At what level do monsters cast cantrips?

Question

A 9 HD Yuan-ti Pureblood can innately cast the poison spray cantrip 3 times per day. The monster description says the save is DC 12, but it does not go into damage. So does it do 1d12 or 2d12 damage?

Answer 1

According to the Monster Manual (p. 10) under Special Traits, Innate Spellcasting:

Unless noted otherwise, an innate spell of 1st level or higher is always cast at its lowest possible level and can't be cast at a higher level. If a monster has a cantrip where its level matters and no level is given, use the monster's challenge rating.

The damage for a Yuan-ti Pureblood's Poison Spray cantrip would be 1d12 since it is a CR 1 monster and no level is specified for the cantrip. MM, p. 310.