@Evil_1993
Try replacing STDIO with $$arcdatafile$$.tmp $$arcpackedfile$$.tmp
e.g.
Code:
[External compressor:zstd]
header = 0
packcmd = zstd\zstd.exe {options} $$arcdatafile$$.tmp $$arcpackedfile$$.tmp
unpackcmd = zstd\zstd.exe -d -f -T2 $$arcpackedfile$$.tmp $$arcdatafile$$.tmp