# Kotlin v2.4.20-RC — Kotlin 2.4.20-RC

- Product: Kotlin (https://whatsnew.fyi/product/kotlin)
- Vendor: JetBrains
- Date: 2026-08-12
- Version: v2.4.20-RC
- Original notes: https://github.com/JetBrains/kotlin/releases/tag/v2.4.20-RC
- Permalink: https://whatsnew.fyi/product/kotlin/releases/v2.4.20-rc
- Labels: Pre-release

What's New is an index, not a publisher: every entry below links to the vendor's own release notes, which are the authoritative source. Entries are labelled where they are hand-curated sample data, pre-releases, or drawn from a secondary source such as a developer blog.

Reuse: the summaries, labels and curation here are © What's New. Quote freely with attribution and a link back; wholesale republication of the corpus is not permitted — terms: https://whatsnew.fyi/terms. The vendors' own release notes remain their publishers'.

---

- **added** — Add support for -XheaderModeType=compilation to improve header compilation performance
- **added** — Add -jvm-target to K2JKlibCompilerArguments
- **added** — Allow disabling firAggressivePrunner for Jklib
- **changed** — Turn on incremental compilation by default for Kotlin Native
- **changed** — Compile against Gradle API 9.7
- **changed** — Run Gradle integration tests against Gradle 9.7
- **changed** — Use DeserializationStrategy.WITH_INLINE_BODIES when deserializing dependencies
- **fixed** — Fix ClassCastException when a suspend function's result is discarded in a when-branch
- **fixed** — Fix ClassCastException on member extension property delegated to bound property reference
- **fixed** — Fix kotlin.NoWhenBranchMatchedException on Kotlin Native with sealed interface
- **fixed** — Fix composable singleton in function with type parameters
- **fixed** — Fix miscompilation of cross-module calls to getters and setters
- **fixed** — Fix KClass.members failing on Java class with self-referential raw type bounds
- **fixed** — Fix MessageCollector in IncrementalCompilerRunner dropping diagnostics IDs
- **fixed** — Fix StringIndexOutOfBoundsException in power-assert when assert() wraps a context-parameter function call
- **fixed** — Fix IndexOutOfBoundsException in ComposableTargetAnnotationsTransformer when overriding a @Composable function with default parameter in another module
- **fixed** — Fix KotlincOutputParser failing to parse compilation errors under Gradle when task path prefixes are enabled
- **fixed** — Fix each build leaving a strong reference to an instance of BuildMetricsService
- **fixed** — Fix dumpXcodebuildArgs task failing if the root build directory is removed
- **fixed** — Fix K2 scripting crash when using class from @file:Import-ed script

##### Changelog

###### Backend. J2KLIB

- [`KT-88165`](https://youtrack.jetbrains.com/issue/KT-88165) Add Support for -XheaderModeType=compilation to improve header compilation performance
- [`KT-86239`](https://youtrack.jetbrains.com/issue/KT-86239) [JKLIB] Investigate which type system context is needed in JklibIrLinker
- [`KT-88306`](https://youtrack.jetbrains.com/issue/KT-88306) [JKLIB] Unbound symbols for kotlin.coroutines.SuspendFunctionN
- [`KT-88120`](https://youtrack.jetbrains.com/issue/KT-88120) [JKLIB] Add -jvm-target to K2JKlibCompilerArguments
- [`KT-88176`](https://youtrack.jetbrains.com/issue/KT-88176) [JKLIB] DeserializationStrategy.WITH_INLINE_BODIES when deserializaing dependencies 
- [`KT-88163`](https://youtrack.jetbrains.com/issue/KT-88163) Allow to disable firAggressivePrunner for Jklib

###### Compiler

- [`KT-87499`](https://youtrack.jetbrains.com/issue/KT-87499) ClassCastException "Float cannot be cast to Unit" when a suspend function's result is discarded in a when-branch
- [`KT-87983`](https://youtrack.jetbrains.com/issue/KT-87983) CCE on member extension property delegated to bound property reference
- [`KT-85403`](https://youtrack.jetbrains.com/issue/KT-85403) kotlin.NoWhenBranchMatchedException on Kotlin Native with sealed interface
- [`KT-87922`](https://youtrack.jetbrains.com/issue/KT-87922) K1FakeDescriptorsForReferencesKt classes duplicated between kotlin-scripting-compiler-embeddable and kotlin-build-tools-impl
- [`KT-86657`](https://youtrack.jetbrains.com/issue/KT-86657) Native: turn on incremental compilation by default

###### Compose compiler

- [`b/543684938`](https://issuetracker.google.com/issues/543684938) Fix composable singleton in function with type parameters
- [`b/537617330`](https://issuetracker.google.com/issues/537617330) Fixed a bug that caused miscompilation of cross-module calls
      to getters and setters, causing spurious runtime errors.

###### JVM. Reflection

- [`KT-87369`](https://youtrack.jetbrains.com/issue/KT-87369) Reflection: KClass.members fails on Java class with self-referential raw type bounds

###### Tools. Build Tools API

- [`KT-88348`](https://youtrack.jetbrains.com/issue/KT-88348) MessageCollector in IncrementalCompilerRunner drops diagnostics IDs
- [`KT-87743`](https://youtrack.jetbrains.com/issue/KT-87743) Make `KotlinCoreEnvironment.disposeApplicationEnvironment()` unnecessary for K2 BTA compilation

###### Tools. CLI

- [`KT-87868`](https://youtrack.jetbrains.com/issue/KT-87868) In-process compilation via CLI/Build Tools API pays a large ThreadMXBean.getCurrentThreadUserTime penalty on Linux: PerformanceManager.measureSideTime measures unconditionally around every class file read

###### Tools. Compiler Plugins

- [`KT-88179`](https://youtrack.jetbrains.com/issue/KT-88179) Power-assert: IrGenerationExtensionException (StringIndexOutOfBoundsException) when assert() wraps a context-parameter function call

###### Tools. Compiler plugins. Compose

- [`KT-88028`](https://youtrack.jetbrains.com/issue/KT-88028) Compose compiler: IndexOutOfBoundsException in ComposableTargetAnnotationsTransformer when overriding a `@Composable` function with default parameter in another module

###### Tools. Gradle

- [`KT-87994`](https://youtrack.jetbrains.com/issue/KT-87994) Run Gradle integration tests against Gradle 9.7
- [`KT-87993`](https://youtrack.jetbrains.com/issue/KT-87993) Compile against Gradle API 9.7
- [`KT-87256`](https://youtrack.jetbrains.com/issue/KT-87256) KotlincOutputParser fails to parse compilation errors under Gradle when task path prefixes are enabled (Kotlin 2.4.20-Beta1)
- [`KT-88084`](https://youtrack.jetbrains.com/issue/KT-88084) Each build leaves a strong reference to an instance of BuildMetricsService
- [`KT-87911`](https://youtrack.jetbrains.com/issue/KT-87911) Report an actionable diagnostic when .kar files reach compilation classpaths in Kotlin 2.4.20
- [`KT-86040`](https://youtrack.je

_[Truncated at 4000 characters — full notes: https://github.com/JetBrains/kotlin/releases/tag/v2.4.20-RC]_