In my experience the biggest multiplier isn't any single variable it's the interaction between them. Fanout × retries × context growth compounds in ways that linear cost models completely miss.

The fix that worked for us: treat budget as a hard constraint, not a target. When you're approaching limit, degrade gracefully (shorter context, fewer tool calls, fallback to smaller model) rather than letting costs explode and cleaning up later.

Also worth tracking: the 90th percentile request often costs 10x the median. A handful of pathological queries can dominate your bill. Capping max tokens per request is crude but effective.

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.

Background note I wrote (framing + “budget as contract”): https://github.com/teilomillet/enzu/blob/main/docs/BUDGETS_A...