The Bladesinging arcane tradition for wizards (TCE) gains the Extra Attack feature at level 6, whose description states:
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 haste spells allows the target to take an extra action on each of its turns for the spell's duration, with certain limitations:
That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.
Say a Bladesinging wizard is under the effect of a haste spell.
Can the Bladesinger replace the single attack from their additional Attack action (from haste) with a cantrip cast, per the second part of their Extra Attack feature?
Best Answer
Neither rule is more specific than the other. The restriction "one weapon attack only" is written on a specific spell. The option "cast one of your cantrips in place of one of those attacks" is written on a specific class feature. There is no written guidance about the interaction.
I think we are in rulings territory, and I would rule on the side of balance concerns due to how cantrips scale. That is, I would rule against allowing the Hasted Attack action attack to be switched out for a cantrip.
An additional scaling cantrip per turn is equivalent power boost to a whole Attack action (with any Extra attacks or riders). The restriction in the Haste spell looks set to avoid that for martial characters, so it makes sense to keep the same regard for balance when looking at the Bladesinger feature. The bladesinging feature is already very good for overall damage, and the Haste spell a popular choice for Bladesingers because it is also already very good. It doesn't need this further boost.