firebase-auth-basics

The account side of an app: people prove who they are when they sign in, and your stored data then opens or stays shut depending on who is asking.

WHEN TO USE

SCOPE

USE FOR

Let users sign in with email and password, Google, a phone number, or as a guest.
Upgrade a temporary guest account into a permanent one without losing its data.
Look up a signed-in user's ID, email, or display name from your app's code.
Write Firestore or Storage rules that check who is currently signed in.
Fix the Google sign-in popup that opens and immediately closes with an unauthorized-domain error.

This is a saved copy of the skill's SKILL.md instructions. It may differ from the upstream file or the version you install.

In this document (3)

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:

{
  "auth": {
  "authorizedDomains": ["localhost"],
    "providers": {
      "anonymous": true,
      "emailPassword": true,
      "googleSignIn": {
        "oAuthBrandDisplayName": "Your Brand Name",
        "supportEmail": "support@example.com"
      }
    }
  }
}

[!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:

npx -y firebase-tools@latest deploy --only auth

Option 2. Enabling Authentication in Console

Enable other providers in the Firebase Console.

Go to the https://console.firebase.google.com/project/_/authentication/providers
Select your project.
Enable the desired Sign-in providers (e.g., Email/Password, Google).

2. Client Setup & Usage

Web See references/client_sdk_web.md.

Flutter See references/flutter_setup.md. Android (Kotlin) See references/client_sdk_android.md.

3. Security Rules

Secure your data using request.auth in Firestore/Storage rules.

See references/security_rules.md.

Account or credentials required

3 FOR YOU

Items without a tag are required.

01
Create and log into a Firebase project first
Create a Firebase project and log into the Firebase CLI before configuring auth — see the firebase-basics skill.
02
Turn on your chosen sign-in providers
Enable providers (anonymous, email/password, or Google Sign-In) via a firebase.json auth block plus firebase deploy --only auth, or manually in the Firebase console for other providers.
03
Optional Add localhost to authorized domains for local testing
For local development, make sure localhost (no protocol or port) is listed in the Firebase console's Authorized Domains, or Google Sign-In will fail.

Listed as free: Connected services may require an account or charge fees.

This skill ships inside the Firebase Agent Skills plugin. Listed package contents include 1 agent and an MCP server. Installed components and skill versions may vary by AI tool.

Install just this skill

Run this outside your AI tool, using Node.js 22.20.0 or newer. Skills CLI 1.7.2 skips skill selection; prompts cancel installation without an interactive terminal. If choosing Project when asked about installation scope, open your project's folder before running this.

Install checklist

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npx skills add firebase/agent-skills --skill "firebase-auth-basics" --full-depth

After installing for Claude Code, open it in the installation project if you chose Project; Global makes the skill available across projects on this computer. Once Claude Code loads the skill, its description helps Claude decide when to use it.

If installation created a top-level skills folder that did not exist when your Claude Code session started, run /reload-skills in that session. Run it again after later changes in that folder.

Install the whole plugin

This route installs the Firebase Agent Skills plugin package selected for your AI tool.

Add the plugin's marketplace, then install it

Install Claude Code first to get the claude command, then run the commands below in your terminal, outside a Claude Code session.

claude plugin marketplace add firebase/agent-skills
claude plugin install firebase@firebase

Add Firebase's marketplace, then install the plugin

Run both lines in a terminal. They add Firebase's marketplace, which is this repository, and install the firebase plugin: Firebase's skills plus the Firebase MCP server.

codex plugin marketplace add firebase/agent-skills

02
```
codex plugin add firebase@firebase
```

Installation and loading

Start a new Codex session before using it.

Good to know

Plugins work in Codex CLI and the ChatGPT desktop app, not in the Codex IDE extension.
For the desktop app, Firebase has you run the first line, fully restart the app, then install Firebase from Plugins.

Run it in a terminal, not inside a Gemini CLI session. It installs this repository as a Gemini CLI extension with Firebase's skills and the Firebase MCP server.

Install checklist

Installing from GitHub needs git on your computer.
Node.js, because the Firebase MCP server starts through npx.

gemini extensions install https://github.com/firebase/skills

Installation and loading

Restart Gemini CLI if it is already open.

Good to know

The address uses the repository's earlier name, firebase/skills, which GitHub forwards to firebase/agent-skills.

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What's inside

Files included with this skill.

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