Floors at Android 6, Java 21 and the current Kotlin

Status: accepted, 2026-10-09. Serves Two runtimes, one compiler, beside Floors at the 26 releases, the Swift runtime's. Reopen when an adopter needs an older Android, JVM or Kotlin, when a target beyond the JVM and Android is built, or at the 1.0 review.

Context#

The Swift runtime's floor is the newest Apple releases, because the runtime is built on what they added. The Kotlin runtime is built on the Kotlin standard library, kotlinx-coroutines, the Compose runtime and the AndroidX SQLite driver API, which run on an old Android and on any recent JVM, so no platform release sets its floor. What does is the dependencies on Android, the JDK the artifacts are compiled with on the JVM, and the Kotlin the generated code and the hosts need. Until this record the three lived in the version catalog and the build files alone, and the JVM target was whatever JDK ran the build: a local build on JDK 26 wrote Java 26 class files where the release, built on JDK 21, had written Java 21.

Decision#

• Android 6, API 23: the lowest the Compose runtime and AndroidX SQLite allow, so the floor is the dependencies' and moves when theirs does. The runtime compiles against API 36, and the modules with Compose UI, the inspector and the samples, against API 37.
• Java 21: the runtime and its modules, baton, baton-testing, baton-inspector and baton-okhttp, are compiled with a JDK 21 toolchain and carry Java 21 class files on the JVM and on Android alike, the long-term release the release workflow builds with. The build pins it with jvmToolchain(21), so a build on another machine writes the same files or fails saying which JDK it lacks; building needs a JDK 21 and Gradle 9.
• Kotlin 2.4.20, the current release, moved to each new one with the catalog; an adopter compiles with the same or a newer one. The generated Kotlin and the hosts use what the current language has: data object, value classes, and in a host the $$"""…""" multi-dollar string, which Kotlin 2.2 made stable; the runtime reads kotlin.concurrent.atomics behind its experimental opt-in. An older Kotlin reading a newer artifact's metadata is not tested here and not promised.
• The dependencies at this record: Compose Multiplatform 1.12.1 for the Compose runtime, kotlinx-coroutines 1.11.0, AndroidX SQLite 2.7.1, and for baton-okhttp OkHttp 5.5.0. The numbers' home is kotlin/gradle/libs.versions.toml, which a release moves; this record names the policy and the floors it gave on its date.

Evidence#

• The published 0.14.0 artifacts on Maven Central, read 2026-10-09: baton-jvm-0.14.0.jar and the classes.jar inside baton-android-0.14.0.aar carry class files of major version 65, Java 21, and the AAR's manifest declares minSdkVersion="23".
• The manifests of the Compose runtime's runtime-android 1.12.1 and of AndroidX SQLite's sqlite-android and sqlite-framework-android 2.7.1 declare minSdkVersion="23".
• .github/workflows/release.yml and the CI kotlin job set up JDK 21. A local build on JDK 26 before the pin wrote class files of major version 70; after it, 65 on either JDK.
• No device older than Android 17 has run the runtime: the ingest budget and the end-to-end numbers in BENCHMARKS.md are a Pixel 9's. The Android floor is the dependencies' declaration, not a measurement.

Not chosen#

• Java 17 class files, the Gradle plugin's own target and the usual Android choice: the release has built on JDK 21 since the first publication, Compose for Desktop asks 17 and the Android Gradle plugin accepts 21, so 17 would admit no adopter 21 shuts out and would make the artifacts differ from what shipped.
• A JVM target left to the JDK running the build: a local build and the release disagreed, which is how this record was found wanting.
• Android 5, API 21, Compose UI's floor: the Compose runtime 1.12 and AndroidX SQLite 2.7 start at 23, and nothing of Baton would run below it.
• A Kotlin older than the current release held for adopters: the hosts' multi-dollar strings need 2.2 at least, and holding the runtime to an older compiler would cost what each release adds for nothing an adopter has asked for.