The "verify-comments" setting:

This setting determines how much sanity checking, if any, the "fossil commit" and "fossil amend" commands do against check-in comments. Recognized values:

on

(Default) Check for bad syntax and/or broken hyperlinks in check-in comments and offer the user a chance to continue editing for interactive sessions, or simply abort the commit if the comment was entered using -m or -M

off

Do not do syntax checking of any kind

preview

Do all the same checks as "on" but also always preview the check-in comment to the user during interactive sessions even if no obvious errors are found, and provide an opportunity to accept or re-edit