decide-test-meritlisted

# Skill: Decide Test Merit > "A test that cannot fail is a comment that costs a second." `craft-test` decides whether a behaviour deserves a test. This decides whether a test you already have deserves its line. ## The Decision **Keep it when:** - Breaking the code it guards turns it red - It is the only check that would notice - It names the mechanism where a neighbour names only the outcome - It pins something documented — a code, a path, a format **Cut it when:** - A neighbour fails first, for the same reason - It walks the same branch with different inputs - It asserts a thing exists, and the next check reads that thing - It passes without the mechanism it claims to prove ever running ## The Heuristic Ask: *"If I delete this line, what goes red?"* Nothing → it was never a test. ## The Quick Test | Ask | Answer | Then | |-----|--------|------| | Break the code on purpose — does it fail? | No | It is not guarding. Fix it or cut it | | Delete it — is every deliberate break still caught? | Yes | Cut it | | Does another check fail first, same cause? | Yes | Cut the later one | | Would anything else notice? | No | Keep it, whatever it costs | ## Aim The Break A break must change the lines you meant and no others. Prove it before you trust it: apply it, count the changed lines, and check the result still parses. **A break that fires in two places tests neither.** One that must fire in several says so on its own line — otherwise the next reader cannot tell design fr