Property-Based Testing Guide

Use this skill 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

• Parsers: URL parsing, config parsing, protocol parsing

• Normalization: normalize , sanitize , clean , canonicalize

• Validators: is_valid , validate , check_*

• Data structures: Custom collections with add /remove /get operations

• Mathematical/algorithmic: Pure functions, sorting, ordering, comparators

When NOT to Use

• Simple CRUD operations without transformation logic

• One-off scripts or throwaway code

• Code with side effects that cannot be isolated

• Tests where specific example cases are sufficient

• Integration or end-to-end 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

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

Examples by Language

Python (Hypothesis)

from hypothesis import given, strategies as st

@given(st.binary()) def test_roundtrip(data): assert decode(encode(data)) == data

@given(st.text()) def test_normalize_idempotent(s): assert normalize(normalize(s)) == normalize(s)

@given(st.lists(st.integers())) def test_sort_idempotent(xs): assert sorted(sorted(xs)) == sorted(xs)

TypeScript (fast-check)

import fc from 'fast-check';

test('roundtrip', () => { fc.assert(fc.property(fc.uint8Array(), (data) => { expect(decode(encode(data))).toEqual(data); })); });

test('normalize is idempotent', () => { fc.assert(fc.property(fc.string(), (s) => { expect(normalize(normalize(s))).toBe(normalize(s)); })); });

Rust (proptest)

use proptest::prelude::*;

proptest! { #[test] fn roundtrip(data: Vec<u8>) { prop_assert_eq!(decode(&encode(&data)), data); }

#[test]
fn normalize_idempotent(s: String) {
    let once = normalize(&s);
    let twice = normalize(&once);
    prop_assert_eq!(once, twice);
}

}

Go (rapid)

func TestRoundtrip(t *testing.T) { rapid.Check(t, func(t *rapid.T) { data := rapid.SliceOf(rapid.Byte()).Draw(t, "data") decoded := Decode(Encode(data)) if !bytes.Equal(decoded, data) { t.Fatalf("roundtrip failed") } }) }

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), 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.

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

PBT Libraries by Language

Language Library Install

Python Hypothesis pip install hypothesis

TypeScript/JS fast-check npm install fast-check

Rust proptest cargo add proptest --dev

Go rapid go get pgregory.net/rapid

Haskell QuickCheck cabal install QuickCheck

Scala ScalaCheck sbt dependency

Elixir StreamData mix dependency

Attribution

Based on trailofbits/skills property-based-testing skill.