Why do my tests pass locally but fail in continuous integration?

The gap is almost always something present on your machine and absent from the runner. Continuous integration starts from a clean environment every time, which is the point, and that clean start exposes assumptions your local setup has been quietly satisfying.

The usual suspects fall into a few groups.

• Leftover state. A database, cache or file created by an earlier run persists locally and makes a test pass that would otherwise fail on empty. A suite that only passes on the second run is a strong signal.
• Test ordering. Locally, tests may run in a different order or with different parallel grouping. Tests sharing mutable state pass in one order and fail in another, and the runner happens to pick the unlucky one.
• Environment differences. Missing variables, a different time zone, a different locale affecting string comparison or date formatting, or a different operating system where filesystem case sensitivity suddenly matters. The case sensitivity one is especially common when developing on macOS and running Linux in the pipeline.
• Dependency drift. Installing with version ranges rather than from a lockfile lets the runner resolve a newer release than you have. Using a clean install that respects the lockfile exactly removes this class entirely.
• Timing. Runners are often slower and more contended than a laptop, so anything depending on a fixed sleep or an assumption about completion speed becomes flaky.

The most effective way to diagnose it is to reproduce the clean environment rather than stare at the diff. Running the suite in a fresh container, with no existing volumes and only the variables the pipeline provides, usually reproduces the failure on the first attempt.

A test failing only in the pipeline is still a real failure. The environment simply removed the crutch that was hiding it.