I'd say a notch above business casual is acceptable. They have a pretty serious maitre d' and keep guest profiles, so if you plan on returning (or want a decent table), dress for success. Wearing flip flops or shorts will get you 'gray listed' -- same food, same pro service, but not someone they want seated next to a senator.
It's a high-end steak house. A Polo shirt and khakis or nice jeans, or even nice shorts would be OK for men. Tee shirts, flip flops probably would be frowned on but I doubt they would turn you away.
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