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Inspired by a comment in this question, which reads as follows:

It says you must spend your action each turn, but it does not say that if you do not spend an action that the spell fails

And the rules for spells with long casting times are as follows (emphasis mine):

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 (see "Concentration" below). 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.

—Casting Time (Player's Handbook, pg. 202)

Is the comment accurate? If you don't spend your action on a turn casting the spell (but also don't spend your action otherwise), does the casting fail (even if you intend to continue spending your actions on the following turns)? Or does it only fail if concentration is broken?

Since the Player's Handbook quotation only specifies that the casting fails if the caster's concentration is broken, I'm wondering if the

you must spend your action each turn casting the spell

is another way that the casting can fail. To avoid this question being a duplicate of the one I linked, I want to know in a general case how this interacts. The "must" seems to imply that the caster can't just choose to "pause" their casting for a round, but what about being unable to use their action in some other way?

So by my reading, there's two different ways this phrasing could be interpreted:

"you must spend your action each turn casting the spell [and if you don't, the spell fails]"

This is the most straightforward: to continue casting the spell, you must keep using your Action; if you don't, the spell fails. Not much else to say about this.

"you must spend your action each turn casting the spell [literally. The spell physically compels you to continue casting until the spell is complete; you cannot choose to use your Action to do something else until the spell is complete or until your Concentration is broken]"

This is, in my opinion, a valid interpretation of the literal language as well.

^{[without considering intent or how this interacts with other rules, I should stress. I have various reasons for feeling this second interpretation is not how the rule should be interpreted, but that's a different response for a different question.]}

Basically, we have two meanings of the word 'must': in one case, it indicates obligation: you must keep using your Action or the spell will fail. In the other case, it indicates compulsion: you must keep using your Action; you are not allowed to choose otherwise.

Regardless of which interpretation it is, though, the interpretation of the user you quoted, paraphrased below, I believe is very unlikely:

"you must spend your action each turn casting the spell [but if you can't or choose not to, the casting of the spell will continue or suspend]"

Just from a design standpoint, it doesn't make a lot of sense for an effect to say you "must" do something to cause an effect, but imply that if you don't do that thing, the effect will happen anyways. Consider the language of a spell like Animate Dead:

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, you animate or reassert control over two additional undead creatures for each slot level above 3rd. Each of the creatures must come from a different corpse or pile of bones.

—Animate Dead, Player's Handbook, pg. 212

It doesn't expressly tell us what happens if the creatures come from the same pile, but it's still clear from context that the effect doesn't work if this is the case. Other effects in this game are worded quite similarly: if a condition is specified for what the caster 'must' do, those conditions must be satisfied. The absence of an explicitly stated consequence for failing to do that thing isn't required for there to be such a consequence.

So regardless of which interpretation I cited is correct, the user you quoted is almost certainly mistaken. If the spellcaster does not use their Action on a given turn to continue casting their spell, for any reason, the spell will fail. I've already pointed out in my answer to the linked question that under certain circumstances (the Surprised condition, for example) I would personally, at my table, ignore this rule, but as a general rules-as-written principle, this is how the rule works.

If you don't spend your action casting the spell, you have stopped casting the spell

There are only a couple of possibilities for what could happen if you start casting a spell with a long casting time and then don't use your action to continue casting it:

1. The casting continues uninterrupted, and this round counts towards the casting time.


2. The casting is paused, so this round doesn't count towards the casting time, but you can resume casting next round as long as you maintain concentration.


3. The casting is cancelled, and you would have to start over from the beginning if you still want to cast it.

I can't think of any other reasonable interpretations of the rule besides these 3.

Option 1 is directly and unambiguously at odds with the rules, which say that "you must spend your action each turn casting the spell". So we can eliminate it right away.

Option 2 seems plausible, but there are a lot of problems you need to address if you use this interpretation. For example, how many rounds in a row can you "pause" the casting before the spell is lost? Could you pause casting a spell 1 round before you finish, and then wait indefinitely for the opportune time to complete it, thereby allowing you to set a trap with a spell that normally has a prohibitively long casting time? In short, Option 2 implies a whole new set of possible mechanics relating to "paused spellcasting" and its interactions with other mechanics, none of which are addressed at all in the rules. That makes it very unlikely that this is the intended reading.

So, having eliminated those two options, the only reasonable interpretation left is Option 3: the spell's casting is cancelled. This is certainly the most literal reading of the rule: if you spend your action each turn casting the spell, you cast the spell; if you don't spend your action each turn casting the spell, you don't cast the spell. The rules don't say that the spell fails if you don't use your action to cast the spell, because they don't need to. The spell doesn't fail: you simply don't cast it because you stopped casting it. The requirement to maintain concentration through the full casting time is an additional requirement to cast the spell and is unrelated to the requirement to spend your action each turn casting it.

(I agree that this logic gives an absurd result when combined with the rule that a surprised creature cannot take an action on the first turn of combat, but I would argue that the fault lies with the surprise rules, not the spellcasting rules.)

I think the effect of the Slow spell on spellcasting also gives a nice example on how this is supposed to work. It requires you to spend your next action to finish a spell otherwise only requiring a single action:

If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn’t take effect until the creature’s next turn, and the creature must use its action on that turn to complete the spell. If it can’t, the spell is wasted. (Phb. p. 277)

While is is oddly specific and Slow also forbids you from using reactions and your bonus action (if you used your action) you could infer from it that spellcasting is a continuous, uninterrupted process.

Based on the rule quote in the question, the only situation in which your action is not spent is if your concentration is broken, which causes the spell to fail (without expending the spell slot).

Think of it like hanging up the phone halfway through dialing an international number -- the call doesn't go through, but you don't get charged for it, either.

Concentration may be broken by any of the causes listed in the PHB, including injury, but the caster may choose to give up concentration as well -- for instance, to take an opportunity attack on an enemy. This has the same effect as any other concentration break.

You continue casting, up to a point

Technically, you can not "continue casting a spell" but "do not spend your action". You spend an action because you're casting a spell, not vice versa. The same way we use "Attack action" and "attack roll" to simulate combat, for instance — you make these rolls because you're attacking an enemy.

That's why this comment makes sense:

It says you must spend your action each turn, but it does not say that if you do not spend an action that the spell fails

"Casting a spell" takes place in the game world. "Action" does not exist in the game world. We use game mechanics (actions) to model in-game events (spellcasting).

Let's take a specific example. The Wizard is channeling a spell. Suddenly, goblins attack. The Wizard is busy and distracted, so he is "surprised". In the game world that means he is not ready for the fight yet. So, what does he do instead? He either fumbles, or continue the channeling. This is what happens in the game world. This particular situation is quite rare and is not described in the rules, hence, requires DM's adjudication. That's why the game requires a DM, after all.