METR evaluated an early version of Claude Mythos

"We evaluated an early version of Claude Mythos Preview for risk assessment during a limited window in March 2026. We estimated a 50%-time-horizon of at least 16hrs (95% CI 8.5hrs to 55hrs) on our task suite, at the upper end of what we can measure without new tasks. Of the 228 tasks in our suite, only 5 are estimated as 16+ hours long, making measurements at this range unstable and less meaningful than at ranges with better task coverage. Thus, we are not highlighting exact estimates for models above 16 hours measured with our current suite.