This came up during a game this weekend where the rogue player claimed that, since she was taking damage from it and she could see where the "attack" was coming from, she could reduce that damage with this feature. I allowed it at the time, for the pace of the game.
The Rogue feature Uncanny Dodge states:
Starting at 5th level, when an attacker that you can see hits you with an Attack, you can use your reaction to halve the attack's damage against you.
In my mind, this excludes the Magic Missile spell, since nowhere in the spell description does it state the word "attack".
You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range.
In the future, I would have to say "no", based on my reading of the ability and the spell.
Looking for confirmation/counter-arguments.