Async image builds now prepare contexts in a bounded background pool, keeping sandbox and relay requests responsive. Submission deadlines include time waiting for preparation; canceled work cleans up its archive, retries reuse one snapshot, and workers reset safely after a POSIX fork. Sync archive bytes and the HTTP protocol are unchanged.
Validation: 187 tests passed locally in both the standard and Inspect-enabled suites; Python 3.10/3.13 CI checks minimal installation, lint, all extras, packaging and the installed wheel. A local four-context packaging comparison reduced wall time from about 1.31 s to 0.68 s and maximum event-loop heartbeat gap from about 1.30 s to under 9 ms; this is not an end-to-end image-build benchmark.