tdd

Write one failing test that describes observable behavior, then write only enough code to make it pass, and repeat one slice at a time. Refactoring belongs to a separate review stage, not the red-green implementation cycle.

A seam is the public boundary where you observe behavior without reaching inside the code. Tests live at seams, never against internals, and all seams under test should be agreed with the user before writing any test.

Mock only at system boundaries such as external APIs, databases, time, randomness, and the file system. Never mock your own classes or internal collaborators, and prefer dependency injection plus SDK-style interfaces to make boundaries easy to mock.

Tests break during refactoring when they are coupled to implementation details, such as mocking internal collaborators, testing private methods, or asserting on call counts. Tests should verify behavior through public interfaces so they survive internal changes.

A tautological test recomputes the expected value the same way the implementation does, so it passes by construction and can never catch a bug. Expected values must come from an independent source such as a known literal, worked example, or specification.