Capacitor Plugin Build Error AGP 9 Fix (proguard-android.txt Not Found)

If your Capacitor plugin started failing after upgrading to Android Gradle Plugin 9 (AGP 9), you are likely hitting a small but critical Gradle configuration issue.

This post specifically targets common search intents like:

Capacitor plugin build error AGP 9
Android Gradle Plugin 9 plugin build failed
proguard-android.txt not found
AGP 9 getDefaultProguardFile error
Capacitor Android build failed after AGP upgrade

The short version:

proguard-android.txt is no longer the safe default baseline to reference in AGP 9 plugin builds.
Switch to proguard-android-optimize.txt.
Rebuild and verify.

The longer version matters too, especially if you maintain many plugins or large Capacitor workspaces. In this article we cover:

What Android and AGP are in the build chain
What Capacitor is and how plugin builds work
What Capgo is and why this matters for release reliability
The exact AGP 9 change that breaks old plugin templates
A safe migration strategy for one repo or many repos

What is Android in this context?

Android is both an operating system and a build ecosystem. When you ship a Capacitor app or plugin on Android, your project goes through:

Gradle as the build system.
Android Gradle Plugin (AGP) as Android-specific Gradle integration.
The Android SDK toolchain for packaging, shrinking, linting, and producing .aar, .apk, or .aab outputs.

When AGP versions change, some defaults and internal files can change too. A plugin configuration that worked for AGP 8 can fail on AGP 9 if it points to a removed or deprecated baseline.

What is Capacitor?

Capacitor is a cross-platform runtime that lets you build iOS/Android apps with web code (TypeScript, JavaScript, HTML, CSS) while still calling native APIs.

Capacitor apps usually include:

A web layer (your UI and business logic)
Native shells (ios/, android/)
Plugins that expose native features to JavaScript

Each plugin has its own native build configuration. On Android, this means each plugin includes an android/build.gradle file that AGP must parse and compile correctly.

If plugin Gradle settings are outdated, the whole app build can fail, even when your web code is correct.

What is Capgo?

Capgo provides tools around Capacitor delivery and operations:

Live updates for web bundle changes
Plugin ecosystem and native feature packages
CI/CD-friendly update workflows for Capacitor teams

Even with live updates, native build stability is non-negotiable. You still need clean Android builds for:

App Store / Play Store releases
Native plugin upgrades
Platform SDK migrations
Team onboarding and CI reliability

That is why AGP 9 compatibility fixes are important: they keep your plugin layer dependable so delivery pipelines stay predictable.

Why AGP 9 breaks older plugin configs

Many plugin templates historically used:

proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'

In AGP 9 setups, this legacy baseline reference can fail because the old file is no longer guaranteed in the location expected by older templates/configs.

Typical symptoms include Gradle errors during assemble, lint, or build phases, often pointing to missing ProGuard baseline resources or invalid default file references.

Quick background: ProGuard, R8, and baseline files

R8 is the modern code shrinker/optimizer in Android builds.
proguard-rules.pro is your project/plugin custom keep rules.
getDefaultProguardFile(...) injects an Android-provided baseline.

When you reference:

proguard-android.txt -> legacy, minimal baseline
proguard-android-optimize.txt -> modern optimized baseline (recommended default in current setups)

For AGP 9 compatibility, switching to proguard-android-optimize.txt is the practical fix.

The one-line fix

Update plugin and app module Gradle files:

proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'

At minimum, check:

android/build.gradle in each plugin
example-app/android/app/build.gradle in plugin repos
Any generator/template files that create new plugin Gradle config

Migration guide for one plugin

1. Find the old reference

rg -n "proguard-android\\.txt" android example-app

2. Replace it

perl -pi -e "s/proguard-android\\.txt/proguard-android-optimize.txt/g" \
  android/build.gradle example-app/android/app/build.gradle

3. Verify with Bun

bun run verify:android

If your plugin has full verification scripts, run:

bun run verify

Batch update all plugin repos

If you maintain many plugin repositories in one workspace, automate it:

rg -l "proguard-android\\.txt" capacitor-* \
  --glob '!**/node_modules/**' \
  --glob '!**/.gradle/**' \
  --glob '!**/build/**' \
| xargs perl -pi -e "s/proguard-android\\.txt/proguard-android-optimize.txt/g"

Then validate that no tracked plugin source still uses the old file:

for d in capacitor-*; do
  [ -d "$d/.git" ] || continue
  git -C "$d" grep -n "proguard-android\\.txt" -- || true
done

No matches means the old baseline reference is gone from tracked plugin files.

Capgo rollout status

We completed this migration across all official Capgo Capacitor plugin repositories and templates:

Plugin Android modules now reference proguard-android-optimize.txt
Plugin example Android apps were updated too
Plugin scaffolding templates were updated so new plugins are AGP 9-safe by default

This prevents a common class of AGP 9 upgrade failures before they hit CI.

Why this is important even if your build passes today

You might not see failures immediately if:

Your CI cache still masks the issue
You have mixed AGP versions across projects
Only some modules are rebuilt in local dev

But eventually, clean builds, new environments, or upgraded runners expose it. Doing the migration now removes hidden instability.

Troubleshooting if builds still fail after the replacement

Check these points:

Every module is patched. Look at plugin modules, app modules, samples, and template assets.
There is no second reference in shared scripts. Search entire repositories (including custom Gradle scripts).
Caches are clean. Run ./gradlew clean and rebuild.
AGP / Gradle / JDK versions are aligned. Use combinations supported by Android documentation for your AGP version.
CI uses the same versions as local. Pin JDK and Gradle wrapper versions in CI to avoid environment drift.
You are not patching only node_modules. Fix tracked plugin source, not transient dependency directories.

SEO FAQ: AGP 9 Capacitor plugin build errors

How do I fix `proguard-android.txt` not found in AGP 9?

Replace:

getDefaultProguardFile('proguard-android.txt')

With:

getDefaultProguardFile('proguard-android-optimize.txt')

Then run a clean rebuild.

Why does my Capacitor plugin build fail after upgrading to Android Gradle Plugin 9?

Most failures come from legacy Gradle config in plugin android/build.gradle files that still reference proguard-android.txt. AGP 9 projects should use proguard-android-optimize.txt.

What is the fastest AGP 9 migration path for many Capacitor plugins?

Use a workspace-wide search-and-replace command, then validate with git grep and run bun run verify:android on representative plugins.

Is this only a Capacitor issue?

No. Any Android module (app or library) using deprecated ProGuard baseline references can hit similar AGP 9 build errors. It is especially visible in plugin ecosystems because many repositories share old templates.

What keywords are relevant for this migration?

If you are documenting this in internal runbooks or support pages, include terms like:

AGP 9 build error
Android Gradle Plugin 9 ProGuard file missing
Capacitor plugin Android build failed
proguard-android.txt replacement
proguard-android-optimize.txt migration

Android Developers: Build your app overview
Android Gradle Plugin: Release notes
Android code shrinking: R8 and rules
Gradle docs: Build tool fundamentals
Capacitor docs: Official documentation
Capgo docs: Auto update docs

Final takeaway

This AGP 9 issue is simple, but it is easy to miss in multi-plugin workspaces. Once you replace proguard-android.txt with proguard-android-optimize.txt everywhere relevant, Android builds become predictable again.

If you use Capgo plugins, this migration is already applied in official repositories so you can upgrade with fewer surprises.