# Firebase Auth Basics

> 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.

Source: https://skillsagentes.com/skills/firebase/agent-skills/firebase-auth-basics
Repository: https://github.com/firebase/agent-skills
Author: firebase
License: Apache-2.0
Updated: hace 8 días
Context cost: 43 tok installed, 1k tok once triggered, 6.5k tok with every bundled file
Bundle: 6 files, 25 KB
Permissions requested: none declared

## Install

```bash
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent claude-code
```

## What it does

- Configura Firebase Authentication en firebase.json (anonymous, emailPassword, googleSignIn) y despliega esa configuración con firebase-tools
- Explica conceptos base: usuarios (uid, email, displayName), proveedores de identidad y tokens (ID Token, Refresh Token)
- Guía la configuración del SDK cliente en Web, Flutter y Android (Kotlin) mediante archivos de referencia
- Indica cómo asegurar datos con request.auth en reglas de seguridad de Firestore/Storage

## Use it when

- La app necesita inicio de sesión de usuarios, gestión de usuarios o acceso seguro a datos mediante reglas de auth

## What triggers it

- "Configura Firebase Authentication con Google Sign In para mi app"
- "Añade login por email y contraseña usando Firebase Auth"
- "Cómo protejo mis datos de Firestore con reglas basadas en el usuario autenticado"

## Before you install

- Requiere un proyecto Firebase ya creado y Firebase CLI instalado y logueado (ver firebase-basics).
- Needs on PATH: npx

## Files

- SKILL.md — 4 KB
- references/client_sdk_android.md — 5 KB
- references/client_sdk_web.md — 7 KB
- references/flutter_setup.md — 5 KB
- references/ios_setup.md — 2 KB
- references/security_rules.md — 1 KB

## SKILL.md

Reproduced verbatim from firebase/agent-skills under Apache-2.0. This section is the upstream document and is in English.

## 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:

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

#### 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).

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Skills Agentes — https://skillsagentes.com/skills/firebase/agent-skills/firebase-auth-basics