the risk-bucket framing is a useful upgrade from binary pass/fail. we've run into something similar where the failure category changes what you actually do about it - a compliance drift is a rollback, a business logic drift might just need a prompt tweak, a UX drift you might ship anyway and monitor. collapsing those into one score loses the signal that matters for the decision.

the 6-12 turn state drift pattern is real. in browser sessions the equivalent is the fingerprint signal accumulating - each action is fine, but by turn 8 you've built a behavioral profile that reads as bot. same compounding problem, different domain. makes me think the right eval unit for agents in general isn't the turn or even the session, it might be the trajectory shape.