Nine feet with adequate support at two ends is sufficient. I see ne reason for anything in the middle of the nine feet. If there is a reason, a flat steel cleat should have been added atop cabinetry mid point or near that..........prior to the top being installed on the cabinetry. Unseen, just as the cleat at the end, and it needn't extend the full ten inches. Post a photo.

This is a photo of our quartz bar. The dimensions are 9' long x 16" depth with 10" overhang. The quartz is mitered on the right side and sits on a 2x4 pony wall. Would a wall cleat on the right side wall be sufficient, or do we need metal framing brackets under the counter for adequate support? Our quartz guy finished the job and then mentioned to us on his way out that we needed corbels to support this... (so frustrated)

I meant left side wall in the above comment. (Wall cleat)

Jan, I like your solution for added the wall leg on left side. That would indeed make the bartop look better. Should I worry about the quartz sagging in the middle over a 9' span? The only other solution I can see is pulling the quartz off the bar and starting over with steel supports. Can the quartz be removed without damaging the 9' walnut veneer panel in your opinon?

Your designer and fabricator are idiots on multiple accounts. First, the designer for not designing the bar properly from the beginning. And the fabrication for not raising concerns during templating, when the design could have been altered.

Second, for the fabricator going ahead and installing when he had concerns.

Third, for both of them not even knowing the fabrication specs of the material they are dealing with. Namely, that quartz, being a man made product containing resins, has a longer permissible unsupported span without support than does granite. You can have up to a 15" overhang, unsupported. IF, that 15" represents no more than 1/3 of the depth of the overall top. The Rule of Cantilevering does prohibit your installation without some type of central support in order for that pony wall to not act as a pivot in a lever and have the whole thing come crashing down. If it were counter height, it would be a moot issue, as it would be acceptable for the mass over the cabinets to act as the weight balance for the Cantilever.

At this point, you either add a steel L shaped bracket from the rear, and then reclad the island beck, or you remove the bar top and screw a steel plate to your pony wall top. A bar that people put weight onto by leaning on is not something that you want to risk failing by having only 2 end support points.

Either solution should have the costs evenly split between your designer and the stone fabricator. Because both are equally at fault.

"Should I worry about the quartz sagging in the middle over a 9' span?"

Yes. As installed, that's dangerous. Your fabricator had no business installing this in this manner. At 16" wide and cantilevering 9", you're breaking the 1/3-2/3 rule of cantilevering. You're beyond 50/50. If this top were 27" wide, you'd need no supports because 3cm engineered stone can cantilever up to 14" without additional support.

I'd pull the top and screw 1/2" x 2" extruded steel into dadoes in the wall plate every 16" or so. With end caps and painted black, they'd nearly disappear along with your inevitable sag. No corbels required. Like this:

There are great steel supports for this application and corbels are always knee bangers