Regeneration fails with "add a Gemini API key" although I created the book with OpenRouter (or Claude, Grok). What should I do?

Update to version 1.9.2 or later. Earlier versions did not remember which AI engine a book was created with, so regeneration and the Quality tools always fell back to the free engines (Gemini, then Ollama) and never used a paid engine unless you chose it explicitly. Now every book keeps its engine: regeneration, appendices, references, Quality checks, the editor's AI tools, Codex and translations all use it. You can check or change it in Book details under "AI engine for this book". For a book created before the update, set the engine there once. Paid engines are never used as a silent fallback, so if the chosen engine fails, the error names that engine and what to check.