Error: unrecognized iuse function MUTAGEN

This is kind of a continuation from my other thread about “threshold_mut”… sort of.
So now I am getting the following error on the Arcana mod:

Which I believe I have pinpointed to the “use_action” function for tools.json

But now the question is, why is this giving an error now? And how would I be able to fix it myself?

iuse action was renamed from MUTAGEN to mutagen (in lowercase), so you need to changed that - probably some more changes will be required to make everything to work properly.

See all changes in

Update: I replaced MUTAGEN, in all caps, to mutagen, in all lowers, and that did the trick. Nonetheless I found an error with “MUT_IV” later, but replacing that with “mutagen_iv” was good enough.

Nocts’ Cataclysm++ mod too showed the “MUT_IV” error in tools.json, as well as the mod “Mutation changes” in the file mutations.json (this one also had MUTAGEN in all caps in the same file).