Is Booming Blade + Warcaster redundant with Sentinel?

[Greywander]

Sentinel reduces the speed of enemies hit by an OA to 0.

Warcaster lets you make OAs with cantrips.

Booming Blade encourages a creature not to move or else take extra damage.

But it seems like if I have all three and I make an OA with BB, Sentinel will reduce their speed to 0 anyway, and BB's rider is wasted. There might be a few extremely niche situations, such as if the enemy teleports, or uses some unique feature that allows them to move a certain distance (regardless of their speed) after performing a certain action, but those don't really seem worth worrying about.

I know Sentinel gives other benefits, but I wonder if Warcaster + BB is sufficient by itself to make a tank sticky. Not getting Sentinel would mean being able to get another feat, like Mage Slayer, Lucky, or Resilient.

Or I could swap BB for GFB. I wonder if the second target hit by GFB would also have their speed reduced to 0 because of Sentinel. But fire generally isn't as good of a damage type as thunder.
Wait, Warcaster requires the cantrip to target only one creature, nevermind. Dang, I had just had a really good idea for Thunderclap + Warcaster abuse.

[Kadesh]

A creature will not know the effects of the spell until after the first time they are affected by it, or take an action to identify (or reaction, when it is cast) to identify it.

Sentinel stops you moving, and combines with reach. Booming Blade allows a creature to move with no give away to its effects, and doesn't work with reach unless you take Spell Sniper.

[Lunali]

Sentinel will only trigger if the enemy moves in a way that triggers an opportunity attack, so it's only really useful if you have a new way to trigger OAs or you keep your friends far enough away from you that enemies have to move away from you. Booming Blade triggers if the enemy moves at all, allowing friends to be nearby for things like paladin auras or the protection fighting style.

[Greywander]

A creature will not know the effects of the spell until after the first time they are affected by it, or take an action to identify (or reaction, when it is cast) to identify it.

Sentinel stops you moving, and combines with reach. Booming Blade allows a creature to move with no give away to its effects, and doesn't work with reach unless you take Spell Sniper.
Well, I think they wouldn't move after the first time it happens, unless its something with a really low INT score. I kinda get the issue here, though: some people see BB as control and don't want the target to move, some people see it as a way to deal extra damage when the foe inevitably moves. It's hard to please both groups and maintain internal consistency (i.e. not having enemies deliberately act stupid for the players' benefit).

Your notes about reach are also valid. In my specific case, I'm looking at a sword-n-board build, so reach isn't an option (except maybe a whip?), but I never stated this in the OP. This is still important, especially for people who are also picking up PAM.


Sentinel will only trigger if the enemy moves in a way that triggers an opportunity attack, so it's only really useful if you have a new way to trigger OAs or you keep your friends far enough away from you that enemies have to move away from you. Booming Blade triggers if the enemy moves at all, allowing friends to be nearby for things like paladin auras or the protection fighting style.
So from your perspective, Sentinel would be (mostly) a waste of a feat if you already have BB and Warcaster. When you do get an OA, you can hit them with BB, which doesn't necessarily stop them, but it does punish them if they move. But you can still use BB as an action for when you want to lock someone down, even if they haven't provoked an OA.

Unfortunately, I don't know that there are many more ways to provoke OAs, aside from PAM. Mage Slayer and Sentinel itself both give attacks as a reaction, but I don't think those count as OAs despite looking a lot like one. They could have said, "when this is true, you can make an OA," but instead they said, "when this is true, you can make a melee weapon attack as a reaction." Probably specifically to prevent that sort of abuse.

