I DM for a party that includes a bard and a warlock, both of whom can cast tiny hut. In a typical dangerous area long rest, the bard casts tiny hut to include the entire party, and then the warlock leaves that hut and casts his own. This gives him a private resting place to conduct activities that he doesn't necessarily want the party to know about, but allows him to enter the party's resting place if necessary (at the cost of bringing down his own hut when he leaves it).
The warlock is currently 'served' by an imp-like fiend; this is a free-willed relationship that behooves the warlock's patron and is not granted by find familiar, pact of the chain, or other class feature. As such, the fiend has its own motivations and agenda, some of which it does not share with the warlock.
The fiend would like to, invisibly, stand near the bard when she is casting her tiny hut so as to be included in it without her knowledge, be able to go in and out of her hut at will, and influence the dreams of the party members while they are asleep inside the hut. As the DM, I can just make this happen by fiat, but I am curious as to whether, RAW, the bard would realize that the fiend was included within her hut. The inclusion of the fiend would not put the creatures in the hut to more than nine, and the fiend is size Small.
There are questions on this site about whether the target of a spell knows that it is a target, whether a caster knows that a spell failed because the target is immune, and whether a caster knows why a spell failed, but as far as I can tell nothing about whether a caster knows who has been affected by their spell.
Tiny Hut has a range of Self, which in this context means that the effect emanates from the caster (PHB 202), but does not specify whether the creatures affected by the spell are 'targets' or not (and in any event, 'target' is natural language and not a rigorously-applied game term).
The spell describes itself as affecting "Creatures and objects within the dome when you cast this spell" – it does not say that the caster selects them, or has to see them.
Is there any RAW reason to believe that the caster of the hut would know how many creatures the hut included or their identities?
Best Answer
This would work
Think about what the tiny hut is. It's basically a tent made of nigh-indestructible force energy. It's like taking a tarp, hucking into the air, and spreading it over your group of people. It doesn't target anyone, it doesn't have saves or a chance to evade it or anything. It's a tent.
If you're in the tent when the tent goes up, it 'affects' you in that you can pass freely in and out of the tent.
There's absolutely nothing around this spell that indicates the caster somehow knows who is inside the area of effect, any more than the caster knows who all got caught in a fireball if they set it off near a doorway it could spread through and hit targets on the other side.
But...
Do remember that 'invisible' does not mean 'undetectable.' In terms of stealth, Invisible means:
https://www.dndbeyond.com/sources/phb/appendix-a-conditions#Invisible
Your fiend still makes noise, it still disturbs the air, it can still disturb leaves or dust or leave footprints. If it's flying, you're going to get the sound of the flap of its wings. You can still accidentally bump into it, smack it while moving around, hit it with your backpack when you set it down.
So, yes...assuming the fiend can pass its stealth check to prevent anyone from hearing it or seeing signs of its passage. And assuming it manages to stay out of everyone's way all night long without doing anything that might give away its position....then it can do what you're suggesting.
Honestly, the odds of this invisible fiend remaining completely unnoticed while traveling with the party are quite small unless it usually stays well away from them. An Imp only has a +5 bonus to stealth...it's gonna lose that check eventually...probably sooner rather than later.