Gates whether a new test should exist and forces it to be efficient, protecting CI from low-value test bloat. Use before adding or substantially changing any pytest, Jest, or Playwright test — whenever an agent or engineer is about to write tests for a new feature, bugfix, or PR. Front-loads the value bar (every test must catch a realistic regression no existing test already catches; test behavior through the public interface, not implementation details; collapse near-duplicates into parameterized cases) and the efficiency bar (deterministic, isolated, fast; pick the cheapest test level; Django TestCase over TransactionTestCase; no sleeps, no real network). Includes a "don't write it" decision tree. For fixing an existing flaky test use `/fixing-flaky-tests`; after this gate says a Playwright test is warranted, use `/playwright-test` for mechanics.