1.21.1 beta 1: experimental support for Minecraft 1.9.0 beta

Unlock Mojang's experimental scripting API on 1.9.0 beta.

Known issues:

ModPE scripts are not enabled on 1.9.0 beta.
It will be added before 1.9.0 is released.

Importing .mcworld worlds doesn't work yet.
Please import worlds through Minecraft, then open them in this application.