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.

Go / rapid (Primary for Go Code)

For Go code, always use pgregory.net/rapid as the property-based testing library. Do NOT use testing/quick -- it is limited, lacks proper shrinking, and has poor generator support.

Why rapid over testing/quick

• Automatic shrinking of failing cases to minimal counterexamples.
• Rich generator combinators: IntRange, SliceOf, Map, Custom, OneOf, Ptr, SampledFrom.
• Stateful testing via rapid.StateMachine for testing stateful systems.
• Integrates with standard *testing.T via rapid.Check.
• Actively maintained with good documentation.

rapid Core Pattern

func TestRoundtrip(t *testing.T) {
    rapid.Check(t, func(t *rapid.T) {
        msg := genMessage().Draw(t, "msg")
        encoded := Encode(msg)
        decoded, err := Decode(encoded)
        if err != nil {
            t.Fatal(err)
        }
        if !reflect.DeepEqual(msg, decoded) {
            t.Fatalf("roundtrip failed: got %v, want %v", decoded, msg)
        }
    })
}

rapid Generators

Custom Generators

func genMessage() *rapid.Generator[Message] {
    return rapid.Custom(func(t *rapid.T) Message {
        return Message{
            ID:       rapid.IntRange(1, 10000).Draw(t, "id"),
            Content:  rapid.StringN(0, 1000, -1).Draw(t, "content"),
            Priority: rapid.IntRange(1, 10).Draw(t, "priority"),
            Tags:     rapid.SliceOfN(rapid.StringN(1, 50, -1), 0, 20).Draw(t, "tags"),
        }
    })
}

Stateful Testing with rapid.StateMachine

For testing stateful systems (data structures, databases, protocol state machines):

type queueMachine struct {
    queue Queue           // System under test.
    model []int           // Reference model.
}

func (m *queueMachine) Init(t *rapid.T) {
    m.queue = NewQueue()
    m.model = nil
}

func (m *queueMachine) Push(t *rapid.T) {
    v := rapid.Int().Draw(t, "v")
    m.queue.Push(v)
    m.model = append(m.model, v)
}

func (m *queueMachine) Pop(t *rapid.T) {
    if len(m.model) == 0 {
        t.Skip("empty queue")
    }
    got := m.queue.Pop()
    want := m.model[0]
    m.model = m.model[1:]
    if got != want {
        t.Fatalf("got %d, want %d", got, want)
    }
}

func (m *queueMachine) Check(t *rapid.T) {
    if m.queue.Len() != len(m.model) {
        t.Fatalf("length mismatch: got %d, want %d", m.queue.Len(), len(m.model))
    }
}

func TestQueue(t *testing.T) {
    rapid.Check(t, rapid.Run[*queueMachine]())
}

Go Property Test Patterns

Roundtrip (encode/decode):

func TestCodecRoundtrip(t *testing.T) {
    rapid.Check(t, func(t *rapid.T) {
        original := genMessage().Draw(t, "msg")
        encoded, err := Encode(original)
        if err != nil {
            t.Fatal(err)
        }
        decoded, err := Decode(encoded)
        if err != nil {
            t.Fatal(err)
        }
        if !reflect.DeepEqual(original, decoded) {
            t.Fatalf("roundtrip failed: %v != %v", original, decoded)
        }
    })
}

Idempotence (normalization):

func TestNormalizeIdempotent(t *testing.T) {
    rapid.Check(t, func(t *rapid.T) {
        s := rapid.String().Draw(t, "s")
        once := Normalize(s)
        twice := Normalize(once)
        if once != twice {
            t.Fatalf("not idempotent: Normalize(%q) = %q, Normalize(%q) = %q",
                s, once, once, twice)
        }
    })
}

Sorting properties:

func TestSort(t *testing.T) {
    rapid.Check(t, func(t *rapid.T) {
        xs := rapid.SliceOf(rapid.Int()).Draw(t, "xs")
        result := MySort(xs)

        // Length preserved.
        if len(result) != len(xs) {
            t.Fatalf("length changed: %d -> %d", len(xs), len(result))
        }

        // Ordered.
        for i := 1; i < len(result); i++ {
            if result[i-1] > result[i] {
                t.Fatalf("not sorted at index %d: %d > %d", i, result[i-1], result[i])
            }
        }

        // Idempotent.
        result2 := MySort(result)
        if !reflect.DeepEqual(result, result2) {
            t.Fatal("sort not idempotent")
        }
    })
}

Oracle (reference implementation):

func TestOptimizedMatchesReference(t *testing.T) {
    rapid.Check(t, func(t *rapid.T) {
        input := rapid.SliceOf(rapid.Int()).Draw(t, "input")
        got := OptimizedImpl(input)
        want := ReferenceImpl(input)
        if !reflect.DeepEqual(got, want) {
            t.Fatalf("mismatch: optimized=%v, reference=%v", got, want)
        }
    })
}

When to Invoke (Automatic Detection)

Invoke this skill when you detect:

• Serialization pairs: encode/decode, serialize/deserialize, toJSON/fromJSON, pack/unpack, Marshal/Unmarshal
• 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
• State machines: Protocol state, connection lifecycle, workflow transitions (use rapid.StateMachine)

Priority by pattern:

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)

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/Decode is a serialization pair. Property-based testing with a roundtrip property using rapid.Check would provide stronger coverage than example tests. Want me to use that approach?"

If codebase already uses a PBT library (rapid, Hypothesis, fast-check, proptest, Echidna), be more direct:

"This codebase uses rapid. I'll write property-based tests for this serialization pair using a roundtrip property."

If Go codebase uses testing/quick, suggest migration:

"I see this codebase uses testing/quick. The rapid library (pgregory.net/rapid) provides better shrinking, richer generators, and stateful testing support. Want me to use rapid for the new tests?"

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" - rapid has excellent built-in generators and rapid.Custom makes custom ones trivial
• "The test failed, so it's a bug" - Failures require validation; see {baseDir}/references/interpreting-failures.md
• "No crash means it works" - "No exception" is the weakest property; always push for stronger guarantees
• "testing/quick is good enough" - It lacks proper shrinking, has limited generators, and is effectively unmaintained; use rapid instead