configuring-r8-for-compose

Configuring R8 for Compose — Trust Consumer Rules, Avoid Blanket Keeps

R8 is the only supported shrinker — ProGuard is deprecated. Compose ships consumer ProGuard rules, so most apps need no Compose-specific keep rules. The big modern wins come from R8 full mode (default since AGP 8.0): lambda grouping, sourceInformation() stripping, constant-folding of composable args, and ComposerImpl devirtualization. This skill teaches Claude to set R8 up minimally and refuse over-broad keeps that defeat those wins.

When to use this skill

• Setting up a new Compose app's release variant for the first time.
• A PR adds -keep class androidx.compose.** { *; } or any other Compose-wide keep — push back and apply this skill.
• A release build crashes after enabling minification — the cause is almost always a missing keep on a reflective consumer, not on Compose itself.
• APK size review surfaces a large classes.dex and the developer wants to know why.
• The first time isMinifyEnabled = true is being flipped on, before any release benchmark is run.
• Cross-link from ../../measurement/testing-compose-in-release-mode/SKILL.md whenever release measurement is being set up.

When NOT to use this skill

• The app is not yet ready to ship a release variant — wait until perf and crash budgets are in scope.
• The developer is debugging a code-level bug (logic error, race, wrong state). R8 issues manifest as ClassNotFoundException, NoSuchMethodError, or empty reflection results — not behavioral bugs.
• The release variant has been measured and is fine — do not pre-emptively add keep rules.
• The perf concern is recomposition, layout phase, or stability — see the relevant stability/ or recomposition/ skills.

Prerequisites

• AGP 8.0+ for R8 full mode default. If on older AGP, full mode must be explicitly enabled in gradle.properties with android.enableR8.fullMode=true.
• A buildable release variant (debug-signed is fine while iterating — see ../../measurement/testing-compose-in-release-mode/SKILL.md for measurement signing).
• Kotlin 2.0+ with the org.jetbrains.kotlin.plugin.compose plugin so Compose's bundled consumer rules are wired correctly through dependency resolution.
• Familiarity with the difference between proguard-android.txt (no R8 optimizations) and proguard-android-optimize.txt (R8 optimizations on).

What R8 actually does for Compose (since AGP 8.0)

Surface these to the developer when they ask "why bother" or push back against minification:

1. Lambda grouping. R8 merges many of Compose's generated lambda classes, reducing dex size and method count.
2. sourceInformation() stripping. Each composable emits a string of source-position metadata. R8 removes it from release builds — smaller APK and less work per recomposition.
3. Composable arg constant-folding. Constants that would have been wrapped by Live Literals in debug are now folded back into if (changed and 1 == 0) style checks the recomposer can short-circuit.
4. ComposerImpl devirtualization. R8 devirtualizes hot calls into Compose's runtime, which is a major contributor to the cited frame-render improvement.
5. Resource shrinking. Paired with isShrinkResources = true, R8 strips unreachable drawables, strings, and layout XML.

Cited measurement: roughly 75 percent startup gain and 60 percent frame-render gain debug to release. Source: Ben Trengrove, "Why should you always test Compose performance in release" (Android Developers Medium).

Workflow

• 1. Configure the release buildType. This is the entire happy-path config — no Compose-specific keeps needed:

// app/build.gradle.kts
android {
    buildTypes {
        release {
            isMinifyEnabled = true
            isShrinkResources = true
            proguardFiles(
                getDefaultProguardFile("proguard-android-optimize.txt"),
                "proguard-rules.pro",
            )
        }
    }
}

• 2. Verify the default file name. The single most common mistake is using proguard-android.txt instead of proguard-android-optimize.txt. Only the -optimize variant enables R8's optimization passes.

• 3. Trust Compose's bundled consumer rules. DO NOT add Compose-wide keeps. Each Compose artifact (androidx.compose.runtime, androidx.compose.ui, androidx.compose.foundation, androidx.compose.material3, etc.) ships a consumer-rules.pro that R8 picks up automatically. The developer's proguard-rules.pro should contain no lines starting with -keep class androidx.compose..

• 4. Identify legitimate keep needs from the project. R8 problems are usually outside Compose:

  • @Serializable data classes consumed by kotlinx.serialization (the kotlinx-serialization Gradle plugin emits the right rules — verify, do not duplicate).
  • Hilt / Dagger entry points and generated DI code (the Hilt plugin emits these — verify).
  • Custom rememberSaveable Saver objects only when accessed reflectively from outside the module.
  • Any Class.forName(...) consumer, JNI binding, or service loader.
  • Public API surfaces of library modules consumed by other apps.

