Can the mage hand cantrip be used to trip an enemy who is running away?
How should I rule if my player is attempting to grab hold of the ankle of a humanoid creature that is running away? Would it change anything if the hand was positioned in front of the creature, and as said creature runs past, the hand reaches up and trips it?
I know that mage hand can only lift 10 pounds, but would that be enough to trip a Medium-sized creature?
Spells do what they say they do and mage hand does not grab creatures as creatures are not objects in 5e.
An alternative attempt might be to use mage hand to lift a board, stick, or bit of rope in the path of the running creature. Although it should not pose too much of a difficult to avoid, it is an obstacle that might work. As it's an inventive use and cantrips are cheap, so maybe it's worth a try.
A floating obstacle may be easy to spot and avoid. Use of a the ready action in order to "get the jump on a foe" and have the obstacle rise to meet the running creature at the right moment.
The Dungeon Master's Guide has a section on chases and chase complications. One of the complications that seems reasonable for running into a bit of debris is:
Your path takes you through a rough patch of brush. Make a DC 10 Strength Athletics) or Dexterity (Acrobatics) check (your choice) to get past the brush . On a failed check, the brush counts as 5 feet of difficult terrain.
Which is a reasonable effect for adding a bit of rough brush equivalent cleverly into the path of a running creature.
The description of mage hand specifies what it can be used for:
You can use the hand to manipulate an object, open an unlocked door or container, stow or retrieve an item from an open container, or pour the contents out of a vial.
Note that this does not include interacting with creatures.
Further, grabbing hold or tripping a creature is usually done by Grappling or Shoving a Creature respectively which are both stated to be special attacks. Mage Hand also specifically states that:
The hand can't attack
For both of these reasons, it does not seem reasonable to allow Mage Hand to be used in the way you describe to grab or trip a creature.
Minor Note: Mage hand can actually carry up to 10 pounds
While this is a clever idea, this isn't something that you can do with mage hand. The spell limits your interactions with (emphasis mine):
You can use your action to control the hand. You can use the hand to manipulate an object, open an unlocked door or container, stow or retrieve an item from an open container, or pour the contents out of a vial. You can move the hand up to 30 feet each time you use it.
The hand can't attack, activate magic items, or carry more than 10 pounds.
Mechanically, I think there is only one ability that 'trips': The Battle Master Fighter's Trip Attack
When you hit a creature with a weapon attack, you can expend one superiority die to attempt to knock the target down. You add the superiority die to the attack’s damage roll, and if the target is Large or smaller, it must make a Strength saving throw. On a failed save, you knock the target prone.
Functionally, this is basically adding a shove attack added to a weapon attack. But the Shove is a specific attack action that requires an attack action.
Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.
The target must be no more than one size larger than you and must be within your reach. Instead of making an attack roll, you make a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you either knock the target prone or push it 5 feet away from you.
The mage hand can't attack, and the target is definitely going to not fulfill the size requirements either.
There's a lots of things you can do with mage hand, but this isn't going to be one of them.
Let's say that your character wants to do this, though (just as an exercise.) In order to trip them as they flee, you'd need to ready your action to do this as they passed by. This allows you to use your reaction as they run by you. But if you're in range to trip, then you're also in range to attack. Attacking for damage may be the better option. If the trip is successful, all you've done is required them to use half their movement to stand up. This will prevent them from moving as far (by half their movement), but it won't stop them from leaving in general.
The general rule here is one that Jeremy Crawford (the sage who gives the advice in "Sage Advice", and the official rules guy for D&D) has repeated quite often on his twitter feed and elsewhere: "Spells do exactly what they say they do."
A mage hand can "manipulate an object, open an unlocked door or container, stow or retrieve an item from an open container, or pour the contents out of a vial", and can't "attack, activate magic items, or carry more than 10 pounds." A Rogue with the Arcane Trickster archetype can do a few extra things with mage hand that basically boil down to pickpocketing, picking locks, and disarming traps.
Since trip, grapple, shove, etc. aren't listed as things the hand can do, it can't do those things.
Furthermore, a trip is a kind of attack (it's a Shove action where you choose to knock the target prone), and attacks are on the list of things you specifically can't do with mage hand, so it's even more explicitly excluded.
Mage Hand can only do what it says on the tin: grabbing an object up to ten pounds is included in that list, and the pants of most humanoids weigh less than ten pounds.
So we can unambiguously say that it is possible to grab a running creature’s clothes with Mage Hand. The question then is “what happens when a creature tries to contest a Mage Hand’s grip?”
This SE question addresses that: Can another character physically take something that Mage Hand is carrying/holding?
Basically, it’s up to the DM! I would treat this as a Trap: Setback. Based on the table for traps in the DMG, that gives it a DC of 10-11, and probably a dex saving throw. On a failed save, the creature goes prone.
I might limit this to only work outside of combat, just to avoid concerns about it being similar to attacks with similar effects.
