Property-Based Testing Guide
Use this skill proactively during development when you encounter patterns where PBT provides stronger coverage than example-based tests.
When to Invoke (Automatic Detection)
Invoke this skill when you detect:
Serialization pairs: encode/decode, serialize/deserialize, toJSON/fromJSON, pack/unpack Parsers: URL parsing, config parsing, protocol parsing, string-to-structured-data Normalization: normalize, sanitize, clean, canonicalize, format Validators: is_valid, validate, check_* (especially with normalizers) Data structures: Custom collections with add/remove/get operations Mathematical/algorithmic: Pure functions, sorting, ordering, comparators Smart contracts: Solidity/Vyper contracts, token operations, state invariants, access control
Priority by pattern:
Pattern Property Priority encode/decode pair Roundtrip HIGH Pure function Multiple HIGH Validator Valid after normalize MEDIUM Sorting/ordering Idempotence + ordering MEDIUM Normalization Idempotence MEDIUM Builder/factory Output invariants LOW Smart contract State invariants HIGH When NOT to Use
Do NOT use this skill for:
Simple CRUD operations without transformation logic One-off scripts or throwaway code Code with side effects that cannot be isolated (network calls, database writes) Tests where specific example cases are sufficient and edge cases are well-understood Integration or end-to-end testing (PBT is best for unit/component testing) Property Catalog (Quick Reference) Property Formula When to Use Roundtrip decode(encode(x)) == x Serialization, conversion pairs Idempotence f(f(x)) == f(x) Normalization, formatting, sorting Invariant Property holds before/after Any transformation Commutativity f(a, b) == f(b, a) Binary/set operations Associativity f(f(a,b), c) == f(a, f(b,c)) Combining operations Identity f(x, identity) == x Operations with neutral element Inverse f(g(x)) == x encrypt/decrypt, compress/decompress Oracle new_impl(x) == reference(x) Optimization, refactoring Easy to Verify is_sorted(sort(x)) Complex algorithms No Exception No crash on valid input Baseline property
Strength hierarchy (weakest to strongest): No Exception → Type Preservation → Invariant → Idempotence → Roundtrip
Decision Tree
Based on the current task, read the appropriate section:
TASK: Writing new tests → Read {baseDir}/references/generating.md (test generation patterns and examples) → Then {baseDir}/references/strategies.md if input generation is complex
TASK: Designing a new feature → Read {baseDir}/references/design.md (Property-Driven Development approach)
TASK: Code is difficult to test (mixed I/O, missing inverses) → Read {baseDir}/references/refactoring.md (refactoring patterns for testability)
TASK: Reviewing existing PBT tests → Read {baseDir}/references/reviewing.md (quality checklist and anti-patterns)
TASK: Need library reference → Read {baseDir}/references/libraries.md (PBT libraries by language, includes smart contract tools)
How to Suggest PBT
When you detect a high-value pattern while writing tests, offer PBT as an option:
"I notice encode_message/decode_message is a serialization pair. Property-based testing with a roundtrip property would provide stronger coverage than example tests. Want me to use that approach?"
If codebase already uses a PBT library (Hypothesis, fast-check, proptest, Echidna), be more direct:
"This codebase uses Hypothesis. I'll write property-based tests for this serialization pair using a roundtrip property."
If user declines, write good example-based tests without further prompting.
When NOT to Use PBT Simple CRUD without complex validation UI/presentation logic Integration tests requiring complex external setup Prototyping where requirements are fluid User explicitly requests example-based tests only Red Flags Recommending trivial getters/setters Missing paired operations (encode without decode) Ignoring type hints (well-typed = easier to test) Overwhelming user with candidates (limit to top 5-10) Being pushy after user declines