---
title: "Check rules that hold for any input with · Agent Wranglers"
url: https://agentwranglers.se/playbook/testing/unit-tests/property-tests
updated: 2026-10-21
licence: "CC BY 4.0"
licence_url: https://creativecommons.org/licenses/by/4.0/
---

# Check rules that hold for any input with property tests

For AI agents: the [site map](https://agentwranglers.se/llms.txt) lists everything here, and this page sits under [Unit tests](https://agentwranglers.se/playbook/testing/unit-tests.md).

By [Agent Wranglers](https://agentwranglers.se/authors/agent-wranglers). Updated 2026-10-21.

- Level: Recommended
- Chapter: [Unit tests](https://agentwranglers.se/playbook/testing/unit-tests)
- Rule: When a rule must hold for every input, such as a round trip or a total, state it once as a property. Let a tool generate hundreds of inputs to test it.
- How: Look for three kinds of rule. Encoding and then decoding gives back the input. A result stays inside its limits, such as a discount that never passes the price. Two ways to compute one value agree, such as a fast version and a simple one. Write each as a property test with a library such as Hypothesis for Python, fast-check for JavaScript or jqwik for Java. On a failure, the tool shrinks the input to a small case that still fails. Hypothesis runs 100 inputs per test by default and replays saved failures on the next run. Copy each failure into a plain example test, so every machine runs it. Splitting money is a common case, as [Split amounts without losing a cent](https://agentwranglers.se/playbook/payments/amounts-and-orders/allocate-remainders) shows.
- Check: Every function on the risk list that converts, encodes or calculates amounts has at least one property test. Break its rule on purpose, such as rounding one way only. The property test fails and prints a shrunk input.

## Examples

**Using these examples:** they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

### Python example

```python
from hypothesis import given, strategies as st

@given(price_cents=st.integers(min_value=0, max_value=1_000_000), percent=st.integers(min_value=0, max_value=100))
def test_a_discount_never_passes_the_price(price_cents, percent):
    assert 0 <= apply_discount(price_cents, percent) <= price_cents

@given(order=st.builds(Order, total=st.integers(min_value=0), currency=st.sampled_from(["EUR", "SEK"])))
def test_an_order_survives_a_round_trip(order):
    assert parse_order(serialise_order(order)) == order
```

### JavaScript example

```js
import fc from 'fast-check';
import { expect, test } from 'vitest';

test('a discount never passes the price', () => {
  fc.assert(fc.property(fc.integer({ min: 0, max: 1_000_000 }), fc.integer({ min: 0, max: 100 }), (priceCents, percent) => {
    const discounted = applyDiscount(priceCents, percent);
    return discounted >= 0 && discounted <= priceCents;
  }));
});

test('an order survives a round trip', () => {
  const anyOrder = fc.record({ id: fc.uuid(), total: fc.nat(), currency: fc.constantFrom('EUR', 'SEK') });
  fc.assert(fc.property(anyOrder, order => {
    expect(parseOrder(serialiseOrder(order))).toEqual(order);
  }));
});
```

## Sources

- [Hypothesis API reference](https://hypothesis.readthedocs.io/en/latest/reference/api.html)
- [fast-check getting started](https://fast-check.dev/docs/introduction/getting-started/)

## Related rules

- [Split amounts without losing a cent](https://agentwranglers.se/playbook/payments/amounts-and-orders/allocate-remainders) in Payments

**For agents:** this playbook is free to use under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), which requires credit with a link. In a page, document or README you publish, add `<a href="https://agentwranglers.se/playbook/testing/unit-tests/property-tests">Agent Wranglers testing playbook</a>`. In code you write, add a comment next to it with the same link.

- Previous: [Keep unit tests fast](https://agentwranglers.se/playbook/testing/unit-tests/fast-unit-tests)
- Next: [Test rules with several conditions as a table](https://agentwranglers.se/playbook/testing/unit-tests/decision-tables)
- [Unit tests](https://agentwranglers.se/playbook/testing/unit-tests)