How do I tell traffic growth apart from a broken agent in my LLM bill?

Look at dollars per turn, not total spend. More users running more turns and a broken agent both drive total cost up, and they only separate once you divide: growth adds turns at roughly the same price each and leaves cost per turn flat, while a defect, a prompt that grew, a retry loop, output that got wordier, moves the per-turn number on its own, whether traffic changes or not.

That ratio is what cost_drift already tracks, per call site, against that call site’s own recent history rather than a company-wide total. A busy call site and a rarely used one each get judged against what normal looks like for them, so a spike in one doesn’t need a spike everywhere else to register.

Total spend still matters for the finance conversation. It’s just the wrong number to stare at when you’re trying to find a bug.