diff --git a/src/pf1e_simulator/combat.py b/src/pf1e_simulator/combat.py
index 16e044f..051f20e 100644
--- a/src/pf1e_simulator/combat.py
+++ b/src/pf1e_simulator/combat.py
@@ -9,9 +9,13 @@ Phase 0 documented deviations from PF1e (conventions):
   straight-line charge path exists, `(5foot_step, full_attack)` when a single
   step brings the target into range, or `(move, attack)` otherwise (move at
   full speed along the Dijkstra path, then single attack). Immediate actions
-  (off-turn, consume next swift) are deferred; flanking, full attack, charge,
-  withdraw, and 5-foot step are modeled; attacks of opportunity are resolved
-  inline (see below).
+  (off-turn, consume next swift) are modeled via an opt-in ``reaction_policy``
+  hook: each resolved attack, movement step, spell cast, and damage application
+  emits a ``ReactionEvent``; the first combatant whose policy returns an
+  ``Action(kind="immediate")`` executes it immediately, and its next turn's
+  swift action is consumed. Flanking, full attack, charge, withdraw, and
+  5-foot step are modeled; attacks of opportunity are resolved inline (see
+  below).
 - A weapon with `count` > 1 ("2x Talons") resolves `count` independent attacks
   in one attack action; remaining swings are lost once the target drops
   (down or dead) mid-routine. BAB iterative attacks (full-round action
@@ -246,6 +250,7 @@ class CombatantState:
     power_attack: bool = False
     grappled_with: str | None = None
     deed_up_close_and_deadly: bool = False
+    immediate_used: bool = False
     summon_duration: int = 0
     summon_caster_id: str | None = None
     # Runtime weapon durability (not in the frozen data model): accumulated
@@ -363,6 +368,26 @@ class Action:
     channel_mode: Literal["heal", "harm"] | None = None  # channel energy polarity for this use
 
 
+@dataclass(frozen=True)
+class ReactionEvent:
+    """An off-turn event a ``reaction_policy`` may react to.
+
+    ``kind`` describes what just happened: ``attack_resolved`` (a swing was
+    resolved, ``actor`` is the attacker, ``target`` the defender, ``damage``
+    the damage dealt), ``movement_step`` (``actor`` moved one cell),
+    ``spell_cast`` (``actor`` finished casting at ``target``), or
+    ``damage_taken`` (``target`` lost ``damage`` hp from a non-attack source
+    such as a spell or channel energy). ``action`` is reserved for future
+    use (the triggering action is not threaded through the engine today).
+    """
+
+    kind: Literal["attack_resolved", "movement_step", "spell_cast", "damage_taken"]
+    actor: CombatantState
+    target: CombatantState | None = None
+    damage: int = 0
+    action: Action | None = None
+
+
 @dataclass(frozen=True)
 class CombatResult:
     """Outcome of a full battle: winner, rounds, log, and per-combatant stats."""
@@ -403,6 +428,7 @@ class CombatEngine:
         round_cap: int = 100,
         policy: Policy | None = None,
         spell_registry: dict[str, SpellSpec] | None = None,
+        reaction_policy: ReactionPolicy | None = None,
     ) -> None:
         self._rng = rng
         self._grid = grid
@@ -410,6 +436,8 @@ class CombatEngine:
         self._round_cap = round_cap
         self._policy = policy if policy is not None else default_policy
         self._spell_registry = spell_registry or {}
+        self._reaction_policy = reaction_policy
+        self._in_reaction = False
         self._transcript: list[str] = []
         self._stats: dict[str, _LiveStats] = {s.combatant.id: _LiveStats() for s in states}
         self._current_round = 0
@@ -443,6 +471,9 @@ class CombatEngine:
         state.moved_this_turn = False
         state.power_attack = False
         actions = self._policy(self, state)
+        if state.immediate_used:
+            actions = _without_swift_bonus(actions)
+            state.immediate_used = False
         if self._has_condition_named(state, "staggered"):
             actions = _staggered_filter(actions)
         elif self.has_condition_flag(state, "cant_attack"):
@@ -1407,6 +1438,11 @@ class CombatEngine:
                 break
             attacker.pos = step
             actual_path.append(step)
+            self._check_reactions(
+                ReactionEvent(kind="movement_step", actor=attacker)
+            )
+            if not attacker.active:
+                break
         if actual_path:
             attacker.moved_this_turn = True
             coords = "->".join(f"({p[0]},{p[1]})" for p in (start, *actual_path))
@@ -1440,6 +1476,8 @@ class CombatEngine:
             self._withdraw(state, target)
         elif kind in ("attack", "full_attack"):
             self._execute_attack(state, target, kind)
+        elif kind == "immediate":
+            self._execute_immediate(state, target, action)
         else:
             self._execute_movement(state, target, kind)
 
