auth/invalid-api-key
what this Firebase error means
How it shows up
FirebaseError: Firebase: Error (auth/invalid-api-key).
What it means
Firebase Auth rejected the API key in the app's Firebase config as invalid. The key has to be copied exactly from the Firebase console.
The usual cause in an AI-built app
AI-built apps usually read the Firebase config from environment variables. On the live host those variables were never set, or were set under a different name, so the app starts Firebase with an empty or wrong API key.
Check this first
Check that the apiKey your deployed app passes to initializeApp is the same as the apiKey in the web app config shown in your Firebase project settings.