I was getting a 403 error with the message "Request had insufficient authentication scopes" when using the Google Generative AI (Gemini) API with OAuth. The issue was caused by using an outdated scope: https://www.googleapis.com/auth/generative-language.peruserquota. To fix it, I replaced it with the correct scope: https://www.googleapis.com/auth/generative-language.retriever. I also made sure to include the header x-goog-user-project with my Google Cloud Project ID in the API call. After updating the scope and adding the header, the API started working as expected. Make sure your OAuth consent screen is set up properly and the Generative Language API is enabled in your project.