If agents, retries, and backfills can each trigger billing work, a “current…

If agents, retries, and backfills can each trigger billing work, a “current usage” row is a revenue bug waiting to happen. Mutable counters conflate intent and effect—retries, backfills, and human fixes all increment counters and turn operational noise into customer charges. Treat billable facts as immutable events and compute invoices by reconciling an append-only event log (event sourcing/audit logs); derive charges from settled events, not from a field someone keeps incrementing.