Can a Magic Jar-possessed body level up?

Question

I was looking over the Magic Jar spell, and there are lots of ways to simulacrum and cheese and clone shenanigans. But I haven't seen this question yet and I was wondering...

Simulacrums specifically "cannot learn", which means they can't gain levels. Magic Jar specifies that if you possess another creature you can't use their existing class levels/features, should they have any.

What if they don't have any class levels/features at the time of possession, but you stay in them long enough for them to level up as whatever their chosen class would be?

In my mind, the reason you can't use class features or levels of something you possessed is because you don't know how. That's something the creature learned and you don't know it. But if you possess the body and then, with that possession in that body, start over as a level 1 say, fighter. Can you justify learning new class features and essentially reincarnating as a new character with the knowledge/personality of an older one?

Answer 1

Bodies don't level, characters do

When you possess another body with Magic Jar,

you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can't use any of its class features.

It seems to me that it is still you who is gaining experience in the other body, because you keep your mental stats and access to your class features that you otherwise would not have.

So, if you adventure in the other body, you gain experience. You add levels, of whatever class you choose, while you are in the host body. Once you leave the body and return to your own, you will have those levels in your own body. The host body will not have gained any levels.

What if you take an ASI to increase a physical Ability score like Strength or Dexterity in the host body? You still could justify this narratively by saying it is not your muscles that get physically better, it is your willpower and you learning to move and use them in a better way.

The issue with most questions about magic jar is that the game system and rules are not designed for a mental character being separated from the physical body that it belongs to, and do not really spell any of this out explictly. So, in your game, ask your DM how they handle it. It's their job to resolve such questions.

You cannot possess a Simulacrum

Part of your question is about simulacrum. Magic Jar allows you to possess a humanoid:

You can attempt to possess any humanoid within 100 feet of you (...)

While Simulacrum allows you to copy a beast or humanoid, it is neither, it is a construct, and therefore not a valid target for Magic Jar.

Otherwise, the illusion uses all the statistics of the creature it duplicates, except that it is a construct.

You can see from the list on page 6 of the Monster Manual, that Constructs and Humanoids are different creature types.

Answer 2

Probably, but why?

Per the Magic jar Spell:

Once you possess a creature's body, you control it. Your game statistics are replaced by the statistics of the creature, though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can't use any of its class features.

If the host body already has class levels, you cannot count them. If they don't have class levels, then they are a nondescript extra. So either way, you are basically a blank slate. We need to find rules on someone that had a life before and has just started their path as an adventurer. Like an NPC?

NPC Party Members

NPCs might join the adventuring party because they want a share of the loot and are willing to accept an equal share of the risk, or they might follow the adventurers because of a bond of loyalty, gratitude, or love.
[...]
Any NPC that accompanies the adventurers acts as a party member and earns a full share of experience points.

Adventure NPCs

These NPCs should be the same level as the lowest-level adventurer in the party and built (either by you or your players) using the character creation and advancement rules in the Player’s Handbook.

Per the rules, an NPC can use the character creation rules and become 1st level in some class. And since they gain an equal share of experience, they also follow the advancement rules.

So you decide that, per the rules, this host is going to become a 1st level Cleric. That's awesome because as a 1st level Cleric, they gain spells, cantrips, some divine powers... But wait...

If the target has any class levels, you can't use any of its class features

That rule doesn't change just because you chose a class to level up. You could spend the rest of your days gaining Cleric levels in the host, but you would never see any benefit from it sans the every-four-level-ASI and more hit points (maybe take levels in Barbarian?). And then the host dies, or the container is broken, your soul comes rushing back to your original body... and you lose all that experience (including the ASI and hp).

Experience points are an abstraction of the game to quantify learning and muscle memory. Same way hit points are an abstraction of health, well being, and fortitude. Since it wasn't your body that learned how to better swing a sword, cast a spell, or sing a song, you don't get the "experience" of having done so. Even though you leveled up the host through the ranks of a class, you could not use any of its features and therefore never learned how to do any of those features.

You got to vicariously live the life of a Cleric, but that's all you really gained.