Tests pass locally but fail in CI. Why?

Definition:

A local/CI mismatch means the test depends on something that differs between environments. Common differences include runtime versions, installed files, operating-system behavior, time, service availability or test order. Reproducing the CI command in a clean setup exposes these hidden assumptions.

Key points:

• Compare runtime versions, lockfiles, environment variables and working directories.

• Check timezone, case-sensitive filenames, test ordering and unavailable services.

• Reproduce the CI command in a clean environment before changing the test.

• Use the lockfile and avoid depending on locally generated files that are not committed or built by the pipeline.

Interview point:

• Do not simply retry or skip the failing test; determine whether it found a real bug, a missing dependency or unstable test design.