War Caster Feat: Opportunity Attack Casting following a Bonus Action spell

Question

The war caster feat includes the following feature:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature. [PHB:???]

First, if the spell must have a casting time of 1 action, does this preclude the use of spells with a casting time of 1 bonus action, as previously discussed here?

Also, when you cast a spell as a reaction, is this influenced at all by the one spell per turn rule?

Bonus Action: A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action. [PHB:203]

Thus:

• Would you be prevented from casting during this reaction if you had already cast a spell on your current turn?
• Would you be prevented from casting a spell on your next turn if you cast a spell during this reaction? Or,
• Are reactions outside of the timing window of both your current turn and your next turn?

Answer 1

The first part of the answer to your questions lies in the definition of a combat turn:

A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn. (PHB 189)

The limitations imposed by casting a spell with a bonus action, as you quote, extend only into that turn, not that round. Once you declare that your turn has ended, you are no longer restricted by this rule.

The other part is in the text you quote:

You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

Cantrips are a class of spells and are often treated in a special way.

Outside of your turn you are completely free to cast, limited only by the features that enable you to do so (like the War Caster feat) and the resurces they use (your reaction and spell slots usually). If another creature somehow triggers an opportunity attack from you on your own turn, you will not be able to use War Caster to react with a non-cantrip spell if you have already cast a spell as a bonus action.

Answer 2

Outside your turn

The discussion is moot. You cannot use Bonus Action outside your turn, thus you are not bound by the rule. You can use your reaction to cast a spell with 1 action casting time freely.

On your turn

As the rule, if you cast a spell with casting time 1 Bonus Action, you can only use your reaction to cast a cantrip with casting time 1 action, as required by War Caster and the rule.

A possible scenario:

Use a wand to cast Dissonant Whisper on a nearby enemy. The enemy fails its WIS save and provokes your Opportunity Attack.

If you use your reaction to cast a spell other than cantrip, now you cannot cast Bonus Action spell, because it is still in your turn. However, if you cast a cantrip instead, you still can cast Bonus Action spell.

As the scenario in your question happens in your turn, the answer is yes, you will be prevented from casting a Bonus Action spell if you cast a spell other than cantrip using your reaction. You, however, is not prevented to do so on your next turn.

A reaction can happen in anyone's turn, including your turn, it's not outside the window of your turn and your next turn.

Answer 3

Just because War Caster allows you to use your reaction instead of an action, it does not mean that the cantrip no longer has a casting time of an action, per its description. In fact, if it had any casting time other than an action, it would not qualify for war caster in the first place.