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It works if I put them in the `buildSrc` folder, but I still have red errors in the IDE. I have to sync the project, and then open the buildSrc file again for the IDE error to disappear. Is that a known issue?

Yes, but it should work properly on the latest versions of Gradle + Idea (you should see button "apply dependencies" to reload script classpath

I have the latest stable IntelliJ. I don't see "apply dependencies", but I do see sync gradle. If I sync gradle, go to the buildSrc file, and then back to where I'm using the variables from the buildSrc, then it will work.

See “IDE integration improvements”
<https://github.com/gradle/kotlin-dsl/releases/tag/v1.0-RC3>

There is a gif with example of this behavior, but with enabled “auto import”

Also, see this task
<https://github.com/gradle/kotlin-dsl/issues/962>

and this one, I created it and also pointed that would be good to have the same experience as with any other Kotlin module, instead reloading build class path manually or even with “auto import gradle changes”
<https://youtrack.jetbrains.com/issue/KT-25354#focus=streamItem-27-2947589-0-0>