NM> from mystic_bbs import *
 NM> writeln('just a test')
 NM> menucmd('GI')
 NM> 
 NM> When I invoke it with GY from a menu entry I see Just a test but nothing
 NM> else happens.

You are not calling the menucmd function properly.  It has two parameters the first is the menu command and the second is data. 

Change it to this and it should work:

menucmd('GI', '')

... "No comment" is a comment.

--- Mystic BBS v1.12 A49 2023/01/31 (Windows/64)