principle-test-behavior-not-implementation

What problem does it solve? Test suites accumulate tests that cannot fail for a defect: assertions on mock calls, restated constants, or self-referential comparisons that still pass when every imported function returns undefined. These tests waste CI time and review attention while catching nothing. ## Core Features & Use Cases - Behavior Check: Applies a single litmus test—would this test still pass if every imported function returned undefined?—to decide whether a test observes real behavior. - Five Anti-Pattern Detection: Identifies weak assertions, mock-only verification, self-referential expectations, constant pins, and fixture-asserts-fixture shapes. - Concrete Fixes: Rewrites each anti-pattern into a call with one concrete input asserting a literal output, or deletes the test when no such assertion exists. - Use Case: While reviewing a Kotlin pull request, you find verify { repo.save(any()) } with no asserted state; the Skill directs you to assert the state after the call or remove the test. ## Quick Start Review the tests in this change and rewrite or delete any that would still pass if every imported function returned undefined.