Question

Why does Google Gemini API return permission errors after OAuth update

Answer 1

Ok so the most common thing could be , your AUTH may be outdated . After Google OAuth updates , the gemini API got more stricter about which scopes you request If you were using a broad scope like https://www.googleapis.com/auth/cloud-platform before, you might now need to be more explicit:
SCOPES = [

    "https://www.googleapis.com/auth/generative-language",        # for Gemini API

    "https://www.googleapis.com/auth/generative-language.tuning"  # if you're fine-tuning

]

And if you’re using REST API directly then :
import google.auth

from google.oauth2 import service_account

credentials = service_account.Credentials.from_service_account_file(

    "service_account.json",

    scopes=["https://www.googleapis.com/auth/generative-language"]

)