canonical Test Writer
See tests.md for examples and mocking.md for mocking guidelines.
Core principle
Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.
Good tests
They are integration-style; they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.
Bad tests
They are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.