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Kotlin Claude Code Rules: Coroutines, Null Safety and Compose

Path-scoped Claude Code rule for Kotlin. It loads when a matching file is read. It does not replace CLAUDE.md.

Updated for Kotlin 2.4, Nov 2026

Do not upgrade dependencies or the framework version unless the task asks.

Stay on the Kotlin version the Gradle build uses. Kotlin 2.4.20 adds StackTraceRecoverable for coroutine stack traces. That API is experimental and requires @OptIn(ExperimentalStdlibCoroutineSupportApi::class). It is not on by default. Swift export is the other 2.4.20 change. Use either only when the compiler is already on 2.4. A multiplatform target is added only when the project is already multiplatform.

Project shape

Match the source sets. Android UI uses the Compose compiler the project already applies. A ViewModel or the project's presenter owns screen state. A composable displays state and sends events up. Do not fetch the network directly inside a composable.

Coroutines

Launch in a scope that is canceled with the owner: viewModelScope, lifecycleScope, or a scope the class cancels in close. GlobalScope is a bug. runBlocking does not belong inside a suspend function or on the main thread.

A coroutine checks cancellation. Do not catch Exception and ignore CancellationException. Rethrow it. A Flow is collected in repeatOnLifecycle or the project's equivalent so it stops when the screen stops. A hot flow shared with shareIn has a stopping policy.

Null and errors

Prefer ?. and a real branch over !!. !! is a crash you chose. A public function returns a sealed result or throws the project's exception. Do not return null for three different failures.

Compose and Android

State that changes the UI is mutableStateOf or a StateFlow collected as state. Remember a callback that closes over a changing value, or pass the event up. Keys on a lazy list are stable ids.

A permission, a camera, or a location request follows the Activity result API the project uses. Do not call a deprecated start-activity-for-result snippet. Main-thread work stays tiny. Database and network use the dispatcher the app already injects.

Tests

A coroutine test uses runTest and a test dispatcher. Advance time instead of sleeping. A Compose test sets content and asserts semantics. Run the project's unit test task. Record the output. Say when a device was not used.

Mistakes to avoid

• GlobalScope or runBlocking on a suspend path.
• A catch that swallows CancellationException.
• !! on a value that can be null.
• A Flow collected without a lifecycle stop.
• A Kotlin upgrade bundled into a UI fix.

What changed in this major

Kotlin 2.4 keeps CancellationException special. GlobalScope is still the wrong scope.

Deprecated APIs to never generate

BroadcastChannel is deprecated and no longer supported. Use SharedFlow.

Commands

Run the Gradle unit test task.

Example

viewModelScope.launch {
  try {
    repo.refresh()
  } catch (e: CancellationException) {
    throw e
  } catch (e: Exception) {
    show(e)
  }
}