Excuse the unspecific question title. Hard to ask this without putting spoilers.
In D&D 5e Tales of the Yawning Portal, there is an updated version of the legendary Tomb of Horrors. At page 225, the PCs enter room 30, called:
False Treasure Room
The description goes like this:
The room is lined with lead and has antimagic properties, so no spells will work within the room, and no magical properties of items of any sort will properly function except those that detect an aura of magic or a place of desecration.
So far so good. In the room there is…
A bronze urn that, if opened gently, reveals an efreeti that "will grant three wishes for the party and then depart".
So, what happens then? The book give no additional insights on this.
Obviously the PCs have a good chance to anger the efreeti and not get the wishes at all, but it seemed weird to me at first that Acererak would put something that can, in some circumstances, help the party. I ran the adventure in an Adventurer's League this past week and I pondered on this when I reached that point. The PCs decided to trust the efreeti and ask for some reasonable wishes (I suspect some meta on their part, but that's beside the point).
What I did…
I remembered the properties of the room they were in. I interpreted the "grant three wishes" to literally mean "cast the spell Wish" and I roleplayed the efreeti attempting the spell 3 times, failing to, and then leaving, utterly frustrated. It seemed to go along with all the cruel jokes played by Acererak and how nothing is truly helpful in this dungeon.
My issue with this:
Both the room properties and the efreeti's description are short, apart from each others (not even on the same page), and do not reference each others. If what I did was the actual intended behavior, I feel that it was really obfuscated and rely on the DM doing quite a bit of logical thinking. I certainly wouldn't blame another DM to have miss it and grant 3 wishes. And what would have been expected if the PCs had taken the urn outside the room (and the antimagic field) before opening it?
All in all the players and I were satisfied with how it played out at our table, but we were left confused about what was the intended scenario.
Best Answer
Rules-as-written, the efreet cannot use wishes in this room, but this was not the rule or the intent of Tomb of Horrors releases for earlier editions of D&D.
AD&D
In the original AD&D Tomb of Horrors, the room does not explicitly prevent the efreet from using wishes. As in the 5th edition version, no spells or magic items work, except those that detect auras such as magic or evil, but this somehow does not prevent the illusion on 10,000 copper pieces in one of the chests, which have been enspelled to look like platinum.
In the AD&D edition, however, the efreet does not explicitly grant wishes, instead performing three "services", which may be implied to be wishes, as per the Monster Manual entry:
The word "wish" is not in italics, unlike the efreet's other spells, meaning that its ability to grant wishes in this edition was not considered a casting of the spell wish. Since the anti-magic only affects spells and items, the efreet in Gygax's Tomb of Horrors could originally grant wishes in the room.
D&D 3rd edition
In the earlier D&D 3rd edition version of Tomb of Horrors, the Efreeti has a unique special ability to allow its own special abilities, including wish. This explicitly resolves the conflict between the anti-magic effect, which in D&D is a variant antimagic field spell effect, which in that edition blocks spell-like abilities including the efreet's wish.
The illusion on the contents of the chests in the 3e Tomb of Horrors are also explicitly stated to be immune to the effects of the antimagic.
In short, the efreet in Bruce Cordell's third edition Tomb of Horrors could also grant wishes in the room.
D&D 5th edition
In this edition of Tomb of Horrors, no spells function within this room. The description of genie wishes in the Monster Manual specifies that genies specifically grant wishes by casting the wish spell. I strongly suspect that Jeremy Crawford, with his literal rules interpretations, would insist that this is the case.
The efreet is not bound to the room, and clever players might notice the antimagic and invite him outside after the first wish fails. However, the Monster Manual also notes that certain genies will pervert the intent of their wishes by adhering to the letter of the words, and the lawful evil efreet are most likely to do this, especially given earlier lore which says efreet are very specifically the ones who do this.
We have, in essence, a conflict between two statements in the same section: no spells work within the room, but the efreet does grant three wishes, and in earlier versions these were not considered mutually exclusive. Like all ambiguities in D&D, that's up to the DM to adjudicate, meaning of course that your approach was valid and within the rules. In fact, within the lore, the efreet may intentionally remain within the antimagic room just to cheat the player characters out of their wishes.