I might be a little confused about how the ability works. I've never actually played a spellthief before. So far as I can tell, you can name a spell when you use the ability (you didn't) and if the target has that spell, you steal it, otherwise you get a random spell of any level you can steal. When you get Discover Spells at 13th level, you learn the target's spells prepared/known after the first time you use the ability.

I'm open to the idea I'm wrong on this, though. Can you explain exactly how you think the ability works?