They only don’t work in extensions. If you are building a whole copy of App Inventor, you can use aar files. You just need to extract the classes.jar file from it and reference both the .jar and .aar in the @UsesLibraries annotation. The problem with extensions is that the code that produces the extension doesn’t know how to properly handle .aar dependencies, so it fails to pull in everything that would be needed and the buildserver doesn’t know how to process aix files containing aars. It’s an unowned project if anyone would like to contribute it to the open source repo.
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