Mountain weather can be brutal, so you need top-notch protection from a technical shell. You want a jacket with fabric that boasts a high waterproof rating (20K+) and a highly breathable membrane, ample durability, and a construction that includes fully taped or welded seams.
Technical shells intended for backpacking don't need many bells and whistles, but pockets that are placed higher so you can access them when you're wearing a backpack can come in handy. Jackets you'll wear climbing should also have helmet-compatible hoods and good wrist closures to seal out storms.
Because technical shells have to fit comfortably when you're wearing a backpack or a climbing harness, they tend to feature a fit that's on the slimmer side of regular (this varies by brand). Be careful not to choose a fit that's so slim or a size so small that you can't add extra baselayers or an insulating layer.