Every "LLM cost calculator" answers the question nobody gets paged for. Cost is the easy half. You multiply tokens by a price and you get a number that is wrong by a rounding error. Nobody's on-call rotation has ever blown up because the monthly bill was 8% higher than the spreadsheet said. What blows up the rotation is a 429 cascade : traffic ramps, one rate-limit dimension saturates, retries pile on, the queue backs up, and the thing that was "fine in the load test" is now a red dashboard. Datadog's 2026 State of AI Engineering puts rate-limit errors at roughly 60% of all errored LLM call spans — millions of them a month, industry-wide. It is the dominant failure mode of LLM apps in production, and almost no planning tool models it. So I built one that does, and this post is the reasoning behind it: llmcapplanner.vercel.app — a single client-side page, no signup, nothing leaves your browser. "Tokens per minute" is not one number Here's the trap.…