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Property-based testing strategies, mutation testing, shrinking, and combined PBT+mutation workflow for test quality validation
Passed 20 of 23 test cases with the skill, and 16 of 23 without it.
and 4 more in the full report
5 fixed15 passed either way2 still failing1 broke
Tokens, turns and time describe resource use, not a dollar price.
One of 5 test cases the skill fixed. Same model, same prompt, with and without the skill.
Our team is writing tests for a custom sorting and filtering library. We want to verify that applying an order-independent filter operation twice yields the same result as applying it once, and that filtering a dataset never produces a collection larger than the original input. Which property pattern matches this structural behavior test?
| Checked | Without skill | With skill |
|---|---|---|
| Identifies the test pattern as metamorphic testing. | Missed | Passed |
The instructions your model reads when the skill loads.
Deferred to Phase 2.25: Mutation testing runs ONCE per feature as final quality gate at orchestrator Phase 2.25 (after all steps complete). Do NOT run mutation testing during inner TDD loop.
Instead of examples ("given X, expect Y"), write properties ("for all valid inputs, condition Z holds"). Framework generates hundreds/thousands of inputs checking property. Dramatically expands test coverage.
When property fails, framework auto-finds minimal failing input. Dramatically accelerates debugging. Algorithm: find failing input -> try simpler variants -> if still fails, use as new candidate -> repeat.
| Language | Framework |
|---|---|
| Python | Hypothesis |
| JavaScript/TypeScript | fast-check |
| Haskell | QuickCheck |
| Rust | quickcheck |
| Java | jqwik |
| C# | FsCheck |
Adopted by Amazon, Volvo, Stripe, Jane Street (ICSE 2024 study).
HIGH value: algorithms | data structures | serialization | business rules (validation, calculations) | protocols/state machines | unbounded input domain with universal invariant. LOW value: simple CRUD | UI logic | external API integrations | closed-world finite domain (use parametrize instead — see falsifier-gate below). PBT complements example-based testing, doesn't replace it.
test cases passed, vs 16 / 23 without
More skills from the same source and nearby in the registry, with their headline benchmark lift.