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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?

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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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    \$\begingroup\$ It's hilarious that the UA version is perfectly unambiguous and the official published version...isn't. \$\endgroup\$ – Miniman Nov 21 '19 at 13:20
  • \$\begingroup\$ @Miniman yes it is. :) The "voice" that D&D 5e rules are written in IMO contributes to the ambiguity of a number of rules sections. \$\endgroup\$ – KorvinStarmast Nov 21 '19 at 13:28
  • \$\begingroup\$ @Ruse suggest you write an answer \$\endgroup\$ – KorvinStarmast Dec 5 '19 at 12:14
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RAW, scenario #1 is correct; after using your free creation once, doing so again entails:

  1. Expending a spell slot at some point by any means available as part of an unrelated action, restoring your ability to create the cannon
  2. At some later time (though that time could be an action immediately following a bonus action that expended the slot), you can again use your action to create the cannon

For comparison, look at features like the Conjurer's Benign Transposition, which don't explicitly tie the resource expenditure to the renewal of the ability:

Once you use this feature, you can't use it again until you finish a long rest or you cast a conjuration spell of 1st level or higher.

That wording is identical, except "cast a conjuration spell" becomes "expend a spell slot". The Artificer's ability is more flexible, merely requiring you to expend a spell slot by any means, but also more limited, since the Conjurer's ability can be refreshed with ritual casting between fights (it only requires casting, not a slot expenditure), making it effectively an "at least once per fight" ability for the majority of cases where you have a 10+ minute break between battles, since you can just spend 10 minutes ritual casting Unseen Servant after each fight.

But aside from the precise resource expended, it's the same: You have an ability that takes an action, cannon creation/teleportation, that refreshes on a long rest, or when you expend a spell slot/cast a conjuration spell. The Conjurer doesn't cast the spell and teleport simultaneously, they cast the spell and recover the ability to teleport. In neither case is there any implication that the expenditure of said slot directly pays for, or is part of, the action used to create/teleport (any more than the long rest would be "part of" your first free creation/teleport), only that the ability to create/teleport is refreshed by having expended the spell slot/cast the spell.

The general wording for all caster classes (including Artificer UA, and I assume the final version published too) describes casting as (specific text from Wizard, but wording is similar everywhere):

To cast one of these spells, you must expend a slot of the spell's level or higher.

So "you can't do so again until you finish a long rest or until you expend a spell slot of 1st level or higher" means "you can do so after you expend a spell slot of 1st level or higher"; casting a spell is one way to do that, but not the only way.

For contrast, compare to the the Artificer UA, where RAW would involve summoning the cannon using the spell slot, since the wording there was (emphasis added on "by"):

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.

That phrasing makes it quite clear the cannon is created at the same time as the expenditure (prior text makes it clear the summoning itself costs an action), and that the expenditure directly fuels cannon creation, but the new phrasing from the official rules you posted changes both the timing (the cannon creation ability restores after slot expenditure) and the cost (the slot is expended by any means available to you, with the bonus of renewing your cannon creation ability).

This gives you some unusual ways to renew the ability in multiclassing scenarios too; a Paladin's Divine Smite expends a spell slot, so a Palificer (Artidin?) could create new cannons after smiting, without actually casting a spell. A Sorcerer/Artificer could burn a slot for spell points (which still isn't free; beyond the action cost, buying the slot back with spell points always costs more than you get from burning the slot).

To me, it seems clear that the major change in wording from UA to published rules was intentionally to change the balance and availability of the class feature. In the UA, if enemies destroyed your turret, or you commanded it to detonate, you could use your very next action to summon a new one by expending a spell slot. On the one hand, this increased the cost (the slot was otherwise wasted, and if you'd used your low level spells already, the turret became more and more expensive), but it also increased the availability; you could always summon a new turret as long as you had spell slots. With the new version, the cost is lower (the slot is used for something else productive) and fixed (expending a higher level slot got you a higher level spell, with the cannon renewal being a fringe benefit), but it's not always available (you're usually going to have to use an action or bonus action to cast the spell to renew the ability; if you use an action, that delays recreating by a round).


Response to arguments for scenario #2:

Korvin mentions the Primeval Awareness ability for comparison, but that ability explicitly ties the spell slot expenditure to the action/power being used:

you can use your action and expend one Ranger spell slot to focus your awareness on the region around you.

and ties the benefit and the degree of benefit to the slot, incentivizing choosing a higher level slot:

For 1 minute per level of the spell slot you expend,

By contrast, nothing in the Artillerist's ability ties the slot expenditure to the use of the ability itself (it's not an action, it's not part of the creation action), nothing says the expenditure of the spell slot is only for that purpose, and (indicating it's not intended to balance the ability, just limit how often you can use it), nothing ties the strength of the cannon to the slot used to recover your ability to create it. It just says "If you expend a slot, the power becomes available again."

The point is, when a non-spell power/ability burns a slot to activate it (e.g. Primeval Awareness, UA Artificer's Arcane Turret), it's pretty explicit that the slot is burned as part of using the power. When the resource that fuels the ability doesn't burn the resource directly, but is directly dependent on it (e.g. Storm Sorcerer's Tempestuous Magic), the exact nature of the relationship is spelled out in detail. In the case of Tempestuous Magic, it specifies it what sort of action is used (bonus) and when it is used (before or after casting the spell). The final published version of Eldritch Cannon is like none of those abilities, much like Benign Transposition bears only a passing resemblance to any of them.

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