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"?

Question

The description of the Haste spell says, in part (emphasis mine):

Until the spell ends, [the target] gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

And the Bladesinging wizard's 6th-level Extra Attack feature says (emphasis mine):

Starting at 6th level, you can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.

As a 6th-level Bladesinging wizard, can you use the extra action granted by the spell Haste to cast a cantrip? What if that cantrip has you make a weapon attack as part of the cantrip, such as Green-Flame Blade or Booming Blade?

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Here's how I understand it. You take the Attack action with Haste; then the Bladesinger's Extra Attack triggers when you take the Attack action, which allows you to cast a cantrip in place of one attack. Haste says "(one weapon attack only)" if you choose the Attack action, but it seems like you can satisfy this by making one weapon attack as part of a cantrip, as Green-Flame Blade or Booming Blade specifically have you make a weapon attack.

Haste only seems to care that you:

• Take the Attack action (you do).
• Make one weapon attack only as part of it (you do).

It doesn't have any specific language about the means of how that ends up happening or if a cantrip gets involved in some way.

For reference, the description of the Booming Blade cantrip says (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 then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.

Likewise, the description of the Green-Flame Blade cantrip says (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.

Answer 1

Booming blade and Green-flame blade are not "one weapon attack only".

You write:

But haste has in parenthesis (one weapon attack only), but you can satisfy this by making one weapon attack as part of the cantrip, as Green-flame Blade or Booming Blade specifically have you make a weapon attack.

The trouble with this reasoning is that making a weapon attack as part of a cantrip is not "one weapon attack only", it is "casting a spell and making one weapon attack". You wrote:

It doesn't have any specific language about the means of how that ends up happening or if a cantrip gets involved in some way.

But it does - the word "only". "Only" restricts the action to including a weapon attack and nothing else. Making the weapon attack as part of casting a cantrip violates the "only" portion of "one weapon attack only".

Answer 2

They don't interact at all:

• The extra Attack action you get from haste only allows one weapon attack. Thus Extra Attack wouldn't apply, nor would its cantrip replacement clause.
• Even though these cantrips involve making a melee weapon attack as part of their effects, they're still cantrips, which are performed with the Cast a Spell action and not the Attack action. Thus, haste's additional action couldn't be used to cast them.

Answer 3

You can cast two cantrips, but not for the reasons you state.

The reasons you state rely on Booming Blade and Green Flame Blade involving a weapon attack. However, this has nothing to do with the reason that a Haste'd Bladesinger can attack with a dagger, and cast two cantrips. The cantrips could be Shocking Grasp and Firebolt, for example. They do this by taking the Attack action (dagger then Shocking Grasp), then taking the modified restricted Attack action granted by haste.

The Bladesinger's feature is a specific rule that states, "whenever you take the Attack action on your turn [...] you can cast one of your cantrips in place of one of those attacks." The Haste spell states that you can "take the Attack (one weapon attack only) [...] action." Thus the Haste spell is granting an Attack action on your turn, which can be modified by the feature.

Because the Bladesinger allows you one thing "in place of" another, it interacts with Haste by allowing you to replace "one weapon attack" (the text present in the spell) with "cast one of your cantrips" (the text present in the feature). Thus, a Bladesinger with that feature under the effects of Haste can {take the Attack (cast one of your cantrips only) [...] action}.

Answer 4

The Haste spell's extra action is indeed a valid trigger for the Bladesinger's Extra Attack feature. Level 6 Bladesingers can cast any of their cantrips in place of their Haste Attack action. It is irrelevant whether the cantrip involves a weapon attack as part of casting because the cantrip is not and never becomes a weapon attack, it merely replaces one.

Bladesinger's 6th-level Extra Attack feature:

Starting at 6th level, you can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.

The key mechanic here is that Extra Attack cantrips are order agnostic. The cantrip can replace either the first or the second attack granted by Extra Attack. For example, if the wizard takes the Attack action to swing their scimitar and cast Light, they can cast Light in place of their first attack and then swing their scimitar with the benefit of bright light. In layman's terms, level 6 Bladesingers can cast their cantrips as a full action (Cast a Spell), a bonus action (when applicable), or either half of their Attack action (through Extra Attack).

Haste allows the wizard to take an additional Attack action:

Until the spell ends, [the target] gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

The "one weapon attack only" option is a valid Attack action, so it satisfies the conditions of Extra Attack. Extra Attack grants two distinct features: a second attack, and the option to replace one attack with a cantrip. "One weapon attack only" specifically nullifies the second attack, but does not negate the "you can cast one of your cantrips in place of one of those attacks" option. As such, the rules as written allow the wizard to replace that one weapon attack with a cantrip. This feature does not require the wizard to actually make a second attack thereafter.