tdd

Start by agreeing on the public seams to test, then write one failing test describing observable behavior, implement only enough code to pass it, and repeat one vertical slice at a time. Avoid writing all tests before any implementation.

Mock only at system boundaries such as external APIs, time, randomness, and sometimes databases or the file system. Never mock your own classes or internal collaborators, since that couples tests to implementation details.

A seam is the public boundary where you observe behavior without reaching inside the code. Tests live at seams, never against internals, so they survive refactors that change structure but not behavior.

Tests break on refactor when they are coupled to implementation details, such as mocking internal collaborators, testing private methods, or asserting on call counts. Rewrite them to verify behavior through public interfaces only.

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