The exact limits of what witches on the Disc are able to do haven't really been defined, and it's probably safe to say that theoretically, there are none. There are practical limits, however, because all magic has its cost. You can't just do something without paying the price, and if you try to, it'll catch up with you eventually. Agnes is just a beginner, anyway, so she can hardly do anything. She's had some instruction from Granny and Nanny -- mostly them yelling at her when she's doing it wrong -- and has a whole library of books and notes in the cottage to learn from, but she's only been at it for a few months. She knows some herb lore, can do some simple spells, that sort of thing. It doesn't matter much, anyway, because Diamanda had been right about one thing -- "real" witchery was about three quarters trickery, one quarter intimidation, and only the tiniest fraction of the time did they do any real, actual magic.

In addition to more normal sorts of things -- brewing potions, flying broomsticks, etc. -- Agnes's magic has enhanced her vocal talents. She can sing in harmony with herself, mimic anyone, throw her voice anywhere in the room, hit notes that only dogs can hear, and all sorts of other things.

Since coming to Fandom, she's been reading up on magic from books in the library and the Magic Box, and has been learning quite a bit, although she hasn't actually practiced much.