Stairs are a whole different beast when it comes to carpet tucking-tight spaces, odd angles, and if you mess up, everyone sees it. I learned this when I re-carpeted my basement stairs last year; the first knife I used was totally wrong for the job. So here's what you need to look for:
First, a compact, narrow blade. Stairs have those tiny gaps between the tread (the part you step on) and the riser (the vertical part), so a big, wide blade won't fit. Look for blades that are 2 to 3 inches long-small enough to squeeze into those tight spots but still sharp enough to get the job done. The Roberts 10-292 I mentioned earlier works, but if you want something even more stair-specific, the Stanley FatMax Carpet Tucking Knife has a super slim blade that's perfect.
Then, a ergonomic grip is even more important here. When you're tucking stairs, you're leaning over, twisting your wrist, and reaching awkwardly-if the grip is uncomfortable, your hand will cramp fast. Foam or rubber grips that contour to your hand are best; they reduce strain, even when you're tucking 10+ stairs.
Also, look for a knife with a stiff blade. Stairs need firm tucks-if the blade bends when you push, you won't get the carpet into the gap properly. A stainless steel blade that's a little thicker (but still slim!) will hold its shape. And avoid cheap plastic handles-they'll break when you put pressure on them, which you will do with stairs.
Pro tip: Don't use a retractable blade for stairs! You don't want the blade sliding in and out mid-tuck-trust me, that's how you end up with a loose edge that trips people. Go for a fixed blade that's secure; it'll give you way more control.
