So in the game I'm in, a cleric wanted to use Zone of Truth to ascertain the actions of a group of prisoners they were going to release. He wanted to use Zone of Truth to guarantee the course of action the prisoners would take in the future and not to uncover facts or to see through deception. It basically went as follows:
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Cleric wanted to make sure prisoners, upon being released, would not inform the guards or the authorities of the whereabouts of the party.
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At that particular moment in time while under the spell, the prisoners agreed to this.
My question is that after being released, and after the spell had ended, are the prisoners still bound to the effects of the spell cast before and would they still act according to what they said while under the influence of the spell? Using the spell to ascertain facts is one thing, but using it to dictate actions in the future seems iffy.
Best Answer
It is not binding.
From the PHB, p289:
The spell merely makes those affected tell the truth as they know it. As far as the prisoners know at that time, they won't tell any authorities about the party. They're not planning to tell the authorities, but if they're recaptured and tortured for information...
On the other hand, if any of the prisoners can see the future then that would be somewhat more binding (to the extent that the future is fixed): on the path they've seen the future take, they won't tell the authorities.
On the gripping hand, if they've said they're not going to "tell the authorities", they're still free to write a letter to them or tell some 3rd party to tell them, whether they're seeing the future or just truthfully reporting their intentions.
TLDR: Zone of Truth doesn't bind people to keep their promises, it merely verifies their intent to keep the letter of their promise (not necessarily the spirit) at the moment they make it.