@@ -1476,6 +1514,46 @@ class CombatEngine:
         elif kind == "wait":
             self._log(f"round {self._current_round} {state.combatant.id}: wait")
 
+    def _execute_immediate(
+        self, state: CombatantState, target: CombatantState | None, action: Action
+    ) -> None:
+        """Execute an immediate action (off-turn or in-turn).
+
+        The immediate action consumes the combatant's next swift action
+        (``immediate_used``), which suppresses swift/free bonus actions on the
+        following turn. The effect is resolved by the action's carrier fields:
+        a spell, a maneuver, a class ability, or a single attack on ``target``.
+        """
+        state.immediate_used = True
+        if action.spell_name is not None:
+            self._execute_cast_spell(state, target, action.spell_name)
+        elif action.maneuver is not None:
+            self._execute_maneuver(state, target, action.maneuver)
+        elif action.class_ability is not None:
+            self._execute_class_ability(state, target, action)
+        elif target is not None:
+            self._execute_attack(state, target, "attack")
+
+    def _check_reactions(self, event: ReactionEvent) -> None:
+        """Offer an event to the reaction policy; execute the first immediate.
+
+        One immediate per event: the first active combatant (in state order)
+        whose policy returns an action gets to act. Reactions never cascade —
+        while an immediate is executing, further events are ignored.
+        """
+        if self._reaction_policy is None or self._in_reaction:
+            return
+        for state in self._states:
+            if not state.active or state.immediate_used:
+                continue
+            action = self._reaction_policy(self, state, event)
+            if action is None:
+                continue
+            self._in_reaction = True
+            self._execute(state, action)
+            self._in_reaction = False
+            return
+
     def _execute_cast_spell(
         self,
         state: CombatantState,
@@ -1887,6 +1965,14 @@ class CombatEngine:
                 f"round {self._current_round} {state.combatant.id}: "
                 f"{affected_state.combatant.id} dead"
             )
+        self._check_reactions(
+            ReactionEvent(
+                kind="damage_taken",
+                actor=state,
+                target=affected_state,
+                damage=dmg,
+            )
+        )
 
     def _execute_hex(
         self, state: CombatantState, target: CombatantState | None, hex_name: str | None
@@ -2182,6 +2268,10 @@ class CombatEngine:
                 f"round {self._current_round} {caster.combatant.id}: "
                 f"{target.combatant.id} dead"
             )
+        self._check_reactions(
+            ReactionEvent(kind="spell_cast", actor=caster, target=target)
+        )
+
     def _apply_spell_effects(
         self,
         spell: SpellSpec,
@@ -2246,7 +2336,7 @@ class CombatEngine:
         if isinstance(effect, DamageEffect):
             return self._apply_damage_effect(
                 effect, target, caster_live, target_live,
-                saved=saved, empower=empower, maximize=maximize,
+                saved=saved, empower=empower, maximize=maximize, caster=caster,
             )
         if isinstance(effect, ConditionEffect):
             return self._apply_condition_effect(effect, target, saved=saved, spell_name=spell_name)
@@ -2264,6 +2354,7 @@ class CombatEngine:
         saved: bool,
         empower: bool,
         maximize: bool,
+        caster: CombatantState,
     ) -> str:
         raw = effect.formula.roll(self._rng)
         if maximize:
@@ -2279,6 +2370,11 @@ class CombatEngine:
         target.hp -= dmg
         caster_live.damage_dealt += dmg
         target_live.damage_taken += dmg
+        self._check_reactions(
+            ReactionEvent(
+                