second chunk

#1 · message · id 41 · data "first chunk"
#2 · message · id 41 · data "second chunk"
reconnection time 3,000 ms · last event ID "41"

: keep-alive

: keep-alive

data: {"ready":true}

#1 · message · id — · data "{\"ready\":true}"
1 event dispatched · 2 comment lines ignored

Every example on this page is executed against the same engine that powers the tool, on every build.

Common errors

Line 1: the stream ends with buffered fields but no blank line, so nothing was dispatched — the classic missing final blank line. Add one empty line to deliver the event.

The transcript stops after its data lines. A real client would buffer them forever and deliver nothing, which is exactly what a stalled-looking SSE integration usually is.

Line 1: `retry` must be ASCII digits only, so "3000x" is ignored and the reconnection time is unchanged.

retry accepts nothing but digits — no units, no spaces, no sign. Anything else is discarded whole rather than partially parsed.

Line 1: this `id` value contains U+0000 NULL, so the whole field is ignored and the last event ID is unchanged.

A NULL anywhere in an id makes a spec client ignore the entire field — the previous id keeps standing, which surprises servers that pack binary junk into ids.

This transcript is 2,097,153 characters, past the 2,097,152-character (2 MB) limit this page parses in one tab — nothing was read. Trim the capture or split it.

Past the cap the transcript is declined whole rather than read in part: half a timeline with wrong totals would be worse than an honest refusal. Both numbers are printed.

Keyboard shortcuts

⌘/Ctrl + Enter

Parse the pasted transcript.