Does Cast a Spell trigger an Attack of Opportunity?

Question

Attack of Opportunity (CRB, p142) does not list Cast a Spell as a trigger:

Trigger A creature within your reach uses a manipulate action or a move action, makes a ranged attack, or leaves a square during a move action it’s using.

Is it intentional, an oversight, or is Cast a Spell somehow part of a Manipulate action?

Answer 1

Cast a Spell does not inherently trigger Attacks of Opportunity.

Cast a Spell, on its own, is not a Manipulate action (it does not have the manipulate trait), it is not inherently a ranged attack, it is not a move action (it does not have the move trait), therefore it does not provoke attacks of opportunity on its own.

But Components can change that

Components add traits to the Cast a Spell activity, as follows:

• Material (manipulate)
• Somatic (manipulate)
• Verbal (concentrate)
• Focus (manipulate)

Therefor, if a spell requires one or more of Material, Somatic, or Focus components, the Cast a Spell activity gains the manipulate trait. And gaining the manipulate trait makes it trigger Attack of Opportunity. Note that this means spells which only require no components or Verbal components only do not satisfy the trigger.

All information comes from the section on Casting Spells, CRB 302, which includes a sub-section giving more information on the requirements of spell components.

Also note that being hit during the Cast a Spell activity does not on its own cause you to lose the spell or even make a save for the same. Attack of Opportunity does, however, disrupt the action (thus forcing you to lose the spell) if it critically hits. Other similar mechanics may or may not have similar clauses - Ranger's Disrupt Prey does disrupt the action on a critical hit like Attack of Opportunity, but Lesser Death's Lurking Death reaction disrupts the action if it hits.

Answer 2

Cast a Spell can sometimes trigger an Attack of Opportunity

The Cast a Spell action can sometimes have the manipulate trait, depending on the components needed to cast the spell:

The spell components, described in detail below, add traits and requirements to the Cast a Spell activity. If you can’t provide the components, you fail to Cast the Spell.

• Material (manipulate)
• Somatic (manipulate)
• Verbal (concentrate)
• Focus (manipulate)

So if a spell being cast requires material, somatic or focus components, it is a manipulate action and, as you already note, being a manipulate action will therefore trigger an Attack of Opportunity