Guide for setting up and using Firebase Authentication. Use this skill when the user's app requires user sign-in, user management, or secure data access using auth rules.

Prerequisites Firebase Project : Created via npx y firebase tools@latest projects:create (see firebase basics ). Firebase CLI : Installed and logged in (see firebase basics ). Core Concepts Firebase Authentication provides backend services, easy to use SDKs, and ready made UI libraries to authenticate users to your app. Users A user is an entity that can sign in to your app. Each user is identified by a unique ID ( uid ) which is guaranteed to be unique across all providers. User properties include: uid : Unique identifier. email : User's email address (if available). displayName : User's display name (if available). photoURL : URL to user's photo (if available). emailVerified : Boolean indicating if the email is verified. Identity Providers Firebase Auth supports multiple ways to sign in: Email/Password : Basic email and password authentication. Federated Identity Providers : Google, Facebook, Twitter, GitHub, Microsoft, Apple, etc. Phone Number : SMS based authentication. Anonymous : Temporary guest accounts that can be linked to permanent accounts later. Custom Auth : Integrate with your existing auth system. Google Sign In is recommended as a good and secure default provider. Tokens When a user signs in, they receive an ID Token (JWT). This token is used to identify the user when making requests to Firebase services (Realtime Database, Cloud Storage, Firestore) or your own backend. ID Token : Short lived (1 hour), verifies identity. Refresh Token : Long lived, used to get new ID tokens. Workflow 1. Provisioning Option 1. Enabling Authentication via CLI Only Google Sign In, anonymous auth, and email/password auth can be enabled via CLI. For other providers, use the Firebase Console. Configure Firebase Authentication in firebase.json by adding an 'auth' block: [!NOTE] If the Google Sign In popup opens and immediately closes with the error [firebase auth/unauthorized domain] , it means the domain is not authorized. For local development, ensure localhost is included in the Authorized Domains list in the Firebase Console or via the authorizedDomains field in firebase.json . CRITICAL : Do NOT include the protocol or port number in the Authorized Domains list (e.g., use localhost , NOT http://localhost:9090 ). CRITICAL : After configuring firebase.json , you MUST deploy the auth configuration to the Firebase backend for the changes to take effect. This is essential for auth providers like Google Sign In, email/password, etc. to auto generate the necessary OAuth clients for your app platforms. Run: Option 2. Enabling Authentication in Console Enable other providers in the Firebase Console. 1. Go to the https://console.firebase.google.com/project/ /authentication/providers 1. Select your project. 1. Enable the desired Sign in providers (e.g., Email/Password, Google). 2. Client Setup & Usage Web See [references/client sdk web.md](references/client sdk web.md). Flutter See [references/flutter setup.md](references/flutter setup.md). Android (Kotlin) See [references/client sdk android.md](references/client sdk android.md). 3. Security Rules Secure your data using request.auth in Firestore/Storage rules. See [references/security rules.md](references/security rules.md).

firebase-auth-basics