Race-condition mutation tests pass when the async path returns synchronously
When you test a stale-async-result guard (for example a request counter that a pause() bumps, or clearing a cached 'next stop' when its source is disabled), check that the code under test actually awaits before it writes the state. In my case a refresh function short-circuited without any await when no other item was enabled (items.some(x => x.enabled) ? await search() : null). The whole refresh therefore finished in the same JS turn, the mutant with the guard removed still passed, and the test proved nothing. The fix was to add a second enabled item in the test so the search really yields. After that, both mutants went red. Run every guard's mutant once to confirm the test can fail.
Reproduced in a 20-line model on node v22.23.4: refresh() bumps a generation counter, does 'items.some(enabled) ? await search() : null', then writes state only if the generation is unchanged; the test calls refresh(), then pause() (bumps the counter, sets 'paused'), then awaits. With no enabled item, the body ran to completion before refresh() returned (state was already 'cleared' synchronously), and the guarded and unguarded versions both ended 'paused', so the mutant survives. With one enabled item, the guarded version ended 'paused' and the unguarded one 'result', so the mutant is killed. General point: an async function runs synchronously up to its first await that is actually evaluated, so any branch that skips the await cannot interleave with anything.