Can caster prove truthfulness with "Zone of Truth"

Question

I've seen a lot of posts about how "Zone of Truth" might be defeated by the target. But my character has the opposite problem: he's extremely truthful but people don't always believe him. Is there any mechanism for using Zone of Truth to make verifiably True statements?

If I cast ZoT, I know (to first order) who in the zone succeeds or fails. I would also have to make a save, which I could fail voluntarily. Is there any way I could reliably convince my interlocutors that I did indeed fail (note: this isn't "convince them I'm telling the truth when I'm lying" but "convince them I'm telling the truth when I am."

The benefit of this is illustrated by a binding oath taken some of the magic users of the Wheel of Time fantasy series:

One of the great powers of the Aes Sedai First Oath was that if an Aes Sedai made a statement without evasion, then all knew it to be true and reliable. And if the Aes Sedai said "I am not trying to mislead you, you understand my point correctly" it was more powerful.

Zone of Truth has precisely the opposite power.

Answer 1

No.

This is pretty straightforward. The only person who necessarily gains knowledge on whether or not a creature within the radius of Zone of Truth has been affected by the spell, and therefore is or is not capable of lying, is the caster.

Since the character in question is not considered reliably honest by the others (that's why there's a question of honesty in the first place), the character's own word about whether or not they are affected by the spell is similarly unreliable. If that would work, they might as well not bother with Zone of Truth at all.

This is particularly true if you are hoping for others' views of this character to include a reputation similar to inability to lie. Aes Sedai cannot lie, with magical effects physically preventing them from directly expressing, verbally or in writing, something that they believe is untrue. Trying to use Zone of Truth in that way may even backfire-- without the spell being in effect, people may assume that the character is lying simply because they are eliding the situation in which lying would be impossible to get away with.

The correct solution involving RAW Zone of Truth would be for the character to submit to another character casting the spell, perhaps repeatedly until the character fails the saving throw (voluntarily or otherwise). If your character is really focused on this, building them to be able to produce spell scrolls for Zone of Truth and then giving those scrolls to others so that the spell is as available as possible to other people might be the closest RAW solution involving this spell.

Other options in this area will certainly exist as well, but lie outside of the scope of this question. As quick examples, someone could cast Wish on a character to produce this effect, homebrew spells/curses/feats/magic items can impose it, and so on.

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Experience Note:

I've run NPCs with inability to lie as a trait, and found mechanics surrounding that to be unimportant (players don't trust them, much like Aes Sedai are not much trusted because they can be misleading without lying). The hard part is actually making sure they never lie, because the players/DM at the table have no constraints on lying whether a character does or not.

Answer 2

Statistically, Yes

There is no game mechanic or loophole in the spell that would allow your interrogator to determine that you have failed your save. However, the spell lasts for 10 minutes and requires you to repeat the save every turn you enter or stay in the spell's radius. Therefore, the following strategy is perfectly viable:

1. Cast the spell
2. Enter the spell's area
3. Repeat the same (true) statement every round, 100 times, until the spell expires

100 repetitions of the saving throw gives you about a 99.5% chance to fail at least once even if you have a 95% chance of success each time. This should be enough to persuade a skeptical interrogator. If not, simply repeat the process.

Of course, this technique requires your interrogator to understand how the spell works and to have a decent understanding of probability. Depending on the type of world your game takes place in and the type of person interrogating you, this may or may not be reasonable to hope for. As a DM, I might allow this trick to work against an intelligent NPC wizard who has studied this particular spell, but probably not anyone else. In the fiction, I would rule that such an NPC has a general understanding of the near-impossible difficulty of resisting the spell for the entire duration, rather than having them calculate the precise probability based on meta-concepts like your saving throw bonus.

As pointed out in a comment by @carcer, if your interrogator doubts that you have actually cast the spell at all, they can simply step into the area with you and experience its effects for themselves. Again, this may only be persuasive to someone who has studied this particular spell themselves.

Answer 3

By itself, zone of truth can't prove your own honesty

The spell only does what it says it does, and though you could affect yourself with your own zone of truth and therefore be forced to be truthful, there is no mechanism in the spell which grants the knowledge you have about whether or not any subject is affected by the spell to anyone else.

However, if you have a way to give your interlocutor the ability to cast zone of truth, then you have reversed the situation and can invite them to subject you to the effect to prove your honesty. The easiest way I can think of to do this off the top of my head would be to lend your interlocutor a ring of spell storing into which you have cast zone of truth already, so they can attune to it and then cast it on you. Any creature can use the ring of spell storing (unlike a scroll of the same) so it doesn't require the interlocutor to be a capable caster themselves, but it does require you to have an hour available so they can attune to the item (and, ideally, another hour so you can attune to it again after they give it back).

If you're in the specific case of dealing with an individual who is a spellcaster with the spell identify prepared, you could subject yourself to a zone of truth and then invite them to cast identify on you, because:

If you instead touch a creature throughout the casting, you learn what spells, if any, are currently affecting it.

Thus, they can verify with their own magic that you are in fact affected by a zone of truth. If they're also inside the zone of truth, it would be obvious to them if the spell ends, as they will suddenly be able to lie.