If you’re trying to estimate before prod, logging these 4 things in a pilot gets you 80% there: - tokens/run (in+out) - tool calls/run (and fanout) - retry rate (timeouts/429s) - context length over turns (P50/P95)

Fanout × retries is the classic “bill exploder”, and P95 context growth is the stealth one. The point of “budget as contract” is deciding in advance what happens at limit (degraded mode / fallback / partial answer / hard fail), not discovering it from the invoice.