One node at a time was hours on a fleet of hundreds. Nodes now go one, then two, four, up to --parallel at once, each wave waited on whole; a wave that does not come back stops it. The other control plane is upgraded first over ssh, so a leader restarted last hands over to a standby on the release. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>