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.
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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).
Indexing, trimming, or fine-tuning Di2 rear derailleurs is done through micro-adjustment. Read how to set up your Di2 rear derailleur perfectly and ride without those grinding noises.
Read how to install the RD-RX805 / RD-RX815 / RD-RX817 Di2 rear derailleurs.
