Vitest mocking patterns and test double strategies. Use when writing tests that need mocks, spies, stubs, or fakes. Triggers include: mocking modules, stubbing globals (fetch, Date, timers), testing composables, mocking imports (default/named), creating test doubles, replacing dependencies, verifying mock calls, cleaning up mocks, or any question about "how do I mock X in Vitest". Also use when reviewing tests for proper mocking practices.