SSE (Server-sent Events)

An HTTP mock replies with server-sent events: a text/event-stream body, sent event by event, each flushed to the client as it goes. Use it to test clients of streamed LLM answers, live feeds, job progress, notifications and MCP servers over Streamable HTTP.

Go

m.MustMock(mockhttp.Getf("/prices").Reply(mockhttp.OK().EventStream(mockhttp.NewEventStream(
	mockhttp.NewEvent().Name("price").ID("1").DataJSON(map[string]any{"value": 10}),
	mockhttp.NewEvent().Name("price").ID("2").DataJSON(map[string]any{"value": 11}).Delay(time.Second),
))))

Mock file

request: {method: GET, path: /prices}
response:
  status: 200
  event_stream:
    events:
      - {event: price, id: "1", data: {value: 10}}
      - {event: price, id: "2", data: {value: 11}, delay: 1s}

CLI

vlatte --addr=127.0.0.1:8080   # with the mock file under testdata/vlatte/_mocks
curl -N http://127.0.0.1:8080/prices

The client gets the first event at once, and the second a second later:

event: price
id: 1
data: {"value":10}

event: price
id: 2
data: {"value":11}

What the reply sends

• Headers. Content-Type: text/event-stream and Cache-Control: no-cache, unless the reply sets them: a Content-Type header can add a charset, or name another type to test a client that refuses it. The headers go out before the first event, so a client opens the stream even when nothing is due yet.
• Flushing. Each event is flushed as it goes, over HTTP/1.1 and HTTP/2, and gzipped too, with Gzip() or encoding: gzip.
• HEAD. A HEAD request gets the headers only.
• Status. A stream needs a status with a body: a 1xx, 204 or 304 fails the mock.
• Ending. A stream ends after its last event, unless it keeps open. The mock counts the request once the stream starts.

A mock with an event stream is a mock as any other: it matches requests, takes part in scenarios, runs callbacks and events, and can be added over the admin API and MCP. Only HTTP mocks reply with events: gRPC has streams of its own.

TIP

The admin API's GET /events and GET /logs are server-sent events too, but of vlatte itself: what the server serves and logs. See Admin API.

Pages

Page	What it covers
Events	The fields of an event, text and JSON data, and templated events.
Timing and lifetime	Delays, heartbeats, streams that keep open, and how a stream counts.
Reconnecting	retry, Last-Event-ID, resuming, and telling clients to stop.
Testing a client's parser	Raw events, cut connections, and other faults.
With other features	Proxying, recording, CORS, the admin API and per-user feeds.

Two guides put the parts together: Testing an LLM streaming client and Mocking a remote MCP server.