** Using WeBWorK **

Yes, you are right to put back the original student answer in that case. (It would be possible to put back the stringified MathObject, if one was actually produced, as is the case when the error message is for reasons other than a syntax error, but it is probably not worth it.)

You can, of course, put your post-filter in a macro file as

   $noMessages = sub {
      ...
    };

and then use

    ANS($ans->cmp->withPostFilter($noMessages));

in the problem, to make it easier.

I understand your pedagogical need, and was not suggesting that you not modify the problems, only that they not be distributed as part of the NPL, where people might not realize their special purpose.

Good luck with the quizzes.

Davide