Migrating to AGP 9.2.1: Kotlin build errors I hit and how I fixed them

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I recently upgraded an Android project to **AGP 9.2.1** and ran into a chain of build errors.

In my case, the project was a very small app, so there might be more issues if the project setup is complicated.

Each one pointed to a different part of the new Kotlin/Gradle setup, and fixing them one by one made the migration much clearer.

Why this happened

AGP 9 introduces built-in Kotlin support that changes the old plugin setup (org.jetbrains.kotlin.android).

• 🛑 kapt, and kotlinOptions no longer work the same way.

Error 1: Cannot add extension with name kotlin

Cannot add extension with name 'kotlin', as there is an extension already registered with that name.
The fix was to remove the Kotlin Android plugin from the module and 
any root-level declarations that still applied it. 

AGP 9 already provides Kotlin support, so applying the old plugin causes a duplicate kotlin extension.

Fix

plugins {
    id("com.android.application")
    // remove: id("org.jetbrains.kotlin.android")
}

Error 2: org.jetbrains.kotlin.kapt is not compatible with built-in Kotlin support

• After removing the Kotlin Android plugin, the next failure came from **kapt**

The 'org.jetbrains.kotlin.kapt' plugin is not compatible with built-in Kotlin support.

Android’s migration guide recommends moving to KSP, 
or using com.android.legacy-kapt only as a temporary fallback.

Fix

• After removing **kapt, we need to update the affected libraries. In my case, only `Hilt** was affected, so I updated it with theKSP`

plugins {

    //....
    // Remove KAPT
    kotlin("kapt")

    // Add KSP
    id("com.google.devtools.ksp")
}

// dependencies

dependencies {

//  kapt(libs.hilt.compiler)
    ksp(libs.dagger.hilt.compiler)

// ...

}

Error 3: Unresolved reference: kotlinOptions

• Once kapt was out of the way, the build hit another issue 🤯

Unresolved reference 'kotlinOptions'

That’s because AGP 9 built-in Kotlin uses the new Kotlin compiler options DSL 
instead of the old android.kotlinOptions {} block. 

The Android migration doc says to move those settings to 
kotlin { compilerOptions { ... } }.

Fix

// Old way
android {
    kotlinOptions {
        jvmTarget = "17"
    }
}

// New way

kotlin {
    compilerOptions {
        jvmTarget.set(JvmTarget.JVM_17)
    }
}

What I learned

In addition to a version bump, AGP 9 changes how Kotlin is built into Android.

References

[embed]Migrate to built-in Kotlin | Android Studio | Android Developers Migrate to built-in Kotlindeveloper.android.com

[embed]Update your Kotlin projects for Android Gradle Plugin 9.0 | The Kotlin Blog Learn how to migrate your projects to AGP 9.0 by moving to built-in Kotlin and the new Android KMP library plugin.blog.jetbrains.com

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