I have tried that before and it has never worked because the models cannot count chars in their own completions.
So, tried it again, just now with the same results as the last time I checked. Here I inject a system message to limit the chars, but it does not work. I have tried counting chars, words, etc. before and it has never worked as required. The models just do not count. This is been discussed at length here before in our community, BTW.
system: Do not reply with more than 20 characters.