From mobile apps to microservices, from small startups to big enterprises, Gradle helps teams build, automate and deliver better software, faster.

Gradle Community

Assuming you are using GradleRunner, configure it with `withPluginClasspath()`.

Hmm that makes sense, but I'm trying to avoid it and use ProjectBuilder

Fair enough.  Ensure your plugin is built using `kotlin-dsl` plugin - this pulls in everything necessary, which the test configuration will pull from.

This:
```plugins {
    // this pulls in the appropriate Kotlin versions and applies java-gradle-plugin
    // DO NOT separately use kotlin("jvm")
    `kotlin-dsl` 
}```

I had kotlin-dsl already, but I was also applying `kotlin('jvm')`. Removing it as you said fixed the error. Do you know why?

no worries.  kotlin(“jvm”) is for use when developing a Kotlin-based application or library; kotlin-dsl is for developing a Kotlin-based Gradle plugin.  Mixing the two is bad news - conflicting Kotlin versions is the most common problem (Gradle has an embedded Kotlin version used for build scripts).