bug-fix

# Skill: bug-fix Fix a defect in the smallest, most root-causing way. The discipline is universal: reproduce before fixing, write the failing test first, falsify rival hypotheses before asserting a cause, identify root vs symptom, close the coverage gap that let it through, minimum diff, commit body documents why. ## When to invoke Even a one-line fix benefits from walking this discipline; it forces the question "is this fixing the cause or hiding it?" For multi-file changes that go beyond fixing one defect — refactors, new features triggered by discovering the bug — stop and use `new-spec` instead. This skill is for bug fixes, not opportunistic restructuring. ## Procedure 1. **Reproduce first.** Don't write a fix until you have one of: a failing test, documented manual reproduction steps that fail reliably, or a captured error / stack trace / log signature. No reproduction = no fix; you might be fixing the wrong thing. 2. **Write the failing test (red).** It should pin the *observable contract being violated*, not the current implementation. Push back on these failure modes: - **Mock-shape assertion.** `expect(mock).toHaveBeenCalledWith(...)` when the observable contract is a returned value or state change. Test the contract, not the implementation. (See `quality-engineer`'s "mock-shape assertions" check.) - **Test passes for the wrong reason.** Run the test against the unfixed code; confirm it fails *because of* the bug, not ...