Can a Bladesinger wizard cast Shillelagh, Magic Stone, or Mending with their Extra Attack feature?

Question

The Bladesinging wizard's Extra Attack feature reads:

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.

Notably, all but three cantrips have a casting time of 1 action. Two cantrips, shillelagh and magic stone, have a casting time of 1 bonus action, and the cantrip mending has a casting time of 1 minute.

Can these cantrips be cast with the Bladesinger's Extra Attack feature, despite not having a casting time of 1 action?
If mending in particular can be cast in this way, does it take effect immediately? Or does it still take a minute to complete casting?

(Never mind how our Wizard learned shillelagh or magic stone; perhaps they took a 1 level dip into druid when they went hiking during a gap year in their studies.)

Answer 1

Yes, you can.

Compare that phrasing to the third benefit of the War Caster feat (PHB, p. 170):

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

You can see that it specifies what type of cantrip you can cast, with the limitations of needing to have a normal casting time of 1 action, and also only targeting the creature that provoked the opportunity attack. It effectively limits your options to mostly just offensive cantrips.

Notably, the Bladesinger's Extra Attack feature lacks any language that restricts what type of cantrip you can cast. It simply states that "you can cast one of your cantrips". Obviously, the intent was for you to cast an attack cantrip, such as booming blade or fire bolt, but this ability has been worded poorly, and allows for any cantrip to be cast, including ones that normally have a casting time of "1 bonus action" or "1 minute".

In the case of mending, it should go into effect immediately, although the benefits of casting mending in the middle of combat are few, unless you have an Artificer ally with an Eldritch Cannon, Steel Defender, or Homunculus Servant in need of repair.

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If I were the DM in this situation, I would restrict it to only cantrips with a casting time of 1 action, and disallow things like the examples you provided. I would still allow non-offensive cantrips, as I like to reward creative uses of spells.

Answer 2

No, you cant.

I know this is an old question with an accepted answer, but I am coming in to highlight that the accepted answer is missing important details and is not, in fact, how the abilities would interact.

First, to reiterate

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 important thing here is that it's just saying you can cast. There is no distinction on cast time, and there doesn't need to be, because of the rules on spellcasting and specifically spells with longer cast times (PHB, p. 203):

Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours. When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so. If your concentration is broken, the spell fails, but you don’t expend a spell slot. If you want to try casting the spell again, you must start over.

This means that your bladesinger extra attack feature can cast one action worth of the spell, but if you wish to complete casting the spell, you must continue to either use your action (or your extra attack action) each turn until the cast time has been satisfied, and for these longer duration spells like mending, combat will usually be over by then. Plus, you're probably going to be concentrating on more important things.