Discovery works both ways. If you want to present an alibi defense at trial, you have to provide the prosecutor with information about that alibi. Defendants have to provide a list of witnesses they intend to call, and the prosecutor may depose those witnesses beforehand. There are certain exceptions - for example, if a prosecution witness lies on the stand, the defense may call a witness not on the list to rebut that witness’s statements and impugn their credibility.

Basically, neither the prosecution nor the defense are allowed a “hole card”. The “hand” is played open.