Hey folks - hitting a weird runtime crash in a KMP...

# compose

[Sagar Khurana]

Hey folks - hitting a weird runtime crash in a KMP project and wondering if anyone's seen this before.
Getting IncompatibleClassChangeError on $stable when kotlinx-serialisation tries to deserialize plain DTO classes at runtime:

java.lang.IncompatibleClassChangeError:
Expected 'int food.boo.bar.data.remote.dto.CategoryDto.$stable'
to be a instance field rather than a static field
  at CategoryRepository.getCategories(CategoryRepository.kt:47)

The offending class is completely vanilla — no Compose annotations, no custom serializers:

@Serializable
data class CategoryDto(
    val id: Int,
    val name: String
)

Reproduces across three different toolchain combos:

Kotlin 2.4.0-Beta2 + Compose MP 1.10.3 + KSP 2.3.7
Kotlin 2.3.21    + Compose MP 1.10.3 + KSP 2.3.7
Kotlin 2.3.10    + Compose MP 1.10.1 + KSP 2.3.6

Other context:
• This is a debug build — no R8, minification is disabled, so it's not a shrinking artifact
• The Compose compiler plugin is applied to the whole app module with no stability configuration, so it's instrumenting every class in commonMain, including these plain DTOs
• The crash only fires during deserialization (construction via the serialization-generated factory), not when constructing the DTO manually
Is this a known interaction between the Compose compiler plugin and the kotlinx-serialization plugin's generated constructor code? Any idea what changed recently that could cause $stable to land as a static field when the serialization-generated init path runs? Happy to file a YouTrack issue with a reproducer if helpful.

[Asapha]

Ah, I've just filed a bug report for it!
See https://youtrack.jetbrains.com/issue/KT-85963

[Sagar Khurana]

😭 its blocking my release really bad

[Asapha]

It doesn't happen on versions prior to 2.4.0-Beta2 on my end, though

[Sagar Khurana]

lme try that again i recently switching to SPM from cocoapods thats why i bumped the kotlin version

[Asapha]

Try to see if applying the serialization plugin before the compose one changes anything

plugins {
  […]
  alias(libs.plugins.kotlinSerialization) 
  alias(libs.plugins.composeCompiler)
}

[Sagar Khurana]

trying that out

[Sagar Khurana]

That fixed it! 🎉 Swapped kotlinSerialization to apply before composeCompiler in the plugins {} block of my composeApp/build.gradle.kts, did a clean install, and the IncompatibleClassChangeError

Confirming on Kotlin 2.3.21 + Compose MP 1.10.3 + KSP 2.3.7

@Asapha thanks, god may bless you