Rear Derailleur (2x)
If you've been looking at GRX Di2 components you may have seen that there are two rear deraileurs. This RD-RX815 and the RD-RX817. While the RD-RX817 supports a greater cassette range of 11-42T, that specific derailleur is not meant to be used with a front derailleur. It is the 1x-only GRX rear derailleur.
So if you want to use GRX Di2 and run a front derailleur, this RD-RX815 is the way to go. It has a total capacity of 38T, with a top sprocket max of 11T and a low sprocket max of 34T (min 30T).
Both GRX rear derailleurs feature a clutch mechanism, just like the Ultegra RD-RX805 and the mountain bike rear derailleurs. Shimano call this the chain stabiliser. It can be adjusted and prevents chain slapping (or worse, dropping your chain).
Details & Specifications
E-Tube Port type:
GRX RX815 (Gravel)