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Yes under the 2014 rules, but only for Sentinel’s Disengage-related opportunity attack. War Caster can replace that opportunity attack with a qualifying spell. It cannot replace Sentinel’s separate reaction attack against a creature that attacks your ally, and the replacement spell does not inherit Sentinel’s speed reduction. Under the 2024 rules, War Caster generally does not apply to Sentinel’s new Guardian triggers because Reactive Spell requires an opportunity attack provoked specifically by leaving your reach.
Quick verdict by rules edition
The answer changes because both feats were rewritten. Treat “Sentinel and War Caster” as two different rules questions: first identify why Sentinel grants the reaction, then check whether that reaction is an opportunity attack with the trigger War Caster requires.
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| Situation | 2014 rules | 2024 rules |
|---|---|---|
| Creature leaves your reach normally | War Caster can replace the opportunity attack with a qualifying spell. | War Caster can replace the opportunity attack if the spell meets Reactive Spell’s requirements. |
| Creature uses Disengage and leaves your reach | Sentinel makes it provoke an opportunity attack anyway; War Caster can replace that attack with a qualifying spell. | Sentinel’s Guardian reaction happens immediately after the Disengage action, before a later movement trigger; the “by leaving your reach” condition is not met at that moment. |
| Creature attacks someone else while adjacent | Sentinel grants a separate reaction melee attack, not an opportunity attack; War Caster cannot substitute a spell. | Sentinel grants an opportunity attack after the hit, but its trigger is the attack against another target, not leaving your reach; Reactive Spell does not apply. |
| Sentinel’s speed reduction | Applies only when you hit with an opportunity attack, not when War Caster substitutes a spell. | Applies only when you hit with an opportunity attack, not when a spell is substituted for one. |
What War Caster actually replaces
War Caster does not turn a spell into an opportunity attack. It gives you an exception to the normal casting-time rule: instead of making the opportunity attack, you cast a permitted spell. The Sage Advice Compendium, Version 1.14 identifies War Caster as a feature that lets a character cast a spell in place of an opportunity attack, even though the spell normally has a one-action casting time. The same clarification is available in the online Sage Advice Compendium.
That distinction controls every Sentinel interaction. If Sentinel gives you an opportunity attack, War Caster may be able to substitute a spell for it. If Sentinel gives you some other reaction attack, there is no opportunity attack for War Caster to replace. And when the substitution occurs, the result is a spell, not a hit from an opportunity attack; riders that require you to hit with an opportunity attack do not automatically follow.
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2014 War Caster requirements
Under the 2014 feat, the spell must have a casting time of one action and target only the creature that provoked the opportunity attack. A cantrip such as booming blade can qualify if its current wording and targeting satisfy those conditions at your table. War Caster consumes your reaction, just as the original opportunity attack would.
2024 Reactive Spell requirements
The 2024 wording is narrower in its trigger. As reproduced in the D&D Beyond discussion of Sentinel and War Caster, Reactive Spell says that when a creature provokes an opportunity attack from you by leaving your reach, you can take a reaction to cast a spell at that creature rather than making the opportunity attack. The spell still needs a one-action casting time and must target only that creature. The quoted wording is clear about the required cause: the opportunity attack must arise from the creature leaving your reach.
How Sentinel works under the 2014 rules
The 2014 feat has three relevant benefits, and they do not all create the same kind of attack.
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A creature that takes the Disengage action normally does not provoke opportunity attacks from its movement. Sentinel overrides that protection for you. If the creature then moves out of your reach, it provokes an opportunity attack from you despite having Disengaged. Because this is an opportunity attack, War Caster can replace it with a qualifying spell.
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Hitting an ally creates a different reaction
When a creature within 5 feet of you makes an attack against a target other than you, Sentinel lets you use your reaction to make a melee weapon attack against the attacking creature. This is not an opportunity attack. It is a separate reaction whose trigger is the creature’s attack, so War Caster cannot exchange it for a spell.
The Sage Advice Compendium distinguishes this kind of reaction from an opportunity attack and explains that opportunity attacks occur in relation to a movement trigger, whereas an unspecified reaction occurs after its stated trigger. Calling both results “a reaction” does not make them interchangeable.
Stopping speed is tied to an opportunity-attack hit
Sentinel’s speed-stopping benefit says that when you hit a creature with an opportunity attack, its speed becomes 0 for the rest of the turn. If you use War Caster, you do not make that melee attack; you cast the spell instead. The spell therefore does not satisfy the condition “hit with an opportunity attack,” even if it hits, deals damage, or is a cantrip that resembles a weapon attack.
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2014 examples at the table
Normal movement away from you
An enemy starts within your reach and moves away without Disengaging. Its movement provokes an opportunity attack under the ordinary rules. You may make the attack or use War Caster to cast a qualifying one-action spell that targets only that enemy. If you cast, Sentinel’s speed reduction does not apply because you did not hit with an opportunity attack.
