Diagnosing Compose Stability — Read the Compiler Reports First

Compose skips recomposition by comparing parameters. When a parameter is unstable, skipping is disabled — this skill tells Claude how to find out which parameters are unstable and why. The output is a prioritized list of unstable types and non-skippable composables; the fix lives in ../stabilizing-compose-types/SKILL.md.

When to use this skill

• The developer asks "why does this recompose?", reports jank, dropped frames, or scroll stutter.
• A @TraceRecomposition log shows recomposition counts that exceed the number of meaningful state changes.
• The developer mentions Compose Compiler Reports, classes.txt, composables.txt, composables.csv, module.json, or "non-skippable".
• The developer asks how to find unstable parameters, or whether List<Foo>, LocalDateTime, or a domain type is stable.
• A reviewer asks for evidence that a perf-sensitive composable is skippable.

When NOT to use this skill

• The unstable types are already known — jump straight to ../stabilizing-compose-types/SKILL.md.
• The symptom is a wrong-phase state read (Modifier.alpha(state.value)); use ../../recomposition/deferring-state-reads/SKILL.md.
• The symptom is a derivedStateOf misuse; use ../../recomposition/choosing-derivedstateof/SKILL.md.
• The developer wants CI gating instead of one-shot diagnosis; use ../enforcing-stability-in-ci/SKILL.md.

Prerequisites

• Kotlin 2.0.0+ with the Compose Compiler Gradle plugin applied: id("org.jetbrains.kotlin.plugin.compose"). The pre-2.0 kotlinCompilerExtensionVersion flow is obsolete.
• A buildable release variant of the target module. Reports MUST be produced in release; debug adds Live Literals which makes constants look dynamic and skews every report.
• Apply the org.jetbrains.kotlin.plugin.compose Gradle plugin (Kotlin 2.0+). The composeCompiler { … } extension is owned by that plugin; AGP version is incidental.
• Optional but PREFERRED: the developer has a stability baseline goal, not a 100-percent-skippable goal (skydoves hot take #1 — skippability is a diagnostic, not a KPI).

Workflow

• ] 1. Enable the reports in the module's build.gradle.kts. Scope to release only — debug builds emit misleading data.

kotlin
// app/build.gradle.kts (or any compose module)
plugins {
    id("com.android.application")
    id("org.jetbrains.kotlin.android")
    id("org.jetbrains.kotlin.plugin.compose")
}

composeCompiler {
    // Only emit reports for release to avoid Live Literals noise.
    val isReleaseBuild = providers.gradleProperty("composeCompilerReports").orNull == "true"
    if (isReleaseBuild) {
        reportsDestination = layout.buildDirectory.dir("compose_compiler")
        metricsDestination = layout.buildDirectory.dir("compose_compiler")
    }
}

Or the always-on form:

kotlin
composeCompiler {
    reportsDestination = layout.buildDirectory.dir("compose_compiler")
    metricsDestination = layout.buildDirectory.dir("compose_compiler")
    // Optional — opt mutable third-party types into stability:
    stabilityConfigurationFiles.add(
        rootProject.layout.projectDirectory.file("stability_config.conf")
    )
}

• ] 2. Build the release variant so Live Literals and interpreted-mode noise are absent:

bash
./gradlew :app:assembleRelease -PcomposeCompilerReports=true

For a library module: ./gradlew :feature-feed:assembleRelease. The release flavor is required — see RIGHT/WRONG below.

• ] 3. Locate the four output files. They are written to <module>/build/compose_compiler/:

app/build/compose_compiler/
├── app_release-classes.txt        # per-class stability
├── app_release-composables.txt    # per-composable signatures
├── app_release-composables.csv    # CSV mirror of the above (CI-friendly)
└── app_release-module.json        # aggregate counts

If any file is missing, the plugin did not run for that variant — re-check that composeCompiler { reportsDestination = ... } is on the right module and that the build was release.

• ] 4. Open composables.txt first. This is the highest-signal file. Search for restartable lines that are not followed by skippable. Each one is a recomposition entry point that cannot be skipped. Inside each block, read the per-parameter prefix: stable, unstable, @static, @dynamic. Any unstable parameter blocks skipping. See references/reading-composables-txt.md for the full grammar.

• ] 5. Open classes.txt to learn _why_ a class is unstable. For every type flagged unstable in composables.txt, find its declaration in classes.txt. The line tells you whether a var field, a generic parameter, or an unstable nested type is the cause. The runtime stable class Box { stable val value: T } shape means "this class is stable iff the runtime $stable: Int field of the substituted T says so" — see references/reading-classes-txt.md.

• ] 6. Open module.json for triage numbers. Counts of skippable composables, restartable composables, stable classes, etc. Use this to compare before/after a fix or to decide which module to attack first. DO NOT treat these counts as a target — they exist to spot regressions, not to chase 100 percent.

• ] 7. Prioritize by hot path. A non-skippable composable that runs once at startup is irrelevant; one inside a LazyColumn item is critical. Cross-reference with measurement (@TraceRecomposition from skydoves/compose-stability-analyzer, or Macrobenchmark FrameTimingMetric) before fixing — see ../../measurement/tracing-recompositions-at-runtime/SKILL.md.

• ] 8. Hand off to the fix skill. Produce a list of unstable types + offending composables and apply ../stabilizing-compose-types/SKILL.md.

