Share module initialization only for explicitly safe tests

Share module initialization only for explicitly safe tests

Linear added an opt-in Vitest project with isolate: false, so safe files share a module registry within each worker. Each eligible file carries an opt-in comment, shared state gets explicit teardown, and because agents now write most of Linear’s tests, its agent skills follow the same opt-in by default. Linear calls it its largest single performance improvement, worth roughly 17% in monthly savings.

Use it when

Import and transform overhead dominate eligible Vitest files.

Before applying it

This was Linear’s highest correctness risk. Files that use fake timers or shared state stay in the isolated project.