Updated September 28, 2026
Usually the builder does. Most builders pay a co op commission to the brokerage that brings a buyer, so the buyer does not write a cheque to their agent. Whether that is the case for a given project, and what happens if the builder's commission is less than your agreement says, is set out in the representation agreement you sign with the brokerage.
In Ontario you sign that agreement with a brokerage before making an offer through them. Read the part about how the brokerage is paid.
Whether your agent is registering you with the builder before your first visit, because many builders only pay a co op commission on buyers an agent brought in. Whether the builder for that project pays one at all. What your agreement says if it does not.
The agreement of purchase and sale for a new build is long, the deposits are staged, and the closing costs are set by clauses most buyers have never seen. An agent who works new construction reads those for a living.