Before a start date is agreed, if possible. A mixed offer can require the employer to redefine the position, which takes internal approvals, and that process is generally slower than gathering documents. Raising it early keeps the option open.
Raise the duty question while the offer is still being shaped
Employers can usually adjust a role before it is posted internally or budgeted, and much less easily afterward. Bringing the question up during offer discussions frames it as clarifying scope rather than challenging a signed agreement. If the position cannot change, that is useful to know early as well, since it lets the applicant consider other routes without a start date already committed.
Build a dated review file around the intended start date, current authorization record, employer changes, travel plan, and any I-129 filing evidence. A USCIS petition takes weeks or longer, can be expedited for a fee, and produces an approval notice that an employer can rely on before the applicant resigns. Canadians generally request TN admission at a port of entry or have an employer submit Form I-129; neither occurs automatically when a scheduling email names a site. The employer should revise the start date or duties instead of asking anyone to backdate a transcript or support letter.