Error: listen EADDRINUSE

EADDRINUSE in CI environments almost always stems from parallel test execution. Jest, Mocha, and Vitest with --concurrent or --workers flags can start multiple test suites simultaneously, each trying to launch a server on the same port. The default port 3000 or 8080 hardcoded in test setup files becomes a conflict when two worker processes reach the server-start line simultaneously. The fix: use dynamic port assignment in tests. server.listen(0) assigns the next available port, which the test can retrieve with server.address().port. A test helper wrapper that manages port assignment prevents all inter-worker conflicts. GitHub Actions runners share the network namespace across jobs in the same workflow. If two parallel jobs both start servers on port 8080 — even when the jobs seem independent — EADDRINUSE appears. Using matrix strategies with different port offsets, or passing --port=0 to your dev server command, solves this. Self-hosted runners that reuse the same host machine across pipeline runs carry an additional risk: a crashed previous run may have left a process holding the port. A pre-test step running npx kill-port 3000 8080 (or the platform equivalent) on the runner prevents carry-over conflicts between pipeline runs.