57% of frontier agent runs take a planted shortcut, even when told not to.

BAITBENCH hides an optional shortcut in three synthetic tabular ML tasks that inflates the public test score and fails the hidden set (arXiv 2608.30724). Because using it breaks no stated rule, this measures voluntary reward hacking. Across seven frontier agents, 57.1% of runs hack, five of seven above 50%, and the mean stays above 50% under an explicit instruction not to cheat. The judge implementation and an annotated transcript dataset are released, which makes it a usable testbed rather than a one-off headline.