Does prowess trigger when you cast a creature spell?

Prowess reads: "Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn." The keyword specifically excludes creature spells from triggering it. This is defined in CR 702.107a.

So if you cast an instant, sorcery, enchantment, artifact, planeswalker, or battle spell, prowess will trigger. But casting a creature spell — even one with other types, like an artifact creature — does not trigger prowess.

There is one nuance: a permanent with multiple types matters based on whether it is a creature spell on the stack. A Darksteel Citadel is an artifact land, not a creature, so it would trigger prowess if cast. But an Ornithopter (an artifact creature) would not trigger prowess, because it is a creature spell.

Example: You control a Monastery Swiftspear (which has prowess). You cast Lightning Bolt targeting an opponent's creature. Prowess triggers, giving Monastery Swiftspear +1/+1 until end of turn. If instead you had cast a Goblin Guide (a creature spell), prowess would not trigger at all.

Remember that prowess triggers when you cast the spell (CR 702.107a), meaning the trigger goes on the stack after you announce the spell and pay costs, but before it resolves.