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.
> 
> 
> 

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.
This does not work.
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As of the [November 10th, 2020 errata to the *Sword Coast Adventurer's Guide*](https://media.wizards.com/2020/dnd/downloads/SCAG-Errata.pdf), the [*green-flame blade*](https://www.dndbeyond.com/spells/green-flame-blade) cantrip (SCAG, p. 142; TCoE, p. 107) has a range of "Self (5-foot radius)".

However, the third benefit of the [War Caster](https://www.dndbeyond.com/feats/war-caster) feat says (PHB, p. 170; emphasis mine):

> 
> 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**.
> 
> 
> 

Since *green-flame blade* has a range of self, it targets the caster, not "only that creature" which provoked the opportunity attack.

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It should be noted that rules designer Jeremy Crawford has [stated on Twitter that War Caster still works with *green-flame blade*](https://twitter.com/JeremyECrawford/status/1326598236937674752). I disagree with this ruling, based on my explanation above, and think he should have made his rules clear enough to not need unofficial rulings on Twitter.