compose-stability-diagnostics

Compose stability diagnostics

Core principle

Compose performance problems from parameters are about whether inputs compare cheaply and predictably across recompositions. With Kotlin 2.0.20+ strong skipping is enabled by default, so unstable parameters no longer automatically make restartable composables non-skippable. That does not make stability irrelevant: unstable parameters are compared by instance identity (===), stable parameters by equality (equals), and churny instances can still defeat skipping.

First identify the compiler mode you are on, then read reports in that context.

When to use this skill

• A composable or screen recomposes more than expected and parameter churn is suspected.
• A UI-state/model class is passed to composables and contains List, Set, Map, ranges, Java time/money types, or third-party types.
• composables.txt / classes.txt shows unstable parameters or non-skippable composables.
• A project uses Kotlin < 2.0.20, disables strong skipping, or has old Compose compiler report guidance.

1. Start with strong skipping

On Kotlin 2.0.20+, strong skipping is enabled by default. In that mode:

• Restartable composables are skippable even when parameters are unstable, unless explicitly opted out.
• Stable parameters compare with equals.
• Unstable parameters compare with instance equality (===).
• Lambdas inside composables are automatically remembered based on captures.

That means the question changes from "is this composable skippable at all?" to "will these parameters compare the way I expect, and are callers creating new unstable instances every frame?"

For older compiler setups or strong skipping disabled, the legacy rule still matters: a restartable composable with unstable parameters may be restartable but not skippable.

2. Generate compiler reports

With Kotlin 2.0+ the Compose Compiler is configured through the Kotlin Gradle plugin:

plugins {
    alias(libs.plugins.android.application) // or android.library / jvm
    alias(libs.plugins.kotlin.android)      // or kotlin.multiplatform / kotlin.jvm
    alias(libs.plugins.compose.compiler)
}

if (providers.gradleProperty("composeReports").orNull == "true") {
    composeCompiler {
        reportsDestination = layout.buildDirectory.dir("compose_compiler")
        metricsDestination = layout.buildDirectory.dir("compose_compiler")
    }
}

Then build the variant whose compiler configuration you care about, for example:

./gradlew :app:assembleRelease -PcomposeReports=true

Use release/non-debuggable builds for runtime profiling. Compiler reports are build-time outputs, so the important thing is matching the variant and compiler flags you ship.

Key files:

File	What it tells you
<module>-classes.txt	Stability of classes and properties
<module>-composables.txt	Restartable/skippable status and parameter stability
<module>-composables.csv	Same data in sortable form
<module>-module.json	Aggregate metrics

3. Fix stability where semantics need it

Pick the lightest fix that makes the type's immutability or equality semantics true.

Immutable collections

kotlin.collections.List is an interface; Compose cannot know the runtime implementation is immutable. Prefer kotlinx.collections.immutable at UI-state boundaries:

// Before: unstable collection interfaces
data class UiState(val items: List<Item>, val tags: Set<String>)

// After: immutable collection contracts
import kotlinx.collections.immutable.ImmutableList
import kotlinx.collections.immutable.ImmutableSet

data class UiState(val items: ImmutableList<Item>, val tags: ImmutableSet<String>)

Producers convert once at the boundary with .toImmutableList() / .toImmutableSet().

@Immutable / @Stable

• Use @Immutable when every property is effectively immutable and equality describes all observable state.
• Use @Stable for types whose mutable state is observable by Compose, typically via MutableState.

Do not annotate to silence a report. A false stability promise can produce stale UI.

Third-party immutable types

For types you cannot annotate, use stabilityConfigurationFiles:

composeCompiler {
    stabilityConfigurationFiles.add(
        rootProject.layout.projectDirectory.file("compose_stability.conf"),
    )
}

java.math.BigDecimal
java.math.BigInteger
java.time.*
kotlinx.datetime.*

Only list types you are willing to promise are immutable. Do not list mutable types such as java.util.Date.

4. Stabilize lazy item inputs

Lazy list items recompose when their lambda inputs change identity, even if the visible data is unchanged.

Hoist and remember per-item inputs that are stable for the item's lifetime:

// ❌ BAD — new lambda instances when parent recomposes
items(list, key = { it.id }) { item ->
    RowCard(
        onClick = { onItemClick(item.id) },
        isHighlighted = { item.id == selectedId },
    )
}

// ✅ GOOD — stable captures for this item instance
items(list, key = { it.id }) { item ->
    val onClick = remember(item.id) { { onItemClick(item.id) } }
    val isHighlighted = remember(item.id, selectedId) { item.id == selectedId }
    RowCard(onClick = onClick, isHighlighted = isHighlighted)
}

Also hoist row position metadata (isFirst, isLast, corner radii) with remember(index) { … } when the value depends only on index — but do not expect this alone to fix back-writing or cross-row measurement bugs.

Verify focus moves and insertions with recomposition-count assertions after hoisting.

Quick reference

Symptom	Diagnosis	Fix
Kotlin 2.0.20+ but old docs say unstable means non-skippable	Strong skipping changed the default	Check comparison semantics and instance churn instead
unstable val items: List<Item>	Interface collection	Use ImmutableList<Item> or another true immutable wrapper
unstable val price: BigDecimal	External immutable type	Add to stability config
@Immutable on a type with mutable internals	False promise	Fix the model or remove the annotation
Composable skips poorly despite strong skipping	New unstable instance each recomposition	Remember, hoist, or make the type stable/equality-based
Lazy items recompose on parent recompose despite unchanged data	New lambda or derived-value instance per parent recompose (§4)	Hoist per-item with remember(item.id) { … }
Reports not generated	Compose compiler plugin missing or flag not set	Apply org.jetbrains.kotlin.plugin.compose and enable destinations

When NOT to apply

• The issue is back-writing across phases or cross-row measurement reads. Use compose-state-deferred-reads.
• The issue is a fast-changing State read in composition, such as scroll or animation. Use compose-state-deferred-reads.
• The recomposition count matches real data changes.
• The bug is wrong data or stale state, not excess work.
• The code is test-only and readability is more important than report cleanliness.

Related

• compose-state-deferred-reads - frame-rate state should often be read in layout/draw rather than composition.
• compose-recomposition-performance - entry point when you are not sure which recomposition axis is involved.