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tests/io.test.cjs's "review fix: pending-outcome cell lifetime" describe block deliberately drives runMain()/output() in-process (needed to observe a cross-invocation state leak) instead of via a subprocess. output() ends with a raw synchronous fs.writeSync(1, ...) to the real stdout fd, and Node's --test-isolation=process (default since Node 22) uses that same fd for the file's own parent-child reporter protocol. The two writes racing produced an intermittent "Unable to deserialize cloned data" that killed the whole file — observed twice on next's macOS lane, most recently on the commit that merged PR #4428 (unrelated to that PR's content; io.test.cjs isn't part of its diff). Empirically validated locally (gsd-test can't reach macOS): built a repro loop running N parallel copies of `node --test tests/io.test.cjs` to recreate CI-like contention. Baseline: ~13-17% of runs hit the corruption (12/90, 15/90 across two samples). A first fix attempt wrapped the writes in captureFdAsync (an await-aware twin of the existing captureFdSync, added because runMain() defers main() through a microtask chain, so a synchronous wrap restores before the real write fires) — but captureFdAsync always forwards to the real fs.writeSync by design (matching captureFdSync's "never swallow" contract, tests/helpers.cjs, #4306). Re-ran the same loop against that fix: 15/90, statistically unchanged. Forwarding the write doesn't stop it from reaching the fd node:test's own IPC also uses. Replaced it with suppressFdAsync: a narrow, deliberate exception to the never-swallow contract for a window the caller has verified is fully controlled (a single runMain() call plus its promise-chain settling, where nothing else can legitimately need that fd). It records the bytes for the test's own assertions but never lets them reach the real fd. Re-ran the loop: 0/300 across three samples (90+120+90), including one round at 8-way parallelism. Also caught and fixed a real bug surfaced by the same loop: the new regression assertion checked for compact-JSON `"error":"x"` but output() pretty-prints, so it failed 100% of runs deterministically until fixed to parse and check the structured value instead (io.test.cjs, matching this repo's "assert on structured output, not raw text" convention). Co-authored-by: sim <sim@local> Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
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