A 202 means the sync window expired while the job was still running. Tests treated it as a terminal pass: `if (isAsyncFallback(res)) return;` checked the envelope and returned, asserting nothing about the outcome and leaving the job running into the next test, which is the leak cancelAcceptedJobAndWait exists to prevent.
Because the window only expires under load, coverage tracked runner load. On CI 44 tests took this path and verified nothing; the same tests on a dev machine asserted in full (one measured 6.4s locally against 31s on CI).
settleAsyncFallback waits for a terminal state and asserts the job finished, and that a failure carries a message rather than being a crash. A clean failure stays valid, since the exotic-format fixtures are meant to be rejected. All 82 call sites moved over.
per-fork-env no longer floors SYNC_WAIT_MS, so forcing it to 0 drives every request through its 202 path. 570 tests were validated that way and matched their normal-window results exactly.
The 29-34s band dropped from 44 tests (23.4% of test time) to 6 (3.0%). Total test time rose 5.8% and CI wall went 12.8 to 13.1 min: the forks were doing real work during that wait, so this buys determinism, not speed. Per-shard totals unchanged at 9903 tests, 9435 passed, 468 skipped.
Vitest shards by file and runs a file's tests serially in one fork, so a single spec set the floor for the whole Integration job no matter how many shards or forks it got. Cost-aware sharding (#650) balanced the shards but could not get under that floor.
Split the three specs that exceeded it: format-matrix-comprehensive (1365s) into 4 by describe, format-matrix (1130s) into 4 with Cross-format matrix striped over FORMAT_SAMPLES, and format-matrix-generated (779s) into 3 striped over TOOLS. Largest spec is now 370s. Each preamble moved verbatim into a sibling .shared.ts exposing setupMatrixApp().
Integration shards went from 20m59s/17m55s/16m33s/9m19s to 11m44s/12m31s/10m37s/11m1s.
Coverage checked, not assumed: the set of test names collected by vitest list is byte-identical across the split, 2151 before and 2151 after. Per-shard totals matched the baseline exactly at 9903 tests, 9435 passed, 468 skipped.
