Image: JetBrains
Why it mattersLibrary authors who publish for JVM, Android, iOS, and Wasm from one codebase get an official publishing path, and Kotlin gains a real command-line build process instead of depending on IDE actions.
JetBrains released Kotlin Toolchain 0.12 on 3 September 2026. The version brings first-party multiplatform library publishing, a preview of building Wasm applications, and Compose Hot Reload driven from the command line. It also raises the minimum requirements: JDK 17 or higher, Kotlin compiler 2.2.20, and IntelliJ IDEA 2026.2.1 with the latest Kotlin Toolchain plugin.
Multiplatform library publishing
Authors who publish for more than one target can now publish a common API and one artifact per platform through a single configuration in the toolchain, along with sources and module publication metadata. JetBrains says cinterop bindings are supported and published both commonized and per-platform. The klibs.io service is now built on the same toolchain, which is a useful signal that the pipeline is meant for real library work rather than sample apps.
The release also changes what the toolchain publishes by default. To comply with new Sonatype quotas on Maven Central, .asc.sha1 signature checksums are no longer written by default. Only .md5 and .sha1 are automatic; anyone who needs the older checksums has to opt in.
Wasm apps you can run
wasm-js/app modules can now build into ready-to-use web applications. That means resource customization, npm dependency fetching, and execution through kotlin run. This is a preview, so treat it as a place to try Wasm workflows rather than a production target, but it is the first time Wasm apps are a fully supported output from the toolchain.
Compose Hot Reload from the CLI, with MCP for agents
Compose Hot Reload now runs from the command line with kotlin run --compose-hot-reload-mode. Initial reloads are faster than before, and the release adds an MCP server that lets AI agents control hot reload sessions. For anyone using Claude Code, Codex, or another MCP-aware agent inside a Compose project, that is the difference between an agent that can only edit files and one that can trigger the reload and observe the result.
Default versions and requirements
Kotlin 2.4.10, JDK 25, Spring Boot 4.1.0, Ktor 3.5.2, KSP 2.3.11, and DataFrame 1.0.0-rc01 are the new defaults. The increase to JDK 17 as a minimum will cause problems for anyone still running JDK 11 or 8 in their build chain.
The main change is the publishing path. Kotlin Multiplatform has been usable for library work for a long time, but the setup for actually publishing across JVM, Android, iOS, and now Wasm has been a job in itself, built from community plugins and a lot of Gradle configuration. Having JetBrains own that path removes a class of maintenance work that never really belonged to any team.
The MCP support in Compose Hot Reload is the smaller but more immediate change. If a team is already using coding agents on a Compose project, an agent that can only edit the file and wait for a human to trigger a reload is a slower agent. Giving it the reload as a tool call shortens the loop, and that makes the difference between a demo and a workflow.
The Maven Central checksum change is a maintenance note rather than a feature, but it is the kind of unannounced change to a default that breaks some CI job. Anyone whose release pipeline verifies .asc.sha1 should adjust their configuration before the next publish.
Source
Primary source: Kotlin Toolchain 0.12: Multiplatform Library Publishing, Wasm Apps, and More.
This item was written by an AI system from the linked source. Reveneau is responsible for what it publishes.
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