Are they compatible?

Yep

What he said.

good. i’ve already ordered one.

Yes

… but might depend on what mech you’re using – will it take the extra chain links (assuming you’re having a bigger range). In fact it might not clear a very big cassette at all if it’s an oldish road one

already got an 11-32 on with plenty to spare. its for one more climbing gear for the mrs.

I have used Sora STI’s with an Ultgra mech and MTB cassette for the last few years on my CX bike. No issue. I’m set up with road chain rings (double, no granny chain ring).

Might need a bodge if its a MTB front mech, rear will be fine.

I have just such a set up on my Uncle John. The rear mech (Deore) works fine with the shifters although the front mech (also Deore and a MTB chainset) never really shifts cleanly onto the big ring; the shifter just can’t seem to pull enough cable.

the front mech (also Deore and a MTB chainset) never really shifts cleanly onto the big ring; the shifter just can’t seem to pull enough cable.

There is a bodge – something to do with cable routing or clamping at the mech. I’m sure someone will come along shortly and explain it.