Testing on the Toilet

Distilled from Google's Testing on the Toilet episodes (2007–2026). Episode index: references/episodes.md.

Run this loop on every new test and every test you review. Completion: each test that ships would fail on a real behavior bug and would survive a behavior-preserving refactor.

1. Change-detector gate

A change-detector restates production structure (especially ordered verify of every collaborator call) without checking an observable result. Correct and incorrect implementations pass equally; any rename/extract/reorder fails the suite.

Rewrite it to assert state, return value, rendered output, or persisted data. If you cannot name the user-visible contract, delete the test and say why. Do not "fix" a refactor by mechanically updating fifty mirrors.

Interaction verify is allowed only when the call itself is the contract (send mail once, do not double-charge, presenter tells the view to show X). If unsure, rewrite to a state/return assertion; if that is impossible, stop and ask which observable outcome the test protects.

2. Pick the cheapest layer that still has fidelity

Name the bug/risk first. Pick the topmost row in the table that can catch it. Escalate a row only when a cheaper layer would miss that risk (state the miss). SMURF (Speed, Maintainability, Utilization/cost, Reliability, Fidelity) is the tradeoff language when two rows could both catch it: prefer the cheaper/faster/more reliable one.