Property Based Testing

Property-Based Testing

Generate inputs randomly, verify properties hold for all of them. Finds edge cases example tests miss.

Detection Triggers

Invoke when you see:

| Pattern | Property | Priority | |---------|----------|----------| | encode/decode, serialize/deserialize | Roundtrip | HIGH | | Pure functions (no I/O) | Multiple | HIGH | | State machines, stateful objects | Model-based invariants | HIGH | | Validators (is_valid, validate) | Valid after normalize | MEDIUM | | Sorting, ordering, comparators | Idempotence + ordering | MEDIUM | | Normalization (normalize, sanitize) | Idempotence | MEDIUM | | Parsers (URL, config, protocol) | Roundtrip or no-crash | MEDIUM | | Async code with async callbacks | Race conditions via scheduler | MEDIUM | | Algebraic types (monoids, sets) | Algebraic laws | MEDIUM | | Data migration, ETL | Invariant preservation | MEDIUM | | Builder/factory patterns | Output invariants | LOW |

Property Catalog

| 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, monoids | | Identity | f(x, identity) == x | Neutral element | | Inversion | g(f(x)) == x | encrypt/decrypt, compress/decompress | | Monotonicity | x references/generating.md, then references/strategies.md` for complex inputs

• Designing a feature -> references/design.md
• Code hard to test -> references/refactoring.md
• Reviewing PBT tests -> references/reviewing.md
• Interpreting failures -> references/interpreting-failures.md
• Library reference -> references/libraries.md

How to Suggest PBT

Check for existing PBT usage (see references/libraries.md for detection commands). If present, write property tests directly. Otherwise, offer:

> "This encode/decode pair is a good candidate for property-based testing with a roundtrip property. Want me to use that approach?"

If declined, write example-based tests without further prompting.

When NOT to Use

• Simple CRUD without transformation logic — UI/presentation logic
• Integration tests requiring external setup — prototyping with fluid requirements
• User explicitly requests example-based tests

Red Flags

• Tautological assertions (assert f(x) == f(x)) — reimplementing function logic in test
• Only testing "no crash" when stronger properties exist
• Heavy assume() instead of constrained strategies — being pushy after user declines

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Whamp
• Source: Whamp/skills
• License: MIT

Install and usage instructions live in the source repository linked above.