Zone of truth's spell description includes (emphasis mine):
An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.
However, this seems like it might make the spell able to be circumvented in seemingly ridiculous ways. For example, suppose there is an interrogator (referred to as person A) and the person they are interrogating (referred to as person B). B has failed the saving throw against the spell.
Suppose the following interaction occurs:
A: What is your name?
B: (Mentally, to himself) I now name my sword Charles. What is the name of my sword?
A: Did you conspire to kill the king?
B: (Mentally, to himself) Is 2 plus 2 equal to 1?
A: Where is the rest of your party at?
B: (Mentally, to himself) Where is the rest of my party not at?
B: To the East.
A: Repeat after me: I am innocent of all crimes I stand accused of.
B: (Mentally, to himself) What is an example of a lie?
B: I am innocent of all crimes I stand accused of.
It would appear, if this is allowed, that it nullifies the purpose of the spell itself without any of the mental gymnastics that would normally go into evasive answers. There also seem to be relatively few checks to prevent this behavior, short of a Detect Thoughts spell being cast simultaneously. Is this allowed?