Gemma3 1B seems to be able to choose which tool to use for very simple cases, if you constrain using anyOf, and narrow it down to just a few with RAG first.

It can't understand numbers very well though, "one thousand five" might become "1500".

JSON constraints seem to make them unable to figure it out even if they'd normally get it every time.

Maybe it's different with models above 4B though.