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.

Anthropic doesn't have one rate limit

For Anthropic, a model+tier has three independent ceilings, all enforced per minute:

RPM — requests/minute

ITPM — input tokens/minute

OTPM — output tokens/minute

They are not proportional to each other, and the one that 429s…