Write Property-Based Tests with fast-check and Claude

Unit tests check the cases you thought of — property-based tests check the cases you didn't. Ask Claude to write them and it'll find the invariants you'd never think to enumerate manually.

claude "Read src/utils/currency.ts and write property-based tests using fast-check. Identify the mathematical invariants — commutativity, idempotency, round-trip encoding, boundary behaviours — and test each one with generated inputs including edge cases like zero, negative values, and floating-point precision."

Claude identifies the right properties for your domain: a sorting function should be stable and produce the same length output; a parser should round-trip any valid input; a price formatter should never lose precision.

// Claude generates tests like this
it('formatting then parsing is a round trip', () => {
  fc.assert(
    fc.property(fc.integer({ min: 0, max: 10_000_00 }), (amount) => {
      expect(parseCurrency(formatCurrency(amount))).toBe(amount);
    })
  );
});

For APIs and data pipelines, property tests are especially valuable:

claude "Write fast-check property tests for the UserSchema Zod validator. Test that any object satisfying the schema passes validation, that objects with required fields missing always fail, and that no valid input can cause an unhandled exception."

Claude can also shrink failures for you — when fast-check finds a counterexample, ask Claude to explain why that specific minimal case breaks the property.

Property tests find bugs that 1000 hand-written unit tests won't — and Claude writes them faster than you can dream up the invariants.