[Sception]

The opportunity attack elements of the two feats are somewhat redundant, yes, and to be clear - for a tank looking to be sticky, sentinel's opp attack effect is the better effect. However, both feats also have other affects that are very relevant to tanking gishes. Warcaster's most important affect is letting you bypass the somatic components of spells while carrying a weapon and shield or two weapons, letting you cast spells like Shield and Hellish Rebuke while fully armed. Note that this aspect of the feat doesn't matter if you're two handing a weapon, as you can let go with one hand to cast your spell and then go back to wielding your weapon normally for free. Warcaster also grants advantage on concentration saves, which is valuable for keeping spells like Haste Bless, Wrathful Smite, and so on up while engaged in melee and actively encouraging the enemy to attack you. The concentration boost eventually isn't as big as that granted by proficiency, but the two boosts stack, and again, warcaster does other things.

As for sentinel, it also gives you a reaction to attack adjacent enemies that attack anyone other than you, and prevents enemies from disengaging to escape your opportunity attacks.


So is warcaster worth it, even if you have sentinel? Well, I'd say not if you are wielding a greatsword or polearm. Resilient con is better for concentration saves, and if you're already proficient in con saves then advantage on them probably isn't a big enough boost on its own to be worth an asi slot. If you're dual wielding, or fighting sword and board?

I'd say then it depends heavily on what your available spells and spell slots are. Do you have strong 'somatic-but-not-material' spells like Shield that you would want to cast in combat? Do you have excess spell slots available that you would want to cast those spells with rather than using on other spells that you can already cast like bless (which can be cast at the start of combat before drawing your weapon) or wrathful smite (which has no somatic component) or aura of vitality (which is generally best cast out of combat regardless) or just plain old divine smite?

If so, then Warcaster is probably worth it for bypassing somatic and advantage on concentration, even with the opp attack bit made redundant by sentinel. Shield alone is very good on a tank. That said, you can certainly get by without the feat.


On the other hand, if you already have warcaster, and are considering sentinel, then if you're a tanking type it's probably a good call, presuming your stats are where you want them. Immobilizing opp attacks may not be a huge upgrade over extra-damaging opp attacks that deal even more extra damage if the target moves, but the other aspects of the sentinel feat are still quite useful on a tank, particular the reaction attack to punish adjacent enemies who attack your friends. That said, if your DM doesn't make much use of the disengage action, and you already have some other reaction in response to enemies attacking your friends (say the protection fighting style), then you can probably find a feat that will do more for you than sentinel in that case.


If you have neither warcaster nor sentinel, then it mostly comes down to whether the reaction to punish attacks against friends or the option to cast somatic spells while fully armed is the bigger boon for you. And that will again depend on your other reaction options as well as the spells available for you to learn and the slots available to cast them with (shield is a great first level spell, but if your first and second level slots are all spoken for, well, it's not super great as a third level spell).

In general, I usually lean towards warcaster over (or at least before) sentinel in sword & board hexadin, sorcadin, and eldritch knight, and lean sentinel over (or at least before) warcaster in, well, basically anything else, though for great sword & polearm builds PAM and GWM are both probably better choices than either warcaster or sentinel. You also have to consider stats. Maxing out your attack stat is probably more valuable than either of the feats in question, and in a paladin build, or multiclass build with at least six levels of paladin, maxing out your cha might also be more valuable.

There's also the question of vuman builds bonus feat. A paladin or multiclass build that starts with paladin at first level doesn't qualify for warcaster, so in that case sentinel is obviously better (though there are certainly other strong contenders, in particular half feats like HAM or Resilient), and if you do start with sentinel at level one, then you might put off warcaster a bit longer even if you do eventually take it regardless.

[ad_hoc]

A creature will not know the effects of the spell until after the first time they are affected by it, or take an action to identify (or reaction, when it is cast) to identify it.

Sentinel stops you moving, and combines with reach. Booming Blade allows a creature to move with no give away to its effects, and doesn't work with reach unless you take Spell Sniper.

The creature is 'sheathed by booming energy'. They know.

[Sception]

The creature is 'sheathed by booming energy'. They know.

The spell text is ambiguous so table variation is to be expected and DMs could rule either way, but yeah, at every table I've ever played booming blade at, the target was assumed to understand the effect.