The env knob silently folded 'mailbox' and every typo into the default arm, FIFO_RELAXED was not reachable at all, and clients/session/README.md claimed the default is FIFO (it is MAILBOX with a FIFO fallback). An AMD-on-Windows client always lands on FIFO because that driver offers no MAILBOX — now documented at the picker and in the docs-site client table, next to the ABR probe/ceiling knobs a field report went looking for and couldn't find. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>