How Antithesis Found and Explained a Bombadil Race Condition

A detailed postmortem of a race condition discovered in Bombadil, Antithesis' browser testing framework. The bug manifested as an invalid state machine transition — an ActionAccepted event arriving while the driver was in the Running state instead of Paused. Root cause was a race between the runner selecting a new action (triggered by an immediate JavaScript exception on a bfcache-restored page) and the CDP WebSocket event from Chromium confirming the bfcache restore hadn't arrived yet. Antithesis' deterministic fuzzing environment reliably reproduced the sequence. The fix involved threading a monotonically increasing Generation value through the runner's action selection to detect and discard stale actions.

Source: https://wickstrom.tech/2026-08-07-bombadil-race-condition-antithesis.html. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.