How much support does a deep island overhang need?

I’m trying to balance comfortable seating with keeping the underside of the island visually open. The plan currently shows a 15-inch overhang for stools, using a 3 cm quartz countertop over a seating area that spans the island cabinets rather than an unsupported end.

I’m unclear about when the cabinet structure is enough and when the quartz needs steel brackets, corbels, or reinforcement inside the island. Adding supports could reduce legroom or affect the finished appearance, but I also don’t want the countertop installer to discover too late that the overhang needs a different plan.

For a 15-inch overhang on 3 cm quartz, what should be confirmed with the cabinetmaker and countertop fabricator before the cabinets are installed, and does the answer change if part of the seating extends past an island end panel or unsupported space?