AGP 9 build failure: "Cannot add extension with name 'kotlin'"

[kinggolf-hgm]

Bug

Under Android Gradle Plugin 9 (com.android.tools.build:gradle:9.x), Gradle Sync fails when this plugin's android/build.gradle is loaded:

Build file '.../node_modules/@capacitor/filesystem/android/build.gradle' line: 29
A problem occurred evaluating project ':capacitor-filesystem'.
> Failed to apply plugin 'kotlin-android'.
   > Cannot add extension with name 'kotlin', as there is an extension already registered with that name.

Environment

• Plugin version: 8.1.2
• Capacitor: 8.x
• Android Gradle Plugin: 9.1.1
• Gradle: as required by AGP 9
• Android Studio: Narwhal / Otter
• Kotlin Gradle Plugin: 2.2.x

Root cause

Under AGP 9, the project already has an extension named kotlin registered on the subproject before @capacitor/filesystem/android/build.gradle is evaluated. The plugin's unconditional apply plugin: 'kotlin-android' then fails because the kotlin-android plugin tries to register the kotlin extension a second time.

Notably:

• Guarding with project.plugins.hasPlugin('org.jetbrains.kotlin.android') is not sufficient. The plugin id check returns false (the kotlin-android plugin isn't applied yet) but the extension named kotlin already exists, so the apply still fails.
• The correct guard is on the extension itself: project.extensions.findByName('kotlin').

Proposed fix

Make the kotlin-android apply conditional on the extension not already being registered:

apply plugin: 'com.android.library'
if (project.extensions.findByName('kotlin') == null) {
    apply plugin: 'kotlin-android'
}

This is a minimal, backwards-compatible change:

• Under AGP 8 / older Kotlin, no kotlin extension is pre-registered, so findByName('kotlin') returns null and the apply runs as before.
• Under AGP 9, the extension is pre-registered, the apply is skipped, and Gradle Sync succeeds.

Verified locally

We've been running this patch via patch-package in our app against AGP 9.1.1 with the plugin at 8.1.2. Builds succeed and the plugin functions correctly.

Happy to open a PR with this change if it's helpful — let me know.

[OS-pedrogustavobilro]

Hey @kinggolf-hgm thanks for opening the issue; while that fix for the plugin probably makes sense, it's worth noting that Capacitor 8 does not officially support AGP 9; For Capacitor 8 AGP 8.13.0 was used, and that's what we suggest to be used in migration docs.

Official support for AGP 9 is coming with Capacitor 9, which is slated for later this year.

In the meantime, instead of the patch-package, you can try adding this to your Android app's gradle.properties, if preferable

android.builtInKotlin=false

Thank you!

[kinggolf-hgm]

@OS-pedrogustavobilro Thank you for responding and your recommendation.

[OS-pedrogustavobilro]

FYI here's the PR to fix the issue you were having #83

Opened equivalent PRs to a few other kotlin-based plugins we have:

• fix(android): only apply kotlin-android plugin if not already applied capacitor-barcode-scanner#137
• fix(android): only apply kotlin-android plugin if not already applied capacitor-calendar#2
• fix(android): only apply kotlin-android plugin if not already applied capacitor-camera#71
• fix(android): only apply kotlin-android plugin if not already applied capacitor-contacts#1
• fix(android): only apply kotlin-android plugin if not already applied capacitor-file-transfer#75
• fix(android): only apply kotlin-android plugin if not already applied capacitor-geolocation#97
• fix(android): only apply kotlin-android plugin if not already applied capacitor-os-inappbrowser#120

[kinggolf-hgm]

@OS-pedrogustavobilro Thanks - works great!