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server-sent-events-tests

Test Server-Sent Events (SSE) flows, one-way server-to-client push only (not bidirectional, use websocket-tests for client-to-server messaging): `EventSource` API on the browser side (`onmessage`, `onerror`, `readyState` 0/1/2), event stream format (`data:`, `event:`, `id:`, `retry:`), `Last-Event-ID` reconnect-with-replay header, content-type `text/event-stream`, and HTTP/1.1 connection-pool limits. Use Playwright for browser-side, raw HTTP client for server-side stream tests.

server-sent-events-tests

This skill covers SSE surfaces tests must exercise per the WHATWG SSE spec: event stream format (data: / event: / id: / retry:), readyState lifecycle, reconnect-with-replay via Last-Event-ID, 204 No Content to disable reconnection, and the HTTP/1.1 connection-pool ceiling.

When to use

  • Real-time UI updates that don't need bidirectional communication (notifications, log tailers, build status, score tickers).
  • Mobile-friendly: better than long-polling for battery, simpler than WebSocket for proxies.
  • Pre-deploy gate: SSE retry interval, replay via Last-Event-ID, and HTTP/1.1 connection limits all behave as designed.

Step 1 - Server-side event stream format

Per the WHATWG SSE spec, response must use Content-Type: text/event-stream (UTF-8) and stream lines:

FieldMeaning
data:Appends to message payload (multiple data: lines join with newlines)
event:Custom event type (default = message)
id:Sets last event ID for reconnect replay
retry:Reconnect interval (ms)
:Comment line (kept-alive heartbeat)

Empty line ends a message. Example:

event: order_update
id: 142
data: {"orderId":"o123","status":"shipped"}

event: order_update
id: 143
data: {"orderId":"o124","status":"shipped"}

Step 2 - Browser test (Playwright)

import { test, expect } from '@playwright/test';

test('client receives server-pushed events', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');

  const events = await page.evaluate(() => {
    return new Promise<any[]>((resolve) => {
      const collected: any[] = [];
      const es = new EventSource('/api/orders/stream');
      es.addEventListener('order_update', (e: any) => {
        collected.push(JSON.parse(e.data));
        if (collected.length === 2) {
          es.close();
          resolve(collected);
        }
      });
    });
  });

  expect(events).toHaveLength(2);
  expect(events[0].orderId).toBe('o123');
});

Step 3 - readyState lifecycle

Per the WHATWG SSE spec, readyState values:

ValueState
0CONNECTING
1OPEN
2CLOSED
test('readyState transitions through CONNECTING → OPEN', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');

  const transitions = await page.evaluate(() => {
    return new Promise<number[]>((resolve) => {
      const states: number[] = [];
      const es = new EventSource('/api/stream');
      states.push(es.readyState); // 0
      es.onopen = () => {
        states.push(es.readyState); // 1
        es.close();
        states.push(es.readyState); // 2
        resolve(states);
      };
    });
  });

  expect(transitions).toEqual([0, 1, 2]);
});

Step 4 - Reconnect-with-replay via Last-Event-ID

Per the WHATWG SSE spec, on disconnect the client automatically reconnects with Last-Event-ID: <last-id-seen>. Server uses this to replay missed events.

Server pseudocode:

def stream(request):
    last_id = int(request.headers.get("Last-Event-ID", "0"))
    for evt in fetch_events_since(last_id):
        yield f"id: {evt.id}\nevent: {evt.type}\ndata: {evt.json()}\n\n"

Test (raw HTTP client, simulates reconnect):

import requests

def test_replay_via_last_event_id():
    # First connection — read 5 events, then close
    with requests.get("http://localhost:8080/stream", stream=True) as r:
        events = parse_until_count(r, 5)
        last_id = events[-1]["id"]

    # Reconnect with Last-Event-ID
    headers = {"Last-Event-ID": last_id}
    with requests.get("http://localhost:8080/stream", stream=True, headers=headers) as r:
        replay = parse_until_count(r, 1)
        assert int(replay[0]["id"]) > int(last_id)

Step 5 - Reconnect interval (retry:)

Server hints at reconnect interval:

retry: 10000

Browser will wait ≥ 10s before reconnecting. Test that browser honors:

test('client honors retry: 10000 on disconnect', async ({ page }) => {
  // Server emits retry: 10000, then closes
  const reconnectMs = await page.evaluate(() => {
    return new Promise<number>((resolve) => {
      const es = new EventSource('/api/stream-with-retry');
      let openTime = 0;
      es.onopen = () => {
        if (openTime === 0) {
          openTime = performance.now();
        } else {
          es.close();
          resolve(performance.now() - openTime);
        }
      };
    });
  });
  // Allow ±20% slack
  expect(reconnectMs).toBeGreaterThanOrEqual(8000);
  expect(reconnectMs).toBeLessThanOrEqual(12000);
});

Step 6 - Disable reconnect via 204 No Content

Per the WHATWG SSE spec, server responding 204 No Content disables further reconnection. Useful for "subscription ended" scenarios:

def stream(request):
    if user_unsubscribed(request):
        return Response(status=204)
    # ... event stream ...

Test the client gives up:

test('client stops reconnecting after server returns 204', async ({ page }) => {
  // Server returns 204 immediately
  const states = await page.evaluate(() => {
    return new Promise<number[]>((resolve) => {
      const es = new EventSource('/api/stream-204');
      const seen: number[] = [];
      const interval = setInterval(() => seen.push(es.readyState), 100);
      setTimeout(() => {
        clearInterval(interval);
        resolve(seen);
      }, 2000);
    });
  });
  expect(states[states.length - 1]).toBe(2); // CLOSED
});

Step 7 - HTTP/1.1 connection-pool ceiling

Browsers cap concurrent HTTP/1.1 connections per origin (~6 in Chrome). SSE consumes one persistently - apps with many EventSource connections starve.

test('app uses single EventSource for fan-out', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');

  const eventSourceCount = await page.evaluate(() =>
    performance.getEntriesByType('resource')
      .filter((r) => r.name.includes('/api/stream'))
      .length
  );
  expect(eventSourceCount).toBe(1);
});

HTTP/2 / HTTP/3 lift this limit but verify your CDN supports it end-to-end.

Anti-patterns

Anti-patternWhy it failsFix
Wrong content-typeBrowser doesn't recognize as SSEContent-Type: text/event-stream (Step 1)
Skip newline-newline message terminatorBrowser buffers indefinitelyAlways end messages with \n\n (Step 1)
No id: on eventsLast-Event-ID replay impossibleAlways emit id: (Step 4)
Multiple EventSource per page on HTTP/1.1Connection pool starvationOne stream + multiplex via event: (Step 7)
Use SSE for two-way commsOne-way only; need WebSocket for client→serverUse websocket-tests skill instead

Limitations

  • SSE is HTTP-only; some intermediaries (legacy proxies) buffer responses, breaking real-time push.
  • No native binary support; SSE is text-only (use WebSocket if binary needed).
  • Reconnect uses Last-Event-ID only - server must persist event IDs (or generate from timestamp) for replay to work.

References