The previous attempt opted out of AGP 9's built-in Kotlin (via
android.builtInKotlin=false + explicit kotlin-android plugin) because
the message from Gradle suggested it. But Kotlin 2.2.21's
kotlin-android plugin can't cast AGP 9's new ApplicationExtension to
the removed BaseExtension:
class ApplicationExtensionImpl$AgpDecorated_Decorated
cannot be cast to class com.android.build.gradle.BaseExtension
That suggestion is for projects with an older Kotlin toolchain. The
real fix:
- Kotlin 2.3.21 (latest stable; first line where kotlin-android also
supports AGP 9, but more importantly the built-in path works)
- KSP 2.3.8 — KSP PR #2674 (merged Oct 2025) added AGP 9 built-in
Kotlin support. KSP 1.x and pre-2.3 don't work with built-in Kotlin.
- Re-drop the kotlin-android plugin from both build.gradle.kts files;
AGP 9 enables built-in Kotlin by default and KSP 2.3 cooperates.
- Remove android.builtInKotlin=false from gradle.properties.
compose-compiler plugin tracks the Kotlin version via version.ref, so
no separate bump there.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
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