Properties of Good Tests

From Kent Beck's Test Desiderata — tests should be:

1. Isolated — tests MUST NOT share mutable state or depend on execution order
2. Composable — tests MUST be runnable in any subset and any order
3. Deterministic — tests MUST produce the same result every time, no flakiness
4. Fast — milliseconds per unit test, seconds per integration test
5. Writable — easy to author, low ceremony
6. Readable — a failing test tells you what broke and why
7. Behavioral — tests SHOULD verify what the code does, not how it does it
8. Structure-insensitive — refactoring internals SHOULD NOT break tests
9. Automated — no manual steps, no human judgment needed
10. Specific — a failure points to exactly one cause
11. Predictive — passing tests mean the code works in production
12. Inspiring — confidence to refactor and ship

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