Patterns

Pattern: HighlightedSnacks — read a non-skippable composable

The compiler reports surface the cause directly inside the function signature. Walk the developer through this real shape.

text
restartable scheme("[androidx.compose.ui.UiComposable]") fun HighlightedSnacks(
  stable index: Int,
  unstable snacks: List<Snack>,                  // <-- blocks skipping
  stable onSnackClick: Function1<Long, Unit>,
)

Diagnosis script:

1. The function is restartable but NOT prefixed with skippable. Therefore it always recomposes when its parent does.
2. The cause is the unstable snacks: List<Snack> parameter. kotlin.collections.List is an interface; the compiler cannot prove its implementations are immutable.
3. Open classes.txt and find Snack. If Snack itself is unstable, fix the data class first; if it is stable then only the List wrapper is the problem.
4. Hand off to the fix skill: replace List<Snack> with kotlinx.collections.immutable.ImmutableList<Snack>, or add kotlin.collections.* to stability_config.conf if the developer is comfortable with that contract.

Pattern: "stable but runtime" — what runtime means

text
runtime stable class Box {
  stable val value: T
}

This is not unstable. The compiler emits a synthetic $stable: Int field at runtime and queries it during composition. The class is stable iff the substituted T reports stable. DO NOT annotate runtime classes with @Stable to "promote" them — the runtime check is the correct mechanism.

Pattern: WRONG vs RIGHT — running the build

bash
# WRONG
./gradlew :app:assembleDebug
# WRONG because: debug enables Live Literals; constant 0 dp becomes a getter, every literal looks dynamic, and counts in module.json drift versus what ships to users.

bash
# RIGHT
./gradlew :app:assembleRelease -PcomposeCompilerReports=true

Pattern: WRONG vs RIGHT — reading the report file

text
// WRONG — reading composables.txt without checking the leading flags
fun MyScreen(...)
// WRONG because: skipping the `restartable`/`skippable` prefix discards the only data point that determines whether unstable params actually cost anything.

text
// RIGHT — every diagnosis quotes the full prefix and per-param annotations
restartable skippable scheme("[androidx.compose.ui.UiComposable]") fun MyScreen(
  stable user: User,
  stable onClick: Function0<Unit>,
)

Pattern: empty output directory

If build/compose_compiler/ is missing or empty after a release build:

1. Confirm the org.jetbrains.kotlin.plugin.compose plugin is applied to this module — the extension is per-module.
2. Confirm the build actually compiled Kotlin sources (not an up-to-date no-op). Touch a file or run ./gradlew :app:clean :app:assembleRelease.
3. Confirm reportsDestination is set inside a composeCompiler { } block, not the legacy kotlinOptions freeCompilerArgs flow.

Mandatory rules

• MUST build the release variant. Debug builds emit Live Literals which makes constants look dynamic and inflates the report's "unstable" surface area.
• MUST read composables.txt per-parameter annotations (stable/unstable/@static/@dynamic), not just the function name. The flag prefix is the diagnosis.
• MUST cross-reference unstable params back to classes.txt to identify the root cause (a var, a generic, an unstable field type, or an interface).
• MUST NOT chase 100 percent skippability. Skydoves hot take #1: skippability is a diagnostic, not a KPI. A composable that runs once at startup does not need to be skippable.
• MUST NOT annotate a class as @Stable based on a report alone — that decision belongs to the fix skill, which evaluates the contract.
• MUST NOT read composables.csv line counts and call it done. The counts are a regression sentinel; the per-line annotations are the actual diagnosis.
• PREFERRED: wire the same flags into CI via ../enforcing-stability-in-ci/SKILL.md (skydoves compose-stability-analyzer plugin or the community ComposeGuard plugin) so regressions surface on PR review.
• PREFERRED: record a baseline module.json per release so future regressions are visible by diff.

Verification

• ] ./gradlew :app:assembleRelease (or module-specific) completes successfully with the org.jetbrains.kotlin.plugin.compose plugin applied.
• ] All four files exist: <module>_release-classes.txt, <module>_release-composables.txt, <module>_release-composables.csv, <module>_release-module.json.
• ] At least one of: a list of unstable parameter sites copied from composables.txt, OR a confirmed-zero count from module.json justifying that no fix is needed.
• ] Each unstable parameter has been traced back to a class in classes.txt so the fix skill receives a concrete root cause (a var, a List, a LocalDateTime, etc.).
• ] The developer understands that runtime stable class … is not a problem — it is the compiler's correct lazy-stability emission.

References

• Android Developers — Diagnose stability: https://developer.android.com/develop/ui/compose/performance/stability/diagnose
• Android Developers — Stability overview: https://developer.android.com/develop/ui/compose/performance/stability
• Compose Compiler release notes: https://developer.android.com/jetpack/androidx/releases/compose-compiler
• Ben Trengrove — Jetpack Compose Stability Explained: https://medium.com/androiddevelopers/jetpack-compose-stability-explained-79c10db270c8
• Chris Banes — Composable Metrics: https://chrisbanes.me/posts/composable-metrics/
• Why test perf in release: https://medium.com/androiddevelopers/why-should-you-always-test-compose-performance-in-release-4168dd0f2c71
• skydoves — Optimize App Performance by Mastering Stability: https://medium.com/proandroiddev/optimize-app-performance-by-mastering-stability-in-jetpack-compose-69f40a8c785d
• skydoves — compose-stability-analyzer: https://github.com/skydoves/compose-stability-analyzer
• references/reading-classes-txt.md — full grammar for classes.txt with worked examples (stable / unstable / runtime / generic).
• references/reading-composables-txt.md — full grammar for composables.txt including restartable, skippable, readonly, scheme, and per-parameter stable / unstable / @static / @dynamic.