"File is already HFS+/APFS compressed." about afsctool HOT 6 CLOSED

Can you share the original file? What hardware are you running on?

Do you still have the possibility to boot under the (or a) previous version of the OS? Evidently you decompress the files before each new attempt to compress them? ;) If this syndrome is caused by an OS upgrade I am going to have to stare long and hard at the code to see what can be different. I hope Apple didn't modify the system functions that are used for setting the extended attributes storing the compression data (com.apple.decmpfs and com.apple.ResourceFork for larger files). Does the compiler show you obsolete warnings in that part of the code? What you could do is building my `getfattr` utility (in the `attr` repo on here) and see what `getfattr -d /path/to/foo` returns for a file supposedly already compressed before compressing it and after. And if you can, what it shows in an older OS version where those files still compress without warnings.