When should the builder join the design team?

We are at the beginning of schematic design for a custom home and are trying to define the architect-builder handoff before the drawings advance too far.

Our architect prefers to develop the design through schematic design and much of the construction-document phase before bringing a builder on board. The argument is that the design will be easier to evaluate without early pricing pressure. I am concerned that waiting could mean discovering constructability problems, structural complications, or unrealistic allowances after major decisions are already embedded in the drawings.

We have not finalized the structural system, several site-related issues remain unresolved, and the major exterior and interior finish selections are still open. We are also undecided between a fixed-price contract and a cost-plus arrangement. Those choices seem important because a builder involved during schematic design might provide useful estimating and sequencing input, while a builder brought in later might be able to price a more complete scope with fewer assumptions.

For those who have managed a custom build, when did you formally involve the builder or general contractor? Did you use preconstruction services during schematic design, obtain preliminary estimates at each design milestone, or wait until the construction documents were nearly complete?

I am especially interested in how the timing affected four things: the reliability of early pricing, the ability to change the design, responsibility for revisions caused by constructability issues, and the clarity of the architect’s and builder’s contractual roles. If the builder participates early, how did you prevent that input from quietly turning into design ownership or an implied commitment to a later construction price?