When does the bonus damage of Booming Blade trigger when combined with Warcaster?

Question

Picture the following scenario: I have an enemy in melee range who decides to move away without Disengaging, triggering an Opportunity Attack. However, I have the Warcaster feat:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

For the spell, I choose booming blade, which has this clause:

On a hit, the target (...) becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.

The creature, not wanting to take the extra damage, would like to immediately stop moving. I am unsure if this is actually permitted. Which of the following three interpretations is correct:

1. The creature is already moving, and thus takes the damage.
2. The creature may stop after moving 5 feet, taking no damage.
3. The creature may stop after moving zero feet, similarly to what would happen if I had the Sentinel feat.

This question had previously been marked as a duplicate of another which did not, however, contain an answer to the posed question.

Answer 1

The target cannot decide to modify their movement, once declared.

The most recent opportunity attack rule says:

Opportunity Attacks In a fight, everyone is constantly watching for a chance to strike an enemy who is fleeing or passing by. Such a strike is called an opportunity attack.

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

You can avoid provoking an opportunity attack by taking the Disengage action. You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don’t provoke an opportunity attack if an explosion hurls you out of a foe’s reach or if gravity causes you to fall past an enemy.

In no point of this rule it is specified that once hit by an OA, the moving target can decide to modify its previously declared movement, if there is not any particular feature that specify so (e.g, the Sentinel feat). Hence, if they said "I am running away 25 feet from the enemy without disengaging" they must move the whole distance of 25 feet¹.

Moreover, the Sage Advice Compendium clarifies that being hit during the movement does not halt the latter:

If I use the Ready action to deal damage to someone who’s moving, do I deny the target the rest of its movement?

Dealing damage to a moving target doesn’t halt its movement, unless the damage is accompanied by an ability that stops movement. Things like the Sentinel feat give you such an ability. Reducing a moving creature to 0 hit points is also usually an effective way to stop it!

This means that in the depicted scenario the sequence

• the creature exits the reach of the character without disengaging
• the character casts Booming Blade and hits
• the creature is stopped and then can decide to keep on moving or not

cannot take place.

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¹ Unless another circumstance interrupts and/or modifies it, for example being the victim of a successful Snare spell or trying to pass inside the AoE of an unnoticed Grease spell

Answer 2

The creature may choose to stop or continue

The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

Once the interruption is completed, the creature may reevaluate its life choices.

Just like when you interrupt the photocopier, you can choose to stop or continue photocopying.

Answer 3

The target cannot cancel its move just because you make an opportunity attack

Opportunity attacks (p. 195 PHB) work like this¹:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

The creature cannot decide to cancel its movement after you made your opportunity attack², as the attack happens while it "moves out of your reach". If the attack does not stop its movement (like the Sentinel feat which explicitly says so), the move resolves when the attack is completed. Your replacement effect, of booming blade happens with the same timing.

Option 1 is correct. As the creature is moving out of your reach you stick it with the booming blade effect; booming blade's effect does not stop its movement, so it resolves, and the creature takes damage.

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¹ Strangely, my printing of the PHB has this text on p. 195 under the heading Melee Attacks, subheading Opportunity Attacks:

The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

The latest PHB Errata, dating from 2021, does not show any errata for the Opportunity Attack subheading. At the same time, the text in the online basic rules on D&D Beyond reads as cited, and normally D&D Beyond should have the latest iteration of the rules text.

I'm honestly not quite sure what to make of that, but in any case, even if the language still would contain the "interrupts" claim, I think this only supports the reading that the creature still has to move:

The meaning of interrupt is to "briefly stop something" that afterward continues. Here from the Cambridge Dictionary.

to stop something from happening for a short period

After the short period where you interrupt, the move continues. Otherwise you wouldn't be interrupting, you would be stopping.

² Well, in real life, I had plenty a player appeal to me as the DM that they forgot about opportunity attacks, but their character would not be so stupid and would never have moved away from a foe to risk such. Unless that is only my experience, there may be a way for player characters to get away with canceling their move if they have a lenient DM. I usually tell them to suck it up, unless there are mitigating circumstances, like when it is a new player who does not fully know the rules.