If the Haste spell is cast on a Bladesinging wizard, can the Bladesinger cast three cantrips in a turn using the Extra Attack feature?

Question

If the haste spell is cast on a Bladesinging wizard, can the Bladesinger cast three cantrips in a turn using the Extra Attack feature?

For instance, say I'm a level 6 Bladesinging wizard, with haste already active. Can I do all of the following on the same turn?

1. Cast booming blade to attack a target 5 feet from me
2. Forego my second attack to cast shocking grasp
3. (a) Use the additional action granted by haste to cast infestation

All 3 spells are cantrips.

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Alternately, can I do steps 1 and 2 above, then:

3. (b) use my additional action from haste to cast misty step (instead of infestation)

In this case, it's 2 cantrips and a bonus-action spell.

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I've read a lot about reaction spells from War Caster, or Action Surge spells, or Quickened Spells from a sorcerer, but I haven't heard if it's ok to load up my turn with cantrips as a Bladesinger with haste, as per my 2 examples above.

Answer 1

In order:

Cast Booming Blade to attack a target 5 feet from me

Yes. There's no issue here.

Forego my second attack to cast Shocking Grasp

This will not work. Per the rules on the Bladesinging wizard's Extra Attack feature (Tasha's Cauldron of Everything, pg. 76):

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.

Casting a cantrip doesn't count as "taking the Attack action" – even if, in the course of performing that cantrip, you make an attack against a creature and roll an attack roll.

Use my action from Haste to cast Infestation

The action granted from Haste doesn't allow you to cast spells (Player's Handbook, pg. 250):

[...] it 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.

Use my action from Haste to cast Misty Step.

Sort of. You can't use an action to cast a bonus-action spell. So the action granted by Haste doesn't allow you to cast Misty Step (also, again, that action can't be used to cast spells).

However, as established already, the only other spell(s) cast this turn were cantrips, so you are legally allowed to use a bonus action to cast Misty Step this turn, regardless of the presence of Haste.

Answer 2

The haste spell's additional action cannot be used to cast a spell or to use any bonus action

The haste spell states:

Choose a willing creature [...] it 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 haste spell does not grant an additional bonus action, but instead an action, and because it is restricted to specific action types it cannot be used to take the Cast a Spell action. This is one reason neither of the scenarios you've described can work.

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Casting the booming blade spell does not count as taking the Attack action and so does not activate Extra Attack

The booming blade spell is a spell, so it requires the Cast a Spell action. For more information on this distinction the following two questions exist:

• What does upper-case-A-Attack action vs. lower-case-a-attack mean?
• Can you combo attacks with the Booming Blade cantrip?

Because booming blade does not use the Attack action, you cannot benefit from Extra Attack and cannot cast an additional cantrip.

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The only rule impacting what spells can be cast in a single turn outside of the action economy rules is this:

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

It can be confusing to read, but the bottom line is that so long as you do not cast any bonus action spells, you can cast whatever spells the action economy allows for. Meanwhile, if you cast a bonus action spell, all other spells cast on that turn must have been and be action cantrips. Similarly, if you cast any non-action or non-cantrip spell, you cannot cast any bonus action spells on that turn.

Answer 3

No, you cannot cast all three

You can't cast all three cantrips in the same turn. Not because there is a rule against casting multiple cantrips in a round; there is no such rule. However, casting Booming Blade is an action that allows you to make an attack. It is not an Attack action, so your Bladesinging wizard's Extra Attack feature would not apply. You are not taking an Attack action; you are casting a spell.

Also, haste does not allow you to cast spells at all with its additional action (PHB, pg. 250):

That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide or Use an Object action.

The Cast a Spell action is not included in this list, and the Attack can not be substituted with a cantrip as it specifies weapon attack.

Answer 4

No, this doesn't work

1. Can I cast booming blade to attack a target 5 feet from me.

   Sure. Booming blade is a cantrip, you can either spend your whole action casting it using the Cast a Spell Action, or use the Extra Attack feature to cast it as the first or second attack of the Attack action. I imagine you mean as the first attack of an Attack action.

2. Forego my second attack to cast shocking grasp.

   Here you run into a problem. You can only replace one attack with a cantrip per Attack action.

   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.

3. Use my action from haste to cast misty step.

   It doesn't look like you can. The haste spell specifically says it is just a single attack which it specifies is just a single "weapon attack" and it is arguable more specific because it isn't talking about "any attack action" but just this one granted by this spell. This is a problem with infestation also. Even if it could use haste to cast, misty step is a bonus-action spell so you can't cast it as an action, but you can still cast it using your bonus action.

That would bring you to two spells per turn tops. Multiclassing to gain Metamagic could let you Twin a spell to get to three, or using your reaction during your turn to cast a reaction spell to kind of get three spells (it is just one spell, but it adds an extra target), or maybe some crazy magic item could get you to three... But being a Bladesinger alone can't do it. That said, Attack with sword, cast booming blade and bonus-action misty step away is a pretty powerful combo, even if you can't sneak in the shocking grasp.

Answer 5

Existing answers have done a good job explaining why Extra Attack only works with the Attack action, not casting spells, and that you can't cast a spell with Haste.

So what can you do?

1. With your action Attack, using your Extra Attack feature to cast Booming Blade in place of one of your attacks.
2. With your Haste action Attack, and use your Extra Attack feature to "cast one of your cantrips in place of one of those attacks." (note: you only have one attack, but you only need to replace one to cast a cantrip) - using it to cast Shocking Grasp
3. Cast Misty Step with your bonus action

Unfortunately you won't be able to fit 3 Action cantrips into your turn because you only have 2 Actions and 1 bonus action. The best you can do is 2 cantrips and one bonus action spell.