Every LLM cost calculator goes stale the moment a vendor reprices. PromptSpend re-checks its catalog every morning against vendor pages, flags disagreements instead of averaging over them, and shows the source URL and confirmation date on every number. Paste a real prompt or set your scale, compare up to four models side by side, and see the monthly cost — plus the two numbers every calculator gets wrong: compounding chat history and hidden reasoning tokens. Free, no account, open source.