What happens if a caster is surprised while casting a spell with a long casting time?

Question

Casting a spell with a long casting time requires the caster to use their action each turn casting the spell:

you must spend your action each turn casting the spell

However, what would happen if while casting a long casting time spell (out of combat), a caster is suddenly attacked and is surprised?

Being surprised, the caster won't be able to use his action during his turn to continue casting the spell so is the spell forcefully lost? If it is lost, is the caster still considered to be concentrating on it up until his turn comes up?

This matters in particular for Pock, a level 12 War Magic Wizard who is ritually casting a tiny hut spell when his party gets ambushed. He would like to maintain the spell being cast as long as possible (or at least until he can actually take an action) to benefit from Durable Magic.

Answer 1

Rules as Written, the Spellcaster would lose the spell

This is relatively straightforward. The rules for spellcasting state unambiguously that a caster casting a spell with a long casting time must use their action each turn to cast it:

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

Meanwhile, the rules for the Surprised condition are also quite clear:

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.

—Surprise, Player's Handbook, pg. 189

So these two factors in conjunction show that the spellcaster, being unable to use their Action on their turn, must lose all their progress on the spell they're casting.

"But that seems like a really dumb rule!"

Yes. It does, which is why I don't rule this way in my games.

The Surprised condition in 5th Edition is an abstraction not of a character literally having an emotional paroxysm, but instead of the latency the character experiences in reacting to another character's actions. It doesn't make a lot of narrative sense for a surprise attack—especially if it doesn't hit the spellcaster in question, or wasn't even targeting them—to interrupt their spellcasting.

Certainly, a successful hit against a spellcaster would trigger Concentration checks, which have a chance in their own right of disrupting the spell (since casting a spell with a long cast time requires the maintaining of Concentration), and attacks made against a surprised character are more likely than not to be especially dangerous (Unseen Attackers, First Round bonuses, etc.) so the mechanics are likely to cause a caster's disruption even if they ignore this particular rule. But from my perspective, it doesn't make much sense for the spellcaster to automatically lose the spell.

So in a game that is strictly rules-as-written, or in an Adventurer's League setting, the spellcaster would have to lose the spell. But at my table, this rule would be ignored.

Answer 2

The caster continues casting the spell.

Taking the same quotes from @Xirema's answer, we have the following for casting spells with a long casting time:

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

And the surprised condition:

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.

—Surprise, Player's Handbook, pg. 189

While at first glance it would appear that the casting would end, I would argue that there is difference between spending and taking an action.

Spending vs taking

Both terms are used heavily in the rules, but are never clearly defined beyond their standard English meanings.

However, the words do seem to be consistently used in different ways.

Spending is used to represent the consumption of a resource.

Taking is used to represent the choice to consume a resource.

Example:

A turned creature must spend its turns trying to move as far away from you as it can, and it can’t willingly move to a space within 30 feet of you. It also can’t take reactions. For its action, it can use only the Dash action or try to escape from an effect that prevents it from moving. If there’s nowhere to move, the creature can use the Dodge action.

-- Turn Undead, v1.0 Basic rules PDF, pg. 24

An undead has a turn as a resource, and it must consume it by trying to move away.

It also cannot take a reaction. Why didn't the designers say it can not spend a reaction?

We can't read their minds, but I found many other examples where spend is consistently used to refer to consumption of a resource, whereas take is used to refer to a choice to consume a resource. (Look at class abilities for a lot of "take" examples.)

I would argue that, by RAW, a creature that is casting a spell with a long casting time has already made the choice before being surprised to be casting that spell. As such, they can still have their action consumed by that spell casting when they are surprised. They simply cannot make any choice to have their action consumed in any other way.

This also fits very nicely with what makes sense: If a caster is surprised, they don't really understand that combat is going on, and so would continue to keep concentrating on their spell casting.

I admit it is not perfectly consistent in how the two words are used in the rules, but given that it makes more sense AND is a reasonable reading of the rules, I would definitely say that allowing the caster to continue casting the spell is RAW.

Concentration

Some comments bring up the rules of concentration as an overlooked part of this answer. I do not consider them to be at the core to the question though. The question quotes the following statement:

you must spend your action each turn casting the spell

The word "must" tell us that this is a requirement that must be met, although it doesn't explicitly state what happens if it fails. I believe that the fact that the rules immediately follow with

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.

means that the designers considered failing concentration to be the only way they expected a long casting time to be interrupted, and so did not bother to explicitly say that failing to spend an action was something that had a failure state.

The conditions that may cause you to lose concentration are as follows (Basic Rules PDF v1.0 pg 84):

1. Casting another spell that requires concentration.
2. Taking damage.
3. Being incapacitated or killed.

None of those are triggered by the surprised condition, and so concentration is not broken. However, the question is not asking about whether concentration is broken.

The "must spend your action" statement is what was in question, and so is what my and Xirema's answers are focusing on.

As mentioned above though, I do take the lack of direction on what happens if a creature does not spend an action each turn as further indication that the designers did not intend for the "must spend your action" statement to be something that can ever be involuntarily failed.

As such, despite it admittedly being a bit hair splitting, I believe there is a difference between taking and spending an action, and that an action is still spent while a creature has the surprised condition.

