[RPG] How does the consumption of spell slots affect an Artillerist Artificer’s ability to create Eldritch Cannons

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The Artillerist subclass for the Artificer class has the ability to summon an "Eldritch Cannon" to support them in combat:

At 3rd level, you learn how to create a magical cannon. Using woodcarver's tools or smith's tools, you can take an action to magically create a Small or Tiny eldritch cannon in an unoccupied space on a horizontal surface within 5 feet of you. A Small eldritch cannon occupies its space, and a Tiny one can be held in one hand.

Once you create a cannon, you can't do so again until you finish a long rest or until you expend a spell slot of 1st level or higher.

—Eldritch Cannon, Eberron: Rising from the Last War, pg. 59

The way this feature is written seems to imply that the ability to summon the cannon becomes "unlocked" after consuming a spell slot, which would mean that an Artificer could cast a spell like Cure Wounds and thusly regain the ability to summon their cannon using their action.

Is this a correct interpretation of these rules? Or are Artificers required to spend that spell slot as part of the action used to summon the Cannon (a la Sorcerers burning spell slots for their Font of Magic feature)?

Just to make the example a little clearer, consider two scenarios, one of which I believe to follow the rules, the other does not; but I am uncertain which scenario it is.

  1. Expending a Spell Slot "Unlocks" the re-use of the Cannon
    • Summon a Cannon using an Action
    • Then, an hour later, the Artificer wants to summon a new Cannon
    • They cast Cure Wounds on their ally to heal some incidental damage
    • They can then use their Action to summon a new Cannon
  2. The Spell Slot is spent as part of the summoning of the Cannon
    • Summon a Cannon using an Action
    • Then, an hour later, the Artificer wants to summon a new Cannon
    • They summon a Cannon using their Action, which also consumes a 1st level spell slot

Which scenario is correct?

Best Answer

The second scenario is correct: Spell slots = resources

Once you create a cannon {using an action+tools}, you can't do so again until you finish a long rest or until you expend a spell slot of 1st level or higher

The key to this is the "again", the "until" and the "expend" - there is a resource cost for summoning a subsequent cannon after the free "first cannon of the day." The artificer either waits for a long rest to get another free cannon, or, the artificer expends a spell slot to get another cannon before that long rest recharges the free cannon. The creation of a cannon does not take two actions; - the Artificer spends a spell slot to create the next cannon(s). (If after "or higher" they had added "to create one" it would have better matched how they phrased it in the UA - Jeremy Crawford's tweet indicates that they intended it to work the same way as in UA).

The subsequent cannons resemble the Ranger "Primeval Awareness" class feature ability in that the character expends a spell slot to activate a class feature.

Primeval Awareness
Beginning at 3rd level, you can use your action and expend one Ranger spell slot to focus your awareness on the region around you. For 1 minute per level of the spell slot you expend, you can {snip}...

Sadly for the Ranger, they don't get the first use for free as the Artificer does. We now initiate a mass protest against WoTC for not loving the Ranger class

A spell slot is used to create the next* cannon(s).

Cannons last for an hour or until the cannon reaches 0 HP.

It disappears if it is reduced to 0 hit points or after 1 hour. You can dismiss it early as an action. (RftLW, p. 59)

The order of operations would play out like this:

  1. Create cannon (first cannon of the day). (Uses an action, no spell slot consumed)
  2. Other things happen. (Likely combat, or time expires ... )
  3. Same adventure day, you need another cannon, or, you want to change the kind of cannon you have (there are three kinds).
  4. Expend a spell slot to create the next one

You could run yourself out of spell slots if you go cannon-happy

A likely reason for subsequent cannons costing resources: prevent the creation of multiple cannons that cost no resources. Compare case 2 with case 1, in terms of resources:

  • Case 1: 1 cannon + as many cannons as spell slots + all those spell slots cast as spells.

  • Case 2: 1 cannon + as many cannons as spell slots versus

    Case 1 is a significant resource boost for one adventure day for 6th level Artificer, for example

Designer Commentary/Clarification from Jeremy Crawford

Artillerist artificer's Eldritch Cannon feature says, "Once you create a cannon, you can’t do so again until you finish a long rest or until you expend a spell slot of 1st level or higher." In response to a question about whether spending a spell slot on anything else is sufficient to create a new cannon, Crawford replied: "Eldritch Cannon does let you spend a spell slot to create the cannon. That slot must be spent specifically on the cannon, not on something else." (From this tweet)

Experiential notes:

  1. That's how we did it using the Artillerist from the final Artificer UA. This feature is roughly unchanged from that version of Artillerist.

  2. IMO, this can be very powerful, but if the whole campaign is in Eberron, with its elevated power levels, it probably won't be that big of a deal.

  3. The text from the last Artificer UA; this points toward some design intent.

    Arcane Turret
    {description} It disappears if it is reduced to 0 hit points or after 10 minutes. You can dismiss it early as an action. When you summon the turret, you decide which type it is, choosing from the options on the Arcane Turrets table. {more actions} . You can summon a turret once for free and must finish a long rest before doing so again. You can also summon the turret by expending a spell slot of 1st level or higher.

1 A Protector cannon can be a lot better than cure wounds ...

You mentioned cure wounds as the spell, so I'll point out that the Protector cannon is often a better use of a level 1 spell slot than cure wounds.

Fun Protector Cheese: Set up a Protector with an action, and keep loading up your allies with 1d8+Int THP each round as a bonus action. Even if the Artificer burned a spell slot to do this, that's potentially a lot more HP than a "cure wounds" (1d8+Mod) or a healing word (1d4 +mod) cast once. With three allies, you can offer each of them 1d8+mod of damage prevention (THP) each round for the cost of a bonus action. And, if you have two fights within one hour, you keep covering them in THP to mitigate damage taken - but you didn't have to use any more spell slots. Note that the THP also applies to the cannon, which will reduce the chances that it gets reduced to 0 HP during the fight. (Until you get to higher levels)

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