Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.
\nGood tests 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.
\nBad tests 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.
\nSee tests.md for examples and mocking.md for mocking guidelines.
\nDO NOT write all tests first, then all implementation. This is \"horizontal slicing\" - treating RED as \"write all tests\" and GREEN as \"write all code.\"
\nThis produces crap tests:
\nCorrect approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.
\nWRONG (horizontal):\n RED: test1, test2, test3, test4, test5\n GREEN: impl1, impl2, impl3, impl4, impl5\n\nRIGHT (vertical):\n RED→GREEN: test1→impl1\n RED→GREEN: test2→impl2\n RED→GREEN: test3→impl3\n ...\nWhen exploring the codebase, use the project's domain glossary so that test names and interface vocabulary match the project's language, and respect ADRs in the area you're touching.
\nBefore writing any code:
\nAsk: \"What should the public interface look like? Which behaviors are most important to test?\"
\nYou can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.
\nWrite ONE test that confirms ONE thing about the system:
\nRED: Write test for first behavior → test fails\nGREEN: Write minimal code to pass → test passes\nThis is your tracer bullet - proves the path works end-to-end.
\nFor each remaining behavior:
\nRED: Write next test → fails\nGREEN: Minimal code to pass → passes\nRules:
\nAfter all tests pass, look for refactor candidates:
\nNever refactor while RED. Get to GREEN first.
\n[ ] Test describes behavior, not implementation\n[ ] Test uses public interface only\n[ ] Test would survive internal refactor\n[ ] Code is minimal for this test\n[ ] No speculative features added\n