Reactions in BG3 — How They Work & Which Are Best

Best Reactions in BG3

Automatic vs. Manual Reaction Mode

BG3 offers two reaction processing modes accessible through Settings > Gameplay. In Automatic mode, reactions fire immediately whenever their trigger condition is met — no confirmation required. This sounds convenient but causes significant problems. If you have both Opportunity Attack and Sentinel active, Automatic mode fires Opportunity Attack on the first creature that moves away, burning your reaction before a more important enemy makes their move.

Manual mode pauses the game when a reaction trigger occurs and displays a popup asking whether you want to use your reaction. This adds a tiny time cost but is almost always the correct choice. It lets you decline a reaction against weak enemies to save it for a more important trigger. A Wizard who auto-fires Shield against a 1-HP goblin's final attack has wasted a 1st-level spell slot and their reaction — both of which could have stopped the enemy spellcaster's Fireball two seconds later.

The exception where Automatic mode can work: pure Opportunity Attack builds using the Sentinel feat, where you specifically want to trigger Sentinel on any creature that tries to flee and don't want to deal with popups. Even then, most experienced players prefer manual for the control it provides over harder encounters.

The Shield Spell — Best Reaction in the Game

Shield is a 1st-level Abjuration spell available to Wizards and Sorcerers (and Eldritch Knights via their spellcasting). When you are hit by an attack, you can cast Shield as a reaction using a 1st-level slot. Until the start of your next turn, your AC increases by +5. Critically, this bonus applies retroactively to the triggering attack — meaning if the attack roll would have hit your old AC but misses your new AC+5, the hit is negated.

A Wizard at level 10 might have AC 14 from Mage Armor. Without Shield, a fighter hitting on 13+ has a 40% chance to land. With Shield available, you can negate any hit that falls between 14–18, which covers the vast majority of attacks. Effectively, Shield converts a 1st-level spell slot into damage immunity for one attack, making it one of the best defensive resources per slot in D&D 5e and BG3.

Shield is most powerful early in each long rest cycle when you have abundant 1st-level slots. In the late game, many BG3 casters have 5–6 first-level slots, meaning they can Shield up to six incoming hits before being exhausted. This is why Arcane Recovery (Wizard feature) is so valuable — it restores spell slots on a short rest, refueling your Shield supply.

Counterspell — Shutting Down Enemy Casters

Counterspell is a 3rd-level reaction spell that lets you cancel a spell being cast by an enemy within 60 feet. Against spells of 3rd level or lower, it automatically succeeds — no roll required. Against higher-level spells, you make a spellcasting ability check (DC 10 + spell level) to cancel it. With a high Intelligence or Charisma modifier, you can reliably counter 4th and 5th level spells this way.

The highest-value Counterspell targets in BG3 are Hold Person/Monster (which can lock your entire party), Hypnotic Pattern (area crowd control), and Mass Suggestion. A single well-placed Counterspell can prevent what would have been a total party wipe. Counterspell is particularly valuable in honor mode and tactician difficulty where enemy spellcasters are at their most dangerous.

Multiple party members having Counterspell creates an exceptional defensive setup. If the enemy has their own Counterspell and tries to cancel yours, your ally can Counterspell the Counterspell — though this quickly escalates into a spell slot war. Generally: one Counterspell per party is sufficient, but two creates redundancy for the most important fights.

Sentinel Feat — Lock Down the Battlefield

• Sentinel grants three benefits: Opportunity Attacks against creatures attempting to Disengage succeed anyway, Opportunity Attacks reduce the target's speed to 0 (stopping all movement that turn), and you can make an Opportunity Attack when a creature within reach attacks an ally instead of you.
• The speed reduction to 0 is the critical part: enemies cannot run past your frontliner to reach your wizard or cleric even if they use Disengage. Combined with the Polearm Master feat (which extends reach and grants a Bonus Action attack), Sentinel creates an effective zone of control around your character.
• Sentinal is a feat, so it costs one of your Ability Score Improvement slots. It is best on tanky frontliners (Fighter, Paladin, Barbarian) who want enemies to stay engaged with them and off the backline. Paladins especially benefit because enemies stuck in melee range remain in Aura of Protection range.
• Sentinel does NOT prevent teleportation abilities like Misty Step — creatures that teleport out of your reach don't trigger Opportunity Attacks at all. Keep this in mind against mage-type enemies.

Uncanny Dodge — Rogue's Damage Halving

At Rogue level 5, Uncanny Dodge lets you use your reaction when an attacker you can see hits you to halve the incoming damage. Unlike most reactions, Uncanny Dodge doesn't require a spell slot or resource beyond the reaction itself. This makes it a free damage reduction tool that stacks well with the Rogue's naturally high AC from Dexterity.

Uncanny Dodge is best used against high-damage single hits rather than multiple smaller attacks. Halving 60 damage from a boss strike saves 30 HP; halving a 10-damage goblin attack saves only 5. Learn to identify which incoming hit is most worth halving and save your reaction for it. In manual mode, you can always decline the reaction on weak attacks and wait for a bigger threat.

Reaction Tier List by Value

Verdict: Shield and Counterspell are the two best reactions in BG3. If your class has access to them, prioritize keeping your reaction available for these rather than spending it on Opportunity Attacks.