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property-based-testing

Skill1 starsChanged 9 months agoMITView on GitHub

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.

We split this skill into the rules it gives an agent

4 rules. 0 are common, 4 are distinctive, and 0 are very specific to this project.

  • 0

    Common

    Most of the 15 edited versions we read keep these.

  • 4

    Distinctive

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Use this skill proactively during development when you encounter patterns where PBT provides stronger coverage than example-based tests.

    line 8
  2. Distinctive:

    Do NOT use this skill for:

    line 36
  3. Distinctive:

    No Exception → Type Preservation → Invariant → Idempotence → Roundtrip

    line 59
  4. Distinctive:

    When you detect a high-value pattern while writing tests, offer PBT as an option:

    line 85

“Common” is measured: kept by at least 6 in 10 of the 15 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

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:

PatternPropertyPriority
encode/decode pairRoundtripHIGH
Pure functionMultipleHIGH
ValidatorValid after normalizeMEDIUM
Sorting/orderingIdempotence + orderingMEDIUM
NormalizationIdempotenceMEDIUM
Builder/factoryOutput invariantsLOW
Smart contractState invariantsHIGH

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)

PropertyFormulaWhen to Use
Roundtripdecode(encode(x)) == xSerialization, conversion pairs
Idempotencef(f(x)) == f(x)Normalization, formatting, sorting
InvariantProperty holds before/afterAny transformation
Commutativityf(a, b) == f(b, a)Binary/set operations
Associativityf(f(a,b), c) == f(a, f(b,c))Combining operations
Identityf(x, identity) == xOperations with neutral element
Inversef(g(x)) == xencrypt/decrypt, compress/decompress
Oraclenew_impl(x) == reference(x)Optimization, refactoring
Easy to Verifyis_sorted(sort(x))Complex algorithms
No ExceptionNo crash on valid inputBaseline 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

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