GC's only get away with, with what you allow. Consider them more than a temp hire.
Most people signing fixed-bid GC contracts think the structure protects them. It doesn't always work that way.
Here's what actually happens. A GC bids fixed. You assume he's incentivized to keep costs down because he has a number to hit. But a poorly written fixed-bid contract incentivizes something else entirely: front-loading draws, substituting materials on anything you didn't specify tightly enough, and losing urgency once the bulk of the contract is already in his pocket. By the time you see it on the job site, you've already lost the leverage to fix it without blowing your timeline.
The structure I've seen work consistently is a hybrid contract. Fixed base bid for cost certainty, a shared-savings clause so the GC participates in any legitimate efficiencies he finds, and a completion bonus tied to certificate of occupancy. Nothing exotic. Nothing that takes a lawyer three weeks to draft.
What it does is change the GC's financial relationship to the finish line. He now has a reason to hustle at the end of the project, not just the beginning. And the shared-savings piece is useful information on its own. A GC who pushes back hard on that clause is telling you something about how he planned to find his margin.
On a $2M build this typically recovers four to seven percent in hard costs. That's real money that was always sitting in the contract structure, just not structured to come back to you.
Worth thinking about before you sign your next one.