Cosi’s Trickster’s ability triggers when an opponent shuffles their library because that player was instructed to do so by a spell or ability that specifically contains the word “shuffle” in its text or (in the case of a keyword ability) in its rules.

The cascade ability doesn’t cause a player to shuffle their library, even if that player revealed their entire library. The revealed cards are put on the bottom of their owner’s library in a random order, but they’re not “shuffled.”

If an opponent’s library is empty or has just a single card in it when a spell or ability instructs that player to shuffle their library, Cosi’s Trickster’s ability will still trigger.