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Disengage followed by movement
An enemy takes Disengage while adjacent, then moves out of your reach. Sentinel cancels the Disengage protection for your opportunity attack. At that point you have a genuine opportunity attack, so War Caster can replace it with the qualifying spell. The replacement is still a spell rather than an opportunity attack, so it does not set the enemy’s speed to 0 through Sentinel.
An enemy attacks your ally
An enemy within 5 feet attacks another character. Sentinel gives you its special reaction melee attack. You cannot use War Caster to cast in place of that attack because the reaction is not an opportunity attack. Sentinel’s speed benefit is also unavailable: it requires a hit with an opportunity attack, not a hit with this ally-protection reaction.
How Sentinel works under the 2024 rules
The 2024 Sentinel wording changes the timing and source of its reaction. Its Guardian benefit lets you make an opportunity attack immediately after a creature within 5 feet takes the Disengage action or hits someone other than you. Its Halt benefit sets the creature’s Speed to 0 when you hit it with an opportunity attack.
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After a creature hits someone else
If the adjacent creature hits a target other than you, Guardian grants an opportunity attack immediately after that attack. The attack is an opportunity attack for Sentinel purposes, so Halt can apply if you hit. However, Reactive Spell has an additional requirement: the opportunity attack must have been provoked by the creature leaving your reach. Here, the stated trigger is the creature hitting another target. Because the leaving-reach condition is absent, War Caster does not authorize the spell substitution.
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After a creature takes Disengage
If the creature within 5 feet takes Disengage, Guardian grants an opportunity attack immediately after the action. That timing matters. The reaction is created by the Disengage action itself, before any later movement out of your reach. Since the creature has not yet provoked an opportunity attack by leaving your reach at that trigger point, the literal Reactive Spell condition is not met.
The available wording does not provide an official ruling specifically resolving every possible sequencing argument for Sentinel and War Caster in 2024. A table that reaches a different result should settle the interpretation with the Dungeon Master before relying on the combo. The safest text-first reading is to keep the two triggers separate: Guardian creates the attack after Disengage, while Reactive Spell asks for an opportunity attack caused by leaving reach.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why Sentinel’s Halt does not follow a replacement spell
Both editions phrase Sentinel’s speed reduction around hitting with an opportunity attack. War Caster’s spell is a substitute for that attack, not an opportunity attack itself. Therefore, a spell that hits cannot activate Halt merely because it was cast in response to movement or because Sentinel was involved in creating the opportunity.
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A practical decision procedure
- Identify the edition. Use the 2014 or 2024 feat text, not a mixture of both.
- Name Sentinel’s trigger. Is the creature leaving your reach, taking Disengage, or attacking someone other than you?
- Classify the reaction. Determine whether Sentinel grants an opportunity attack or a separate reaction attack.
- Apply War Caster’s trigger test. In 2014, a qualifying opportunity attack can be replaced. In 2024, Reactive Spell additionally requires that the opportunity attack was provoked by leaving your reach.
- Check the spell. It must use a one-action casting time and target only the provoking creature.
- Resolve Sentinel riders separately. Halt requires a hit with an opportunity attack; it does not attach to the substituted spell.
- Spend your reaction once. You cannot make the opportunity attack and cast the replacement spell from the same trigger, and you cannot also use another reaction during that round unless a rule gives you an additional reaction.
Common table questions
Can I cast Booming Blade from Sentinel?
Under 2014 rules, you can replace a Sentinel-created opportunity attack with booming blade if the spell’s current text meets War Caster’s one-action and single-target requirements. This works for Sentinel’s Disengage-related opportunity attack, not for Sentinel’s separate 2014 reaction against a creature attacking your ally. Under 2024 wording, the spell still needs the Reactive Spell requirements, including an opportunity attack provoked by leaving your reach; a Guardian attack triggered by Disengage or by hitting another target does not clearly satisfy that condition.
Does the spell count as an opportunity attack for other effects?
No. War Caster changes what you do instead of making the opportunity attack; it does not reclassify the spell as one. Any feature that specifically requires you to make or hit with an opportunity attack must be checked independently, and Sentinel’s Halt is the direct example.
Does Sentinel let me cast when the creature never moves?
Under 2014 rules, Sentinel’s ally-protection benefit can grant a reaction attack even when the creature stays in place, but that attack is not an opportunity attack and cannot be replaced by War Caster. Under 2024 rules, Guardian can likewise grant an opportunity attack after a hit against another target without a leaving-reach trigger; Reactive Spell’s wording still requires the leaving-reach cause.
Bottom line for players and Dungeon Masters
For 2014 characters, the reliable combination is Sentinel’s Disengage exception plus War Caster: Sentinel creates a real opportunity attack, and War Caster can substitute a qualifying spell. Do not apply Sentinel’s speed stop to that spell, and do not try to substitute a spell for Sentinel’s ally-protection reaction.
For 2024 characters, read Guardian and Reactive Spell literally and separately. Guardian’s immediate attacks after Disengage or an ally being hit are not described as opportunity attacks provoked by leaving your reach, so War Caster does not clearly apply to those triggers. If your group wants a broader interaction, record that house ruling explicitly; the published wording does not itself establish it.
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