Writes isolated, order-independent tests that assert what the caller sees, not private helpers. Use for tests with shared fixtures, leaked state, run order or platform differences.
Installs into .claude/skills of the current project.
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---
name: test-discipline
description: Writes isolated, order-independent tests that assert what the caller sees, not private helpers. Use for tests with shared fixtures, leaked state, run order or platform differences.
---
<!-- Generated by `basicly skills-build` from skill.yaml. Do not edit; edit the source. -->
# Test Discipline
Keep a test suite true as it grows. These rules catch a test that passes today and lies,
which no formatter, type checker or test runner can see.
## Rules
- **A test owns its inputs.** Build each fixture it needs and remove it after. A test that
reads state it did not make is broken, even when it passes.
- **A test passes alone, in any order and in parallel.** Share no mutable state, and do not
depend on a sibling test that runs first.
- **Assert on behavior, not internals.** Assert on return values, output and visible side
effects. A test that breaks on a pure refactor tests the implementation.
- **Remove nondeterminism at the source.** Pin the clock, seed randomness, and stub the
network and the filesystem. Fix or quarantine a flaky test. Never hide it with a retry.
- **Wait for a condition, not a duration. Measure time with a monotonic clock.** Poll the
predicate with a generous timeout, so that the assertion says that the event happened.
- **Set a wall-clock limit to twice the time on the slowest runner in the CI matrix.** A
limit set from a local run fails on a slower runner.
- **Watch a regression test fail before the fix.** Then you know that it exercises the
defect.
## Read when needed
For fixture inputs, regression mutations, absence controls, test seams or platform differences, read [test controls](references/test-controls.md) before editing the test.
