In addition to my question, I see that gradle task ':copyMainKotlinClasses' is missing in 1.1.51

Hi. It's the intended change: in Gradle 4.0, separate classes directories were introduced (https://docs.gradle.org/4.0/release-notes.html#multiple-class-directories-for-a-single-source-set). Earlier, the classes from all the JVM languages that are used in the project were put into a single directory. With Gradle 4.0+, the previous behavior has been deprecated, each language now has its own separate classes directory, which is useful for incremental builds and caching. Kotlin supports this starting with version 1.1.4 (https://youtrack.jetbrains.com/issue/KT-18544) , and the classes are by default placed in

build/classes/kotlin/<sourceSetName>

. Please consider using the new API and consume classes with

sourceSet.output.classesDirs

and do not expect the Kotlin classes to be put into the Java classes directory.