Reference

Event stream wire format

UTF-8 text/event-stream. Normative: WHATWG parse / interpret.

One event = fields + blank line

event: balance-updated
id: 42
data: {"cents":1200}
retry: 5000

The empty line ends the event and causes dispatch. No blank line → event is not dispatched (incomplete tail is discarded at EOF).

Fields

Field	Effect
`data:`	Append value + LF to data buffer. Multiple lines → one event; trailing LF stripped before dispatch. Client sees string in `event.data`.
`event:`	Event type name. Empty/default → `message` (`onmessage`). Named → `addEventListener(name)` only.
`id:`	Sets last event ID. On reconnect, browser may send `Last-Event-ID`. Empty `id` field clears it.
`retry:`	Reconnect delay in milliseconds (ASCII digits only). Hint to the UA.
`:` comment	Line starting with `:` is ignored. Common keep-alive every ~15s (proxy idle timeouts).

Parsing quirks

Optional single space after : is stripped from value.
Unknown field names are ignored.
Always UTF-8; no charset parameter choice.
Event with empty data buffer after process → no dispatch (buffer cleared).

Resume contract

// Browser reconnects with:
Last-Event-ID: 42

// Server responsibility (not automatic):
// - read header
// - emit only events after 42 (or replay policy you define)
// - keep using id: on subsequent events

Without server support, id only fills event.lastEventId — it does not invent a replay log.

Minimal server response headers

HTTP/1.1 200 OK
Content-Type: text/event-stream
Cache-Control: no-cache
Connection: keep-alive