How can I set free compiler args on a Kotlin multiplatform module?

Is it to add experimental annotations whitelist?

yes

There's first class support, see the docs here: https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#language-settings

That setting is it there that allows me to turn on certain experimental features? progressive mode?

Search for "experimentalAnnotation" in that page (cmd/ctrl + F). Just enable it on the sourceSets, and voilà.

oh got it. I see it now. Thanks 🙂