Does Zone of Truth bind people to promises?

Question

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:

1. Cleric wanted to make sure prisoners, upon being released, would not inform the guards or the authorities of the whereabouts of the party.

2. 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.

Answer 1

It is not binding.

From the PHB, p289:

... On a failed save, a creature can't speak a deliberate lie while in the radius. ... 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 it remains within the boundaries of the truth.

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.

Answer 2

The questioner is asking a person to give a truthful statement about intent. Intent is changeable based on circumstance. When I wake up in the morning, I can honestly intend to go out and mow the lawn this afternoon. However, when afternoon comes around, I can decide the grass is too wet, my muscles too sore, or simply that I'd rather watch a movie and do it tomorrow. However, if asked that morning, I obviously believed I would mow the lawn.

Now while the interrogator is asking about intent, the Dungeonmaster is actually treating it as a pledge, as if the act of stating the promise aloud has more meaning than intent. The pledge is immaterial, though, it's just a formal re-iteration of intent.

The best the interrogator can hope for when asking, "Do you intend to reveal this secret?" is:

a) "Yes", meaning that the creature fully intends to reveal the secret and thus cannot be released.

or

b) "No", indicating that the creature does not have the current intent of revealing the secret.

if he asks a follow up of, "Do you intend to change your mind?", the only rational answer is "No." If you honestly don't intend to now, you don't intend to change your mind later. If he phrases it as "Will you change your mind?", the best he can hope for is "I don't know", based on the current stated intent. Even a statement of "No" here holds no more meaning, since it is based on intent, and the interrogator has no way of knowing how rigidly the prisoner's mind will treat such a promise. He can only deal with the prisoner's self-knowledge and intent.

So, the interrogator can only ferret out active duplicity. A non-commital answer is the best he can hope for and likely indicates a tendency toward keeping the secret. There is no supernatural agency ensuring future action needs to be compliant with present intent.

Answer 3

The oath is somewhat binding:

This is a classic problem called Kavka's toxin puzzle. I deal with it in the following simplistic way:

Some creatures take the oaths they swear under coercion seriously, while others don't. In my campaigns I rule this to be a function of alignment. In either case, though, they will only be able to swear it if they mean it at the time. In order to break the oath, then, they must violate what was previously in accordance with their will-- the oath is exactly as binding as any other fully-committed decision they make.

The following is the alignment-based adjudication I generally use for this, which has worked across a variety of ethical/jurisprudential system implementations.

Lawful:

A lawful creature is strongly bound by its will, and does not lightly change its mind. In general, in the absence of external forces, a lawful creature will continue unchanged indefinitely. A Lawful creature making an oath in the ZoT will not break that oath unless external forces act upon it. Even a lawful creature, however, may break the oath if sufficient new information is obtained, though doing so would be a Chaotic act.

Neutral (undecided/non-commital): A Neutral creature has agreed to the oath, but doesn't have the strong introspective skills regarding law/chaos that are necessary to actually predict ones future actions in this manner, though they may well think they have those skills. Neutral creatures are very likely to change their minds regarding the oath if any new information is revealed or if the oath proves either more or less inconvenient than they expected. Most neutral creatures will find mustering the commitment to actually make such an oath in the ZoT at least a little taxing.

Chaotic: Swearing an oath of future action while in the ZoT is a strongly Lawful act, requiring that the one so swearing truly believe they will do as they are swearing. Chaotic creatures, like Lawful ones, are aware of their nature, and will find swearing such an act all but impossible; torture and other extreme forms of coersion will be needed to force such a creature to make an oath-- even if they want to agree to do something in the future, they know there's a good chance they won't so long as they are thinking rationally. Even if tortured into submission, however, a chaotic creature will, more likely than not, break the oath anyways, even in the absence of external forces, and even without leaving the ZoT.

I haven't run this on a 'True Neutral' character yet, but such characters are usually just Lawful characters disguised behind some 'Nature' stuff.

Also I generally rule using ZoT this way a CE act under most alignment resolution systems I run.