Does BookyAI keep retrying a paid AI engine when it fails?

Not when trying again cannot help. From version 1.9.23, when a paid engine (Claude, OpenAI, OpenRouter, Grok) fails in a way a new try cannot change, such as a refused key, a declined request, or an answer that was all thinking and no text, BookyAI stops asking it for that chapter, because each try is billed. It tells you what happened instead. Problems that do pass, like a busy service or a short outage, are still given another chance. If you chose "Continue with another engine" in Settings, the free engines can still write the chapter.