While building a Rails + Hotwire Native app for Android, errors poured out from the AGP (Android Gradle Plugin) 9.0 and Hotwire Native 1.2.5 combination. Here’s the record of fixing them one by one.

Error 1: kotlin-android plugin is no longer required

Plugin 'kotlin-android' is no longer required for Kotlin support since AGP 9.0

Since AGP 9.0, Kotlin support is built-in and a separate plugin is no longer needed.

// build.gradle.kts — remove
plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.kotlin.android)   // ← delete
}

// keep only this
plugins {
    alias(libs.plugins.android.application)
}

Error 2: kotlinOptions unresolved reference

Unresolved reference: kotlinOptions

kotlinOptions was removed in AGP 9.0. Replace with kotlin { jvmToolchain() }.

// wrong
compileOptions {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}
kotlinOptions {
    jvmTarget = "17"
}

// correct
compileOptions {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}
kotlin {
    jvmToolchain(17)
}

Error 3: HotwireWebBridgeFragment unresolved reference

Unresolved reference: HotwireWebBridgeFragment

Hotwire Native Android 1.2.5 doesn’t have HotwireWebBridgeFragment. Extend HotwireWebFragment instead.

// wrong
class MainFragment : HotwireWebBridgeFragment() {
    override val bridgeComponentFactories = listOf(...)
}

// correct
@HotwireDestinationDeepLink(uri = "myapp://fragment/web")
class MainFragment : HotwireWebFragment()

Bridge component registration should be done in the Application class, not in the Fragment.

class MainApplication : Application() {
    override fun onCreate() {
        super.onCreate()
        Hotwire.registerBridgeComponents(
            BridgeComponentFactory("my-component", ::MyBridgeComponent)
        )
    }
}

Error 4: binding?.webView null

Accessing the WebView via binding?.webView returns null. Hotwire Native provides a callback when the WebView is ready.

// wrong
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    binding?.webView?.settings?.javaScriptEnabled = true
}

// correct
override fun onWebViewAttached(webView: HotwireWebView) {
    super.onWebViewAttached(webView)
    webView.settings.javaScriptEnabled = true
}

Error 5: navigator?.navigateUp() unresolved

Unresolved reference: navigateUp

navigateUp() was removed. Replace with navigator.pop().

// wrong
navigator?.navigateUp()

// correct
navigator.pop()

Error 6: Firebase package name mismatch

Adding applicationIdSuffix = ".debug" to the debug buildType makes the package name (com.myapp.app.debug) different from what’s registered in Firebase (com.myapp.app), causing FCM to stop working.

// This makes the debug build com.myapp.app.debug, which Firebase can't recognize
buildTypes {
    debug {
        applicationIdSuffix = ".debug"   // ← delete
    }
}

Final build.gradle.kts

plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.kotlin.serialization)
    alias(libs.plugins.google.services)
}

android {
    namespace = "com.myapp.app"
    compileSdk = 36

    defaultConfig {
        applicationId = "com.myapp.app"
        minSdk = 28
        targetSdk = 36
        versionCode = 1
        versionName = "1.0.0"
    }

    signingConfigs {
        create("release") {
            storeFile = file("myapp.jks")
            storePassword = "password"
            keyAlias = "myapp"
            keyPassword = "password"
        }
    }

    buildTypes {
        debug {
            buildConfigField("String", "BASE_URL", "\"https://my-server.com\"")
        }
        release {
            isMinifyEnabled = true
            isShrinkResources = true
            signingConfig = signingConfigs.getByName("release")
            proguardFiles(
                getDefaultProguardFile("proguard-android-optimize.txt"),
                "proguard-rules.pro"
            )
            buildConfigField("String", "BASE_URL", "\"https://my-server.com\"")
        }
    }

    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_17
        targetCompatibility = JavaVersion.VERSION_17
    }

    kotlin {
        jvmToolchain(17)
    }

    buildFeatures {
        buildConfig = true
    }
}

Generating Release Signing Keystore

keytool -genkey -v \
  -keystore android/app/myapp.jks \
  -keyalg RSA -keysize 2048 -validity 10000 \
  -alias myapp \
  -storepass yourpassword \
  -keypass yourpassword

Build Automation with Makefile

apk-debug:
	cd android && ./gradlew assembleDebug

apk-release:
	cd android && ./gradlew assembleRelease

aab-release:
	cd android && ./gradlew bundleRelease
  • Debug APK: Direct installation by partners (Settings > Security > Allow unknown apps)
  • Release AAB: For Play Store submission