## TL;DR

Control flags explicitly in tests: set them via API, query params, or config before the test, and test both states. Never let tests depend on the ambient flag state.

## Error

```text
(Not an error; a determinism practice. Symptom: tests pass or fail depending on flag state.)
```

## Steps

1. Inventory which flags each test depends on. Expected: the flag surface mapped.
2. Set flags explicitly in test setup: API call, URL param, or env var. Expected: deterministic state.
3. Test both on and off for flag-gated behavior. Expected: both paths covered.
4. Isolate flag state per test; reset after each. Expected: no leakage.
5. In CI, run the matrix for critical flags. Expected: combinations covered.

## When to use

- Flag-dependent test variance.
- Testing flag-gated features.

## When not to use

- Flag rollout or targeting strategy.
- Tests with no flag dependence.

## Tool compatibility

- Any framework; flag providers (LaunchDarkly, Unleash, homegrown).

## Variant phrasings

### Feature flag test isolation

The core practice; explicit control.

### Testing with flags on and off

The coverage goal; matrix the critical ones.

## Why it happens

Flags are global mutable state. Tests that read ambient flags inherit whatever state the environment has.

## Edge cases

- Flag evaluation caching needs cache-busting between tests.
- Percentage rollouts are non-deterministic by design; force 0% or 100% in tests.
- Stale flags (always on) should be removed, not tested forever.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_9ijl63ySH0JwBv_YIAi4uA