You size an LLM workload by looking at two numbers: the price per million tokens, and the requests-per-minute ceiling on the pricing page. You multiply, you eyeball the RPM limit, you decide you have headroom. Then you scale up and start eating 429 Too Many Requests — and the dimension that's throttling you is not the one you checked. This is not a cost problem. It's a "which constraint binds first" problem, and the binding constraint moves depending on your token mix and your tier. Eyeballing the pricing page cannot tell you which one it is. So I built a deterministic tool that computes it — usable as a web app, and as an MCP server you plug into Claude or your coding agent so it answers capacity questions with arithmetic instead of a guess.…