Che cosa significa i'm up to you ?

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I think you mean "I'm onto you". It means that the speaker has discovered some truth about something you are doing. For example, the listener may be arranging a surprise birthday for the speaker but the speaker has figured out what is going on, and says "I'm onto you!"

It can have negative connotations. It can mean that the speaker has figured out that the listener is doing some illegal or morally dubious activity.

Or maybe you mean, "It' up to you". The speaker is asking the listener to decide what to do. For example, your say:

"What restaurant shall we go to?"

And the other person says:

"It's up to you".

They mean that you should decide which restaurant.