Eater's The Definitive Guide to Tipping at Any Restaurant in America provides some guidance on these situations:
Fast-casual counter service: 20 percent
Cities across the country have seen an explosion of fast-casual
options, where guests order at a counter but perhaps a runner delivers
food and a busser clears it. In restaurants like this, tip 20 percent.
“It seems casual, but everyone is pulling their weight,” says Moonlyn
Tsai, co-owner and operator of the smash-hit fast-casual operation
Kopitiam in New York City. “I just wish people were more aware of the
work they’re doing: they bus tables, they run food, it’s more than
taking orders. They keep the space tidy.” It’s changed how she thinks
of tipping at other similarly set-up restaurants too. “Before working
in this setup, I’d tip between 15 and 18 percent. Now I always tip 20
on the total.”
Tsai says that over half of customers at Kopitiam tip, and it’s not
uncommon to see a guest opt not to tip when ordering, but then leave
cash at the end of the meal, after seeing how much the staff does.
It might feel different at the massive fast-casual chains where diners
are served cafeteria-style, but even then, do 20 percent if you can
stomach it. The people working there deserve it. [NB: Operators, if
you are serving guests from an assembly line, please shift to a
tip-free model.]