| name | property-based-testing |
| description | Provides guidance for property-based testing across multiple languages and smart contracts. Use when writing tests, reviewing code with serialization/validation/parsing patterns, designing features, or when property-based testing would provide stronger coverage than example-based tests. |
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]({baseDir}/references/generating.md) (test generation patterns and examples)
→ Then [{baseDir}/references/strategies.md]({baseDir}/references/strategies.md) if input generation is complex
TASK: Designing a new feature
→ Read [{baseDir}/references/design.md]({baseDir}/references/design.md) (Property-Driven Development approach)
TASK: Code is difficult to test (mixed I/O, missing inverses)
→ Read [{baseDir}/references/refactoring.md]({baseDir}/references/refactoring.md) (refactoring patterns for testability)
TASK: Reviewing existing PBT tests
→ Read [{baseDir}/references/reviewing.md]({baseDir}/references/reviewing.md) (quality checklist and anti-patterns)
TASK: Need library reference
→ Read [{baseDir}/references/libraries.md]({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
