Billing anomaly alerts run on a 24–48 hour lag. The retry loop is already an invoice by the time anyone sees it. The control that catches it is per-session, in-process, and lives in the orchestration layer — profiled envelope, 3x P95 trip, defined degradation.
Two LangChain agents burned $47K in eleven days. The model worked. The budget math didn't. Multi-agent cost is a heavy-tailed distribution, monitoring is structurally too late, and only synchronous SDK-level enforcement stops the spiral.