Does Unspoken Agreement stack with multi-attack or cantrips?

Question

This question is about material from Genuine Fantasy Press's Compedium of Forgotten Secrets, which has a free version available.

Does Unspoken Agreement Spell stack with multi-attack or cantrips?

Unspoken Agreement

4th-level enchantment

Casting Time: 1 bonus action
Range: 30 feet
Components: S
Duration: Instantaneous

Partnership is two minds acting with a singular purpose.

You make a simple gesture, indicating to an allied creature within range that they should perform an action and sending a telepathic message of 25 words or fewer. The target chooses either the Dash, Disengage, Dodge, Attack (single weapon attack only), or Cast a Spell (a cantrip with a casting time of one action) actions. The target uses their reaction to perform that action. You also take the same action as part of casting this spell.

Unspoken Agreement says that you take the same action when casting the spell. So I have 2 questions:

First: If my ally casts a cantrip can I cast booming blade as bonus action and then attack 2 times?

Second: If my ally casts a cantrip can I cast booming blade as bonus action and then cast eldritch blast as my main action?

(My character is a swords bard and a level 1 warlock.)

Answer 1

You get to use one of the prescribed actions regardless of your main action

When the spell says "as part of casting this spell", it is indicating that the bonus action for the spell provides the effect of both you and the target gaining these actions.

As such, your ally can cast a cantrip and you can cast a cantrip (booming blade, or whatever else).

You can then use your action for whatever you like on that turn. This could include 2 attacks via the Attack action (your first case), or eldritch blast via the Cast a Spell action (your second case).

Just remember that since you cast a bonus action spell, you cannot cast another non-cantrip in the same turn.

Answer 2

This works (if it would work)

The sequencing of this spell seems to not be worded that well: a spell’s effect only happens when the spell has been successfully cast. Here the effect is to allow another creature to take one of the described actions. So, what is likely intended (but not written) is that as part of the effect, not the casting of the spell, you then can take the same action. I’ll continue with your questions under that assumption. (Otherwise, it would be too late for you to do anything).

First: yes, you can cast booming blade if they chose to cast a cantrip. It is likewise a 1-action cantrip. As part of booming blades effect, you get to make your attack. You do not cast it as a bonus action. You cast it as part of the effect of Unspoken Agreement (which itself uses a bonus action). It still is a one-action cantrip.

I’m not sure where you get the idea from that you would make two attacks, unless you mean you could then use your action to make a second attack - that would work too, you even could make a full attack action with your action that could involve several attacks if you have the extra attack feature.

Second: yes, you then can cast Eldritch Blast with your action, as you can cast cantrips with a casting time of one action in addition to casting a bonus action spell, and EB is a cantrip with a 1-action casting time.