The spec grows teeth

In Chapter I you learned to write down what right means. A test is that sentence made executable — a spec the machine re-checks in milliseconds, every time, forever.

This matters more with AI, not less. A model can argue with your prose, reinterpret your intent, "improve" your requirements. It cannot argue with assert_eq!. Tests are the spec the machine can't argue with — the one place where a plausible answer and a right answer stop being confusable.

Write them first, and every forge that follows is graded from birth.