It is 11-speed only, but compatible with any of Shimano’s other 11-speed road shifters ... test bikes had a compact 50/34t Dura-Ace crankset paired with an 11-32t rear cassette, which provided a very ...

The GRX RX400 rear mech does not work at all with 10-speed Ultegra because ... plus a spare 10-speed chain and 11-36 cassette, I swapped those and the GRX RX400 mech onto that bike. And there it did ...