Shimano GRX ST-RX400 shifter (10 speed) compatible w/XT RD-M786 (10
 

Is the new*Shimano GRX ST-RX400 shifter (10 speed) compatible with the old mountain XT RD-M786 (10 speed) Shadow+ rear derailleur?

Not without a wolf tooth tanpan for 10/10. You could use the xt 9 speed derailleur with those and you would not need an adapter as the pull is the same as road 9/10.

Edit - the 10 speed that uses 11 speed pull means the above is not accurate. You’d probably want to call wolf tooth but it probably means you’d use an 11 speed MTB derailleur with a 11 speed tanpan and the grx 400 10 speed shifters. I’d definitely call though if that’s what you’re thinking about doing.

Thanks!

What I have discovered is that all 3 GRX groups (RX400, RX600, and RX810) are all based on the Shimano "road" 11 speed derailleurs (just the RX400 is 10 speed but with a pull ratio like Tiagra 4700 which is different than other 10 speed Shimano).*

See: https://productinfo.shimano.com/#/co...-441&cid=C-432

Also, a RX400 rear derailleur can also be used on any Shimano 11 speed road shifter.