Before the agency agreement is signed, if possible. Contract terms about supervision, worksite, and duties are far easier to adjust during negotiation than to explain afterwards, and a signed structure that does not support the profession claimed is expensive to unwind.
Review at the draft stage, not at the border
Agencies frequently work to a client start date and press for quick signatures. Building a short review step into that schedule costs days, while discovering a structural problem after the professional has resigned from other work costs far more. Ask counsel what the weakest point of the arrangement is, and whether adjusted contract wording would strengthen it before anyone commits.
Ask what remains open before the support letter can be finalized, and who controls that missing information. Use the final genuine engagement and explain material changes where relevant. Pause preparation when deliverables change and compare the revised work with the profession originally selected. A credential evaluation agency needs transcripts, which the institution sends when it sends them. Employers sometimes assume a Canadian can appear at the border on any day; that is true only when the record is complete.