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Creatures with Truesight, have a number of benefits, one of which is that they

automatically detect visual illusions and succeed on saving throws against them

How do you determine if a spell creates a visual illusion? Is it as simple as "If the illusion creates something that the target can see, it's visual" or is there a more specific rule?

For some context, this came up into play in our last session where a Wizard PC cast Mental Prison on an enemy with Truesight an we weren't exactly sure how to rule it at the time and decided to research it after the session was over.

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Sage advice helped clarify this, in exactly this circumstance too

Jeremy Crawford, lead rules designer, stated over a twitter Q&A that

Q: Truesight lets you pierce/nullify illusion effects like the ones create by Phantasmal Force (in the mind of a creature) & Mental Prison (the target to perceive itself as being surrounded)? These spells are considered visual illusions for the purpose of Truesight?

A: A visible illusion is something illusory that you can see with your eyes. If an illusion is perceived only within someone's mind, that's not a visible illusion, even if the person thinks they're seeing it.

So, a visible illusion isn't something that affects the mind of the target, it's something, well, visible. It has to be some illusion that creatures actually see with their eyes, and not only see in their mind.

If the text of the effect says that the target "believes" that they see something or that it "takes root in the mind" of a creature, then Truesight won't be effective. But if the illusion is plain for anybody to see, then Truesight would see through it.

In your case, the creature is still affected

As the tweet clarified, since Mental Prison only affects the mind of a creature, it cannot see through the illusion with Truesight. This is because the creature is not witnessing the illusion with their eyes, but rather in their mind.

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