Property-Based Testing Guide
Apply proactively when 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 |
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: 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.
Boundaries
Skip PBT for: simple CRUD, UI logic, integration tests needing complex setup, fluid prototypes, or when user requests example-based tests only. Limit suggestions to top 5–10 candidates; respect user decisions.
Common Misconceptions
- "Example tests are enough" → PBT finds edge cases in serialization/parsing/normalization that examples miss.
- "The function is simple" → Simple functions with complex input domains (strings, floats) benefit most.
- "No crash means it works" → "No exception" is the weakest property; push for roundtrip/idempotence.
1---2name: property-based-testing3description: 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.4---56# Property-Based Testing Guide78Apply proactively when PBT provides stronger coverage than example-based tests.910## When to Invoke (Automatic Detection)1112**Invoke this skill when you detect:**1314- **Serialization pairs**: `encode`/`decode`, `serialize`/`deserialize`, `toJSON`/`fromJSON`, `pack`/`unpack`15- **Parsers**: URL parsing, config parsing, protocol parsing, string-to-structured-data16- **Normalization**: `normalize`, `sanitize`, `clean`, `canonicalize`, `format`17- **Validators**: `is_valid`, `validate`, `check_*` (especially with normalizers)18- **Data structures**: Custom collections with `add`/`remove`/`get` operations19- **Mathematical/algorithmic**: Pure functions, sorting, ordering, comparators20- **Smart contracts**: Solidity/Vyper contracts, token operations, state invariants, access control2122**Priority by pattern:**2324| Pattern | Property | Priority |25|---------|----------|----------|26| encode/decode pair | Roundtrip | HIGH |27| Pure function | Multiple | HIGH |28| Validator | Valid after normalize | MEDIUM |29| Sorting/ordering | Idempotence + ordering | MEDIUM |30| Normalization | Idempotence | MEDIUM |31| Builder/factory | Output invariants | LOW |32| Smart contract | State invariants | HIGH |3334## Property Catalog (Quick Reference)3536| Property | Formula | When to Use |37|----------|---------|-------------|38| **Roundtrip** | `decode(encode(x)) == x` | Serialization, conversion pairs |39| **Idempotence** | `f(f(x)) == f(x)` | Normalization, formatting, sorting |40| **Invariant** | Property holds before/after | Any transformation |41| **Commutativity** | `f(a, b) == f(b, a)` | Binary/set operations |42| **Associativity** | `f(f(a,b), c) == f(a, f(b,c))` | Combining operations |43| **Identity** | `f(x, identity) == x` | Operations with neutral element |44| **Inverse** | `f(g(x)) == x` | encrypt/decrypt, compress/decompress |45| **Oracle** | `new_impl(x) == reference(x)` | Optimization, refactoring |46| **Easy to Verify** | `is_sorted(sort(x))` | Complex algorithms |47| **No Exception** | No crash on valid input | Baseline property |4849**Strength hierarchy** (weakest to strongest):50No Exception → Type Preservation → Invariant → Idempotence → Roundtrip5152## Decision Tree5354Based on the current task, read the appropriate section:5556```57TASK: Writing new tests58 → Read [{baseDir}/references/generating.md]({baseDir}/references/generating.md) (test generation patterns and examples)59 → Then [{baseDir}/references/strategies.md]({baseDir}/references/strategies.md) if input generation is complex6061TASK: Designing a new feature62 → Read [{baseDir}/references/design.md]({baseDir}/references/design.md) (Property-Driven Development approach)6364TASK: Code is difficult to test (mixed I/O, missing inverses)65 → Read [{baseDir}/references/refactoring.md]({baseDir}/references/refactoring.md) (refactoring patterns for testability)6667TASK: Reviewing existing PBT tests68 → Read [{baseDir}/references/reviewing.md]({baseDir}/references/reviewing.md) (quality checklist and anti-patterns)6970TASK: Test failed, need to interpret71 → Read [{baseDir}/references/interpreting-failures.md]({baseDir}/references/interpreting-failures.md) (failure analysis and bug classification)7273TASK: Need library reference74 → Read [{baseDir}/references/libraries.md]({baseDir}/references/libraries.md) (PBT libraries by language, includes smart contract tools)75```7677## How to Suggest PBT7879When you detect a high-value pattern while writing tests, **offer PBT as an option**:8081> "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?"8283**If codebase already uses a PBT library** (Hypothesis, fast-check, proptest, Echidna), be more direct:8485> "This codebase uses Hypothesis. I'll write property-based tests for this serialization pair using a roundtrip property."8687**If user declines**, write good example-based tests without further prompting.8889## Boundaries9091Skip PBT for: simple CRUD, UI logic, integration tests needing complex setup, fluid prototypes, or when user requests example-based tests only. Limit suggestions to top 5–10 candidates; respect user decisions.9293## Common Misconceptions9495- "Example tests are enough" → PBT finds edge cases in serialization/parsing/normalization that examples miss.96- "The function is simple" → Simple functions with complex input domains (strings, floats) benefit most.97- "No crash means it works" → "No exception" is the weakest property; push for roundtrip/idempotence.
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