[RPG] If there is no other enethe within 5 feet of the initial target of the Green-Flame Blade cantrip, must the fire leap to a friendly creature in range

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

In this October 2015 tweet, 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, 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.

As of November 2020, SCAG has received errata to fix this inconsistency. The description of the green-flame blade spell now says that you "can" do it (emphasis mine):

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.