Answer 3

Taking the rules very literally, the spell fails.

The caster doesn't get to take an action, so they don't keep casting the spell.

However, the rules are kind of dumb.

As usual, the 5e rules don't try to be comprehensive. They tell you what happens under "normal" conditions and leave it to the players to work out the edge cases.

Doing nothing while surprised makes sense for usual cases where you're making a sneak attack on a sentry guarding a gate, or a gang of bandits sitting down to eat, or ambushing someone on the road. That is, where the surprised creature is not already doing anything important.

Consider that when you've surprised a creature, you don't actually have to attack. You can sneak away unnoticed (move until you're behind cover, then perform a Hide action). In this case you've had no interaction with them. It makes no sense that this should interrupt casting a spell.

Consider further: If you had run at the wizard screaming like a maniac, then he wouldn't be surprised. It's only because you approached like a sneaky ninja that he mysteriously froze up for a few seconds and botched the spell. That makes even less sense.

How far does this spooky action at a distance extend? You automatically fail Perception checks to see people you don't have line of sight to, so if Lord Voldemort is casting a spell, and I go into a dark room two hundred miles away and "hide" from Lord Voldemort, then declare that I want to "surprise" him, can I mess up his concentration? Can a team of people do this every six seconds for a week and prevent him from eating? The implications are bizarre.

The sensible way to adjudicate surprise is that a surprised creature is unaware of the creature that's surprising it, and can't respond to its presence in any way. This allows most people in the world, who could theoretically be considered "surprised" all the time, to go about their lives. In the normal case where the surprised creature wasn't really doing anything important, they'll continue not doing anything important.

This approach also works for situations such as surprising a creature while it's in combat with someone else, surprising a flying creature, or the start of a battle where only one guy is hiding.

Answer 4

Going with RAW, I have to conclude that the "surprise" rule is of secondary concern. Primacy goes to the spell casting rules, which include concentration conditions.

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The long casting rule:

Longer Casting Times

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.

-- SRD, pg.101 (PDF)

The Concentration rule:

Concentration

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required).

Normal activity, such as moving and attacking, doesn’t interfere with concentration. The following factors can break concentration:

• Casting another spell that requires concentration. You lose concentration on a spell if you cast another spell that requires concentration. You can’t concentrate on two spells at once.
• Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.
• Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.

The GM might also decide that certain environmental phenomena, such as a wave crashing over you while you’re on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.

-- SRD, pg.102 (PDF)

The rules cover maintaining "concentration" as the key to the spell's eventual success. The breaking of concentration has the three standard methods, and the discretionary clause at the end. The event causing the "surprise" might cause a concentration check (Constitution saving throw) at the GM's discretion. If the concentration check fails, the spell is lost, and only then would the surprise effects apply.

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Rationale

The designers seem to have given considerable thought to the spell casting elements of the game, including how a spell caster would be involved in the casting.

While casting a spell the character's concentration is absolute and all encompassing. A cleric communing with their deity, a wizard recalling the precise enunciation of esoteric syllables, or weaving webs of energy, a druid meditating to achieve connection with the forces of Nature, etc. all require that the caster be working on exactly, and only, that. The phrase you must spend your action each turn casting the spell covers that. It also implies that while the casting may involve "actions", such as speaking, using materials, or making gestures, it is still considered a single, long-term, action relative to game play. There is no time, or effort, to spare on other externals.

As noted in the rule for surprise, the "surprise" condition affects each party member individually, not the party as a group.

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.

-- SRD, pg. 90 (PDF)

As the surprise needs to be checked for each character individually, when checking for the spellcaster, the first step is to determine if concentration is broken. If the event causing surprise doesn't break the caster's concentration, it can be presummed that either their concentration is so intense that they completely failed to notice the event, or are so "deep" in their concentration that even though "aware" of the event it is not enough to disturb the caster's efforts. In either case, there is no need to determine if the caster is "surprised" by the event.

As a further enhancement, taking physical damage is something rather difficult to ignore. Even so, damage is not sufficient to automically break the caster's concentration. Rather, taking damage causes a saving throw versus Constitution. As the addendum to concentration shows, other environmental events, a wave crashing over you, might also cause a concentration check. The event which triggers the surprise check could be ruled by the GM as also triggering a concentration check. If, however, concentration is not broken, the spell casting continues, and surprise does not become a factor.

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In the situation descrbed by the OP, Pock is casting a tiny hut spell ritually when the party is ambushed. The ambush is probably insufficient to break concentration, unless Pock is hit as part of the ambush. If Pock is hit, then a Constitution saving role is done. The role passes, Pock maintains concentration, and the spell continues. The role fails, Pock looses concentration, the spell fails, and Pock has to be tested for surprise, with the proper effects if Pock is surprised. If the ambush is sufficiently noisy (a band or raiders yelling war cries as they storm the camp, for example), the GM could call for a check of Pock's concentration as well, with the same results as above for inflicted damage.

Answer 5

By RAW, yes the caster would lose the spell since they can't use their action to continue casting the spell during the first round of combat while surprised. You would be considered still concentrating on it until the end of your first turn when you forfeit the spell.