Question: The War Caster feat (PHB, p. 170) allows a single-target spell ("must target only that creature") to be cast instead of an opportunity attack.

The *Green-Flame Blade* cantrip (SCAG, p. 142) appears to have a single target, and then some damage "leaps" onto a creature 5 feet away. The other creature is never referred to as the target, or even "a target".

**Can the *Green-Flame Blade* cantrip be cast as a reaction using the War Caster feat?**

I'm pretty sure this is solid from the RAW perspective. Is this legit from a RAI perspective?

Answer: 
The RAI on this one is a little uncertain, from what I can tell.

In [this October 2015 tweet](https://twitter.com/JeremyECrawford/status/651801858130898944), rules designer Jeremy Crawford responded to a question about using the *Green-Flame Blade* cantrip for an opportunity attack:

> 
> Without a feature like the War Caster feat, a spell can never be cast in place of an opportunity attack.
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> 

He doesn't make it incredibly clear, but the implication here seems to be that with the feat, this would be possible.

However, this is contradicted by [a later tweet in November 2015](https://twitter.com/JeremyECrawford/status/663510873449762816), in which he stated:

> 
> Twinned Spell doesn't work with a spell that targets more than 1 creature. Green-flame blade targets 2.
> 
> 
> 

This explicitly states that the spell targets multiple creatures (and therefore wouldn't qualify for the War Caster benefit), but again in a different context.

My instinct here is to agree with explicit disqualification, rather than implied exception, so I would say the RAI is that you can't use *Green-Flame Blade* with an opportunity attack.
Yes, as long as the fire doesn't "leap" to a second creature
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Jeremy Crawford has posted some [clarifications regarding spells with a range of "Self" (followed by an area)](https://twitter.com/JeremyECrawford/status/1326595384790056960) after the release of *Tasha's Cauldron of Everything*:

> 
> A note about D&D spells with a range of "Self (XYZ)": the parenthetical—which says "5-foot radius," "15-foot cone," or something else—means you are the spell's point of origin, but you aren't necessarily its target. You're creating an effect that originates in your space.
> 
> 
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He then posted a followup tweet [explicitly stating that the *booming blade* and *green-flame blade* cantrips work with War Caster](https://twitter.com/JeremyECrawford/status/1326598236937674752) (with a caveat, for the latter):

> 
> The Booming Blade spell continues to work with the War Caster feat. The spell targets one creature.
> 
> 
> The Green-Flame Blade spell continues to work with War Caster if you forgo targeting a second creature with the green fire.
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>