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
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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
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</> SKILL.md
property-based-testing.SKILL.md
---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 |
| **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: Test failed, need to interpret
→ Read [{baseDir}/references/interpreting-failures.md]({baseDir}/references/interpreting-failures.md) (failure analysis and bug classification)
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.
