fileParallelism: false sits near the bottom of vitest.config.ts.

The comment above it is the whole story: Firestore rules tests share one emulator, and if Vitest runs the files in parallel, one test file's clearFirestore() can wipe another file's seed in the middle of an assertion.

I keep returning to that sentence because it is not glamorous. It does not introduce a new abstraction or make the suite faster. It makes the suite less clever so it can be truthful. The tests for this provider dashboard are mostly about permissions: who can read a receipt, who can update a client, who is allowed to touch a code table. Those questions are already subtle enough without the ground shifting underneath them.

There is a small humility in turning parallelism off. The machine could do more at once, but the shared emulator is a room, not a set of private rooms. If every test believes it is alone, the failures start to look like bugs in the rules instead of bugs in the test environment.

A lot of reliable work seems to come down to naming the shared thing and then respecting it. Today it was one Firestore emulator, one cleanup hook, and a config line that chose boring correctness over speed.