Stairs - oak risers /tread
 

I put in my last tread a few weeks ago (not that carpet is out of that area) and I cut it a tad short <1/8" :( . Any suggestions on masking this small gap? Since it meets with a wall here I was thinking maybe a little mud to mate then caulk it like all the others..

Stain Putty is a powder mixed with the stain used on the project for a very close match. Although, in your case I would cut a piece of oak to fill the gap and install that.

If it were my home and it were only one tread, I would tear it out and redo it. Mud and putty will eventually crack, particularly on stairs. I know my eye would go to that spot every time I used the stairs.