Agent loop cost is what you pay per task, not per call — and it runs multiples higher than your per-call quote because every tool-call re-bills the whole system prompt plus every tool description. A 40-line offline forecaster reads one JSONL trace and prices the full loop before you ship. On my bloated fixture it measured an 11.29x gap.
On my own bloated fixture, the forecaster measured an effective cost of $2.26 per task against a $0.20 per-invocation quote — an 11.29x gap — with a cumulative-cost curvature of k = 2.14, which is the literal meaning of "the bill grows quadratically with the number of tool-calls." That's my run on my fixture, not a claim about your agent. The whole point of the tool is that it tells you your number, and on a short healthy loop it says the gap is basically zero.
I want to be careful with one number up front. You may have seen the 30x figure going around — Muskan's June 2026 Dev.to writeup, "Why Claude Agent Loops Cost 30x a Single Inference" (dev.to/muskan_8abedcc7e12). That 30x is their aggregate over 10,000 invocations a day, not a single-task measurement, and it's not mine. My single-task gap on a deliberately bloated 14-step loop came out to 11.29x. I'm titling this post with the number I actually measured, rounded down. If your loop is worse, the tool will say 30x or 40x — but I won't put a number in a headline that my own run didn't produce.
