PBT_MST_ENGINE=machine hands operation choice to libhegel's state machine: histories run close to the step bound (39.6 of 40 on average, against about 7 for the weighted table) and get swarm testing, at six to seven times the cost per case. The table stays the default. Corrects the comment on the 5000-case default: with the current generator 100 cases catch each planted dirty-flag bug in 9 or 10 runs of 10, with either driver. Co-Authored-By: Claude <noreply@anthropic.com>