Testing Craft Tests own only what no stronger rung can prove. Every invariant has one owner: the strongest rung of code-craft's proof ladder that can actually prove it. In order: types and linters, effects, proofs, database constraints, unit tests, integration tests, end-to-end tests. Make the bad case fail to compile or construct where you can, then test the remainder at the strongest test rung that proves it, with as few tests as keep that proof. Scope and verification - Identify the behavior being changed and the specific defect each new or modified test should catch. A test need not detec…