I pay for ChatGPT Plus (or Claude Pro). Why does BookyAI still ask for a key and credit?

ChatGPT Plus and Claude Pro are subscriptions for the chat website and app. Programs like BookyAI use the API, which OpenAI and Anthropic bill separately, by use, from a prepaid credit. For OpenAI: create a key at platform.openai.com/api-keys and add credit under Billing (the minimum is usually $5). For Claude: create a key at console.anthropic.com/settings/keys and buy credit under Billing. Then paste the key in BookyAI under Settings, "AI engines", press Save and Test. If Test says "insufficient quota" or "credit balance is too low", the account has no credit yet. You do not need a paid engine at all: the free Gemini key writes whole books.