I asked this question on RPG.SE: How does the Haste spell's extra action interact with the Bladesinger's Extra Attack feature and with cantrips that do "one weapon attack only"?
However, other users closed my question as a duplicate of these 2 questions:
- If the Haste spell is cast on a Bladesinging wizard, can the Bladesinger cast three cantrips in a turn using the Extra Attack feature?
- How does the Bladesinging wizard's Extra Attack feature interact with the additional Attack action from the Haste spell?
Neither of these questions or their answers go over an all-important case: if the cantrip in fact does a weapon attack. Green-Flame Blade and Booming Blade are cantrips that do one weapon attack; their interaction is fundamentally different than all other cantrips, and warrants much attention and different answers.
The crux of the interaction is that Haste has a clause that requires "one weapon attack only" – which these cantrips seem to satisfy. Cantrips that seem to satisfy this clause present wildly different answers and discussion than cantrips that clearly don't. Does it matter how this gets satisfied? This is all very interesting discussion that is completely missing.
Also RAI becomes important to consider here. I'm wondering about the actual intent of the designers behind that "weapon attack" clause. Was adding the word "weapon" just an oversight made in haste?