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It has something called "Custom Words" which might be what you are describing. Haven't tested this feature yet properly.

So is this already in Handy or you are referring to a feature of the underlying models you are still not actively using?

This is already in Handy in Advanced > Custom Words.

There is also Post Processing where you can rerun the output through an LLM and refine it, which is the closest to what Wispr Flow is doing.

This can be found in the debug menu in the GUI (Cmd + Shift + D).