Firebase Authentication - Android Setup Guide (Kotlin)

This guide describes the SDK setup and basic usage patterns for Firebase Authentication in an Android app using Kotlin DSL (build.gradle.kts) and Kotlin code.

Prerequisites

IMPORTANT: Before specifically working with Firebase Authentication, make sure to use the skill and reference firebase_basics/references/android_setup to ensure the following is done.

The Firebase CLI is available and authenticated.
An Android project exists and is registered with a Firebase Project.
The Android project has a Firebase config file (google-services.json) and the Google services Gradle plugin (google-services).

1. Enable Authentication via CLI

Enable the Firebase Authentication service in the Firebase Project using the Firebase CLI:

npx -y firebase-tools@latest init auth

2. Add dependencies to Gradle build files

[!IMPORTANT] [AGENT] RESOLVING THE SDK VERSION DETERMINISTICALLY Never guess or hardcode a memorized out-of-date SDK or plugin version. Before adding dependencies, you MUST query the latest available versions directly from the Google Maven Repository:
# Find the latest Firebase Android BoM version
curl -s https://dl.google.com/dl/android/maven2/com/google/firebase/firebase-bom/maven-metadata.xml | grep -oE '<latest>[^<]+' | cut -d'>' -f2

# Find the latest firebase-auth version
curl -s https://dl.google.com/dl/android/maven2/com/google/firebase/firebase-auth/maven-metadata.xml | grep -oE '<latest>[^<]+' | cut -d'>' -f2
If the dependency is already declared, fetch the exact resolved version currently active in the workspace's build graph:
./gradlew -q :app:dependencyInsight --dependency firebase-auth --configuration releaseRuntimeClasspath

In the module (app-level) build.gradle.kts (usually <project>/<app-module>/build.gradle.kts), add the dependency for Firebase Authentication:

dependencies {
    // [AGENT] Fetch the latest resolved version using the Gradle command above
    implementation(platform("com.google.firebase:firebase-bom:<latest_bom_version>"))

    // Add the dependency for the Firebase Authentication library
    // When using the BoM, don't specify versions in Firebase library dependencies
    implementation("com.google.firebase:firebase-auth")
}

3. Initialize FirebaseAuth

In the Activity or Fragment, initialize the FirebaseAuth instance:

import com.google.firebase.Firebase
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.auth

class MainActivity : AppCompatActivity() {

    private lateinit var auth: FirebaseAuth

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        auth = Firebase.auth

        setContent {
            MaterialTheme {
                Text("Auth initialized!")
            }
        }
    }
}

Jetpack Compose (Modern)

Initialize inside a ComponentActivity using setContent:

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Text
import com.google.firebase.Firebase
import com.google.firebase.auth.auth

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val auth = Firebase.auth

        setContent {
            MaterialTheme {
                Text("Auth initialized!")
            }
        }
    }
}

4. Check current Auth state

Check if a user is already signed in when the activity starts:

public override fun onStart() {
    super.onStart()
    // Check if user is signed in (non-null) and update UI accordingly.
    val currentUser = auth.currentUser
    if (currentUser != null) {
        // User is signed in, navigate to main screen or update UI
    } else {
        // No user is signed in, prompt for login
    }
}

5. Use sign-in providers in the app

Email/Password

Sign up NEW users with Email/Password

Use createUserWithEmailAndPassword to register new users:

fun signUpUser(email: String, password: String) {
    auth.createUserWithEmailAndPassword(email, password)
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                // Sign up success, update UI with the signed-in user's information
                val user = auth.currentUser
                // Navigate to main screen
            } else {
                // If sign up fails, display a message to the user.
                Toast.makeText(baseContext, "Authentication failed.", Toast.LENGTH_SHORT).show()
            }
        }
}

Sign in EXISTING users with Email/Password

Use signInWithEmailAndPassword to sign in existing users:

fun signInUser(email: String, password: String) {
    auth.signInWithEmailAndPassword(email, password)
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                // Sign in success, update UI with the signed-in user's information
                val user = auth.currentUser
                // Navigate to main screen
            } else {
                // If sign in fails, display a message to the user.
                Toast.makeText(baseContext, "Authentication failed.", Toast.LENGTH_SHORT).show()
            }
        }
}

6. Sign out users

To sign out a user, call signOut() on the FirebaseAuth instance:

auth.signOut()
// Navigate to login screen

/firebase-auth-basics

referencesclient_sdk_android.md