[Python] GOOGLE_APPLICATION_CREDENTIALS is null even after setting env_variables in app.yaml

I am deploying a Flask application to Google App Engine (Standard Environment, Python 3.9). I am using Firebase Admin SDK to access Firestore, and I have set GOOGLE_APPLICATION_CREDENTIALS in app.yaml to point to my service account JSON file (firebase_credentials.json).

However, after deploying the app, running:

gcloud app describe --format=json | jq '.envVariables'
returns:

null
i


ndicating that the environment variable has not been set correctly.

Additionally, my Flask debug endpoint:

import os
from flask import jsonify

@app.route("/debug")
def debug():
file_exists = os.path.exists("./firebase_credentials.json")
return jsonify({
"GOOGLE_APPLICATION_CREDENTIALS": os.getenv("GOOGLE_APPLICATION_CREDENTIALS"),
"firebase_credentials_exists": file_exists
}), 200


returns:

{
"GOOGLE_APPLICATION_CREDENTIALS": null,
"firebase_credentials_exists": true
}


This confirms that the service account file is present in the App Engine instance but the environment variable is missing.

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