Principle Test Behavior Not Implementation skill: what it does and how to install it

What it does. Explains how to evaluate tests against two changes: breaking the promised behaviour and changing internals without changing results. It directs tests through user-facing interfaces, uses concrete inputs and requirement-derived expectations, and asserts observable results, errors, state or external effects rather than private helpers or internal call order.

When to use it. When writing, changing or retaining tests, especially to remove implementation-coupled assertions. It also covers cases where no external call, an exact error type or a cross-table invariant is the observable contract.