• 5. Run the AGP 8.x missing-rule reporter to find under-keeps. Do not add speculative keeps. After a build that fails at runtime in release, AGP writes a report at app/build/outputs/mapping/release/missing_rules.txt listing the exact -keep directives R8 inferred from runtime failures. Add only those, narrowed to the specific class/method.

• 6. Use R8 retrace to map crash stacks back to source. When a release crash arrives, run the modern R8 retrace shipped with the Android command-line tools:

# Modern R8 retrace (cmdline-tools 7.0+):
$ANDROID_HOME/cmdline-tools/latest/bin/retrace \
    app/build/outputs/mapping/release/mapping.txt < crash.txt

Or use the Gradle convenience target wired into AGP:

./gradlew :app:retraceR8DebuggingArtifact

(The legacy ProGuard retrace.sh install path under $ANDROID_HOME/tools/ was removed when SDK Tools 26 was sunset; do not look for it.) Always retrace before diagnosing a release crash — the stack frames are otherwise meaningless.

• 7. Verify with an APK analysis. After enabling minification:

./gradlew :app:assembleRelease
# Then in Android Studio: Build → Analyze APK → app-release.apk
# Compare classes.dex method count and resources.arsc size against the same build with isMinifyEnabled = false.

A correctly configured Compose app with no over-broad keeps typically halves the method count vs the un-minified release.

• 8. Re-run Macrobenchmark to capture the perf gain. Cross-link ../../measurement/testing-compose-in-release-mode/SKILL.md. The expected delta vs CompilationMode.None against a debug variant is in the 75 percent / 60 percent range cited above.

Patterns

Pattern: Over-broad Compose keep

# WRONG
-keep class androidx.compose.** { *; }
# WRONG because: Compose's own consumer rules are tighter and correct; a wildcard keep blocks lambda grouping, prevents sourceInformation stripping, kills composable-arg constant folding, defeats ComposerImpl devirtualization, and bloats classes.dex. The cited 75/60 gains evaporate.

# RIGHT — let Compose's consumer rules handle it.
# (no Compose-specific lines in proguard-rules.pro by default)

Pattern: Wrong default ProGuard file

// WRONG
buildTypes.release {
    isMinifyEnabled = true
    proguardFiles(
        getDefaultProguardFile("proguard-android.txt"),
        "proguard-rules.pro",
    )
}
// WRONG because: proguard-android.txt explicitly skips R8's optimization passes. Lambda grouping, devirtualization, constant folding, sourceInformation stripping all do not run. The release variant builds and installs but performs like a poorly minified app.

// RIGHT
buildTypes.release {
    isMinifyEnabled = true
    isShrinkResources = true
    proguardFiles(
        getDefaultProguardFile("proguard-android-optimize.txt"),
        "proguard-rules.pro",
    )
}

Pattern: Defensive kotlin.Metadata keep

# WRONG
-keep class kotlin.Metadata { *; }
# WRONG because: blanket-keeping kotlin.Metadata blocks R8 from shrinking metadata for unreachable Kotlin classes; identify the actual reflective consumer (kotlinx.serialization, Moshi, Gson, Hilt) and add a narrow rule for that consumer instead.

# RIGHT — narrow to the reflective consumer
# Example: kotlinx.serialization needs Companion access on KSerializer implementations.
-keepclassmembers class * implements kotlinx.serialization.KSerializer {
    public static **$Companion Companion;
}

Pattern: -dontobfuscate for "easier debugging"

# WRONG
-dontobfuscate
-dontoptimize
# WRONG because: turning off obfuscation and optimization on the release variant removes most of R8's benefit; the variant being measured is no longer the variant that ships. If readable stacks are needed, ship with R8 on and use retrace + the mapping.txt to deobfuscate.

# RIGHT — leave R8 on, keep mapping.txt for retrace.
# (no -dontobfuscate / -dontoptimize lines)
# Upload mapping.txt to your crash reporter (Crashlytics, Sentry, etc.) so stacks
# are deobfuscated automatically; or run R8 retrace manually:
#   $ANDROID_HOME/cmdline-tools/latest/bin/retrace mapping.txt < crash.txt
# (or: ./gradlew :app:retraceR8DebuggingArtifact)

Pattern: @Serializable keep when the kotlinx-serialization plugin is missing

# RIGHT — when using kotlinx.serialization with reflection-based polymorphism
-keepclassmembers class * implements kotlinx.serialization.KSerializer {
    public static **$Companion Companion;
}
-keepclasseswithmembers class * {
    @kotlinx.serialization.Serializable <init>(...);
}
# But PREFERRED: apply the kotlinx-serialization Gradle plugin so these rules are emitted 

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