BG3's implementation of the War Caster feat is very limited compared to D&D, only allowing you to cast Shocking Grasp instead of a spell of your choice. War Caster 5e supports the 2014 D&D version for BG3, but currently appears abandoned.

Playtest 2 of the 2024 Core Rulebooks for D&D revised the War Caster feat, removing unintended interactions but giving it a +1 ASI.

This mod contains both the 2014 and Playtest 2 versions of the feat.

War Caster (2014)
Source: Player's Handbook (2014)

You have practiced casting spells in the midst of combat, learning techniques that grant you the following benefits:

• You have advantage on Constitution saving throws that you make to maintain your concentration on a spell when you take damage.
  

• You can perform the somatic components of spells even when you have weapons or a shield in one or both hands.
  

• 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.
  

The 2014 version replaces BG3's existing War Caster feat.

Implemented as written, which means that you can not only cast a spell when an enemy moves away from you, but also when they move towards you using the Polearm Master feat, or when they move away from your echo as an Echo Knight.

War Caster (UA2)
Source: Player’s Handbook Playtest 2: Expert Classes

You have practiced casting spells in the midst of combat, granting you the following benefits:

Ability Score Increase. Increase your Intelligence, Wisdom, or Charisma score by 1, to a maximum of 20.

Concentration. You have Advantage on Constitution Saving Throws that you make to maintain your Concentration.

Reactive Spell. When a creature provokes an Opportunity Attack from you by moving out of your Reach, 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 one Action and must target only that creature.

Somatic Components. You can perform the Somatic Components* of Spells even when you have Weapons or a Shield in one or both hands.

The UA2 version is added as a new feat.

Implemented as written, which means that Polearm Master and Echo Knight opportunity attacks can not be replaced with spell casts.



Supported Spells

Cantrips
Bone Chill, Eldritch Blast, Fire Bolt, Poison Spray, Ray of Frost, Sacred Flame, Shocking Grasp, Thorn Whip, Vicious Mockery

Spells
Banishment, Blight, Blindness, Command, Disintegrate, Dissonant Whispers, Dominate Person, Harm, Heat Metal, Hideous Laughter, Hold Person, Inflict Wounds, Polymorph, Resilient Sphere, Vampiric Touch

5e Spells
Frostbite, Lightning Lure, Mind Sliver, Mind Spike, Mind Whip, Primal Savagery, Toll the Dead

5e Spells or SCAG Cantrips
Booming Blade, Green-Flame Blade

Invocations
Agonising Blast, Grasp of Hadar, Lance of Lethargy, Repelling Blast

Other
Mirror Blade, Starry Wisp

Upcasting
Few spells support upcasting when used as a reaction by default, mostly to avoid cluttering the reaction list.
The More Upcasting optional file adds upcasting up to spell level 6 for all spells that can support it.


Extras

Optional Rules
This mod implements the rules as written as faithfully as I can, which includes one interpretation that I find clear but some disagree with, and one I expect some BG3 players will object to. The Lenient Rules optional file can be used to relax conditions.

5ft range spells with Extra Reach weapons. The opportunity attack to be replaced is triggered when the target moves out of weapon range, which means that if you're using an Extra Reach weapon then the target will be out of range of some spells at time of the reaction. Lenient rules allow touch ranged spells to be used even with Extra Reach weapons.

Blade cantrips from Echo. Echo Knights can make an opportunity attack against a creature moving away from the Echo as if in the Echo's space. The spell cast from War Caster replaces an opportunity attack but is not itself an opportunity attack, so is not made from the Echo's space. Lenient rules allow using blade cantrips on reactions from the Echo.


Latency Workaround
In D&D, the spell cast and any attack rolls it makes would be resolved the instant the reaction is triggered. In BG3 the target is often allowed to run for a little while before the spell is cast, and then any rolls may be further delayed by animations. By the time the roll is made you may have disadvantage because the target was allowed to move into melee range. Polearm Master reactions constantly suffer from this.

As a workaround, ranged spell attack rolls made from this mod's reactions will be replaced by melee spell attack rolls if you were not Threatened at the time the reaction was triggered. This prevents Threatened status applied during the delay before the roll causing disadvantage.

It's possible this may cause some unintended interactions with effects that trigger off melee spell attacks, though I'm not aware of any.


Extensibility
Other mods can register their own spells for use as reactions. It's not quite trivial to do, but what would have been the most time-consuming part of the process is automated. See the API Sample under miscellaneous files for an example and some documentation (start from RegisterSpells.lua).




Compatibility

Should be placed after (below) 5e Spells and Community Library in the load order, if used.

Mirror Blade and any other mods that add their own spells for use with this mod should be loaded after (placed below) it.

Compatible with my Echo Knight mod, and replaces the War Caster Support optional file (which is an earlier version of this mod.)
It's safe to switch from the Echo Knight War Caster mod to this one at any time, as long as you don't simultaneously uninstall the main Echo Knight mod. Load order relative to Echo Knight doesn't matter.

Can coexist with War Caster 5e, but only the mod loaded last (placed lower) will control the War Caster (2014) feat.


Known Issues

Eldritch Blast cast via reaction uses one shared attack roll for all beams.

Available reactions don't update automatically when you change spell preparations. They will update on entering turn-based mode or combat, (un)equipping a melee weapon, saving the game, or resting.

Game bug: Reactions costing spell slots do not consistently accept Warlock spell slots. The More Upcasting optional file can be used to partially work around it - Warlock slots generally work when upcasting spells, but not when casting at the original level.

Game bug: Reaction tooltips often get stuck showing outdated information. This mod has contextual error messages that try to tell you exactly why reactions are not available, but because of this bug the information displayed may be incorrect. Save then load the game to refresh the UI if necessary.


Uninstalling

This mod can usually (there was a bug in 1.0) be uninstalled even if you have characters using the 2014 feat, but not the UA2 feat.
Respec to remove feat selections before uninstalling if necessary.