trailofbits/property-based-testing

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: 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.

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

Rationalizations to Reject

Do not accept these shortcuts:

• "Example tests are good enough" - If serialization/parsing/normalization is involved, PBT finds edge cases examples miss
• "The function is simple" - Simple functions with complex input domains (strings, floats, nested structures) benefit most from PBT
• "We don't have time" - PBT tests are often shorter than comprehensive example suites
• "It's too hard to write generators" - Most PBT libraries have excellent built-in strategies; custom generators are rarely needed
• "The test failed, so it's a bug" - Failures require validation; see interpreting-failures.md
• "No crash means it works" - "No exception" is the weakest property; always push for stronger guarantees