If I only have a single % fmt: off without a % fmt: on afterwards, everything after % fmt: off is not printed. This also happens if i use something like % fmt: off fmt: on on the same line.

In the first case i would expect to turn of formatting as long as there is no % fmt: on which would mean if i insert % fmt: off at the beginning of a file the whole file would not be formatted.
In the second case the comment should have no effect at all.

Any chance to also support something like

:- bounds(L,U),0 < L,not counter(1,L). % fmt: skip
% which would be a shorthand for
% fmt: off
:- bounds(L,U),0 < L,not counter(1,L).
% fmt: on

If I only have a single % fmt: off without a % fmt: on afterwards, everything after % fmt: off is not printed.
.. i would expect to turn of formatting as long as there is no % fmt: on which would mean if i insert % fmt: off at the beginning of a file the whole file would not be formatted.

This should be working now as you expect. The unformated code at the end of the file is now written.

This also happens if i use something like % fmt: off fmt: on on the same line.
... the comment should have no effect at all.

I do currently no parsing of the comment, I have a global variable skip: bool that is set to true if a comment contains fmt: off
and if skip is true and a comment contains fmt: on skip is set to false.

What behavior would you expect for % fmt: on fmt: off ?

Any chance to also support something like

:- bounds(L,U),0 < L,not counter(1,L). % fmt: skip
% which would be a shorthand for
% fmt: off
:- bounds(L,U),0 < L,not counter(1,L).
% fmt: on

I guess so. I'm not sure what the semantic would be for a multi line statement should it consider the statement (rule) or the line?

like in

            a(X, Y, Z) :-
       b(X), c(Y) % fmt: skip
    d(Z).

Would the rule stay the same or just the line ?

a(X, Y, Z) :-
       b(X), c(Y) % fmt: skip
    d(Z).

Sry for the delayed answer. I was quite busy.
% fmt: off fmt: on should actually have no effect at all, you could see it as

% fmt: off
% fmt: on

I'm not sure if this case has to be adressed or if it should just print out a warning or something like that.

Regarding 'skip', i would expect that just the line stays the same.