Kotlin 2.5.0-Beta2 Adds Experimental Companion Blocks and Full Value Classes, a .kar Library Format and an Error Ahead of a JDK 17 Compiler Minimum

Overview

Kotlin 2.5.0-Beta2 is an early-access pre-release, not a stable version. JetBrains’ Kotlin early-access page lists it as released on October 21, 2026, and describes it as “A language release with major changes in the language and tooling updates.” The GitHub release is marked as a pre-release. This article summarizes what the What’s new in Kotlin 2.5.0-Beta2 page lists. The Herald has not built or run the release. The previous feature release was covered in Kotlin 2.4.20.

Language

According to the What’s new page, four language features become Stable in this release: improved compile-time constants, the @IntroducedAt annotation for generating version-based overloads for optional parameters, the unused return value checker (now enabled by default), and name-based destructuring in only-syntax mode, which is now the default.

The same page introduces two Experimental features. Companion extensions and companion blocks remove the requirement that a type have a companion object: a top-level extension can take the companion modifier, and a companion {} block inside a class or interface can hold functions and properties without creating an object instance. The page says that on platforms with static members the compiler generates these as static members, so they need no @JvmStatic annotation on the JVM. Opting in requires the -Xcompanion-blocks-and-extensions compiler option.

The page also describes experimental full value classes, where the properties together make up the value and the identity of individual instances does not matter. The compiler generates equals(), hashCode() and toString() for them, similar to data classes, but there are no componentN() functions and no copy() function. Opting in requires -Xfull-value-classes. Because of this, the page says, value class “no longer necessarily means inline.” It describes a staged migration: Kotlin 2.5.20 will report a warning for unmarked single-field value classes, and Kotlin 2.6.0 or 2.7.0 will raise that warning to an error.

A further opt-in feature, enabled with -Xstrict-equality-for-structural-classes, makes the compiler report == comparisons that always return false (or != comparisons that always return true) involving data classes, data objects, final value classes and enum classes, based on the operands’ known types, according to the What’s new page.

Library, platform and tooling changes

The What’s new page lists an Experimental CodePoint inline value class and related APIs in the kotlin.text.unicode package, and Experimental onTrue(), onFalse() and ifOrNull() functions for Boolean values. It also lists support for generating Java 28 bytecode, an es2020 target for Kotlin/JS, incremental compilation of klib artifacts enabled by default on Apple targets, and Swift export moving to Beta with a new Gradle DSL.

For multiplatform library authors, the page describes a new publication format that publishes platform artifacts, except JVM and Android, as a single .kar archive, a ZIP archive additionally compressed with XZ. It says the change addresses publication size limits recently introduced to Maven Central, and that publishing in the new format is still Experimental while consuming such artifacts is available for all projects. As an example, the page says the cryptography-kotlin library’s published artifact set went from 7539 files and 91 MB to 1036 files and 23 MB for the whole multi-module project. That figure is JetBrains’ own example for one library, not a general result.

The page also says cross-module inlining during klib compilation is now enabled by default, and that reduced constraint generation in type inference may change inferred types, overload selection or diagnostics in some edge cases; the previous behavior can be restored with -XXLanguage:-EliminateSecondKindIncorporation.

Breaking changes

The page says Beta2 “introduces an error as the next step to raising the minimum JDK version required to run the Kotlin compiler from JDK 8 to JDK 17.” The error can be disabled with the -Xallow-pre-17-runtime-jdk compiler option, which the page says will be removed in Kotlin 2.5.20 or 2.6.0 when JDK 17 becomes mandatory.

Changelog scale

The Beta2 GitHub release carries a changelog that includes the entries “CodePoint value class” and “Implement Kotlin Archive (.kar) publication format”. In the Herald’s own comparison of the issue IDs listed in that changelog and in the Beta1 release notes, 19 IDs appear in both lists.

What We Don’t Know

The sources do not give a date for the stable Kotlin 2.5.0 release. Experimental features, including companion blocks, full value classes and the .kar publishing format, may change before then. Independent benchmarks or third-party reports on Beta2 were not found in the sources reviewed.