add-property-test

name	add-property-test
description	Model-only playbook for adding a property-based test that pins a function's invariant.
argument-hint
user-invocable	false
disable-model-invocation	false

add-property-test

A model-only playbook for adding a property-based test. There is no slash command; the model invokes this when example-based tests leave a function's invariants under-specified.

When to reach for it

• The function has an algebraic law (round-trip, idempotence, commutativity, monotonicity) that examples only sample.
• The input space is large and edge cases keep slipping through hand- written cases.
• A parser/serialiser pair should satisfy decode(encode(x)) == x.

Canonical procedure

1. State the invariant in one sentence ("encoding then decoding returns the input").
2. Pick the narrowest strategy that generates valid inputs; constrain it so generated values are in-domain rather than filtering after the fact.
3. Assert the law inside the test body; let the framework shrink failures to a minimal counter-example.
4. Keep one or two example-based tests alongside for the named boundary cases (empty, single, max-length) the contract calls out.
5. When a property fails, treat the shrunk counter-example as a new regression fixture before fixing the code.

Guardrails

• Property tests complement, never replace, the boundary-case AND error-path tests every public function owes (test-discipline rule).
• Use pytest.approx for float laws and numpy.testing.assert_allclose for array laws.
• Name the test test_<func>_<invariant> so the law is legible from the test report.