karma disconnected browser errors in CI: how to fix

TL;DR

Karma disconnects browsers that stop responding, usually from timeouts or resource exhaustion. Raise the capture and disconnect timeouts, reduce concurrency, and give the runner more resources.

Error

WARN [launcher]: Chrome have not captured in 60000 ms, killing.
WARN [Chrome]: Disconnected (1 times), because no message in 10000 ms.

Steps

1. Raise captureTimeout and browserNoActivityTimeout in karma.conf.js. Expected: slow CI stops tripping them.
2. Reduce concurrency to match runner CPUs. Expected: less contention.
3. Use ChromeHeadless with --no-sandbox --disable-gpu in containers. Expected: launches reliably.
4. Check memory: browser disconnects under OOM look like timeouts. Expected: resources adequate.
5. Re-run. Expected: browsers stay connected.

When to use

• Disconnected or have not captured in CI.
• Containerized Karma runs.

When not to use

• Test failures (the tests run; the browser is fine).
• You are migrating off Karma (consider Vitest instead).

Tool compatibility

• Karma 6.x; ChromeHeadless launcher.

Variant phrasings

Karma browser disconnected

The general error; timeouts and resources.

Chrome have not captured in Karma

The capture variant; raise captureTimeout.

Why it happens

Karma drives browsers over a socket with timeouts tuned for fast machines. Slow or loaded CI trips them.

Edge cases

• Single-run vs watch mode have different timeout needs.
• Proxy env vars can break the Karma socket; unset them for YOUR_HOST.
• Consider this a migration signal; Karma is in maintenance.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_eRYfeyNL71eagXbBTM0-2w