Calling Firestore Database from another Cloud Project shows “Permission Error”

After reading every piece of question, documentation available online, we couldn’t find a solution, so posting a question here. Our Setup : Project FB: Used only for Firestore in Dev mode. We want to access data from this project in our dev backend server, hosted on a different cloud project. Project GCP: A GCP Project with app deployed in GAE that makes a simple get() call to Project FB’s document. The app is in Python, and it works perfectly in localhost, but not after it’s deployed. Sample Flask code that we are testing with: Note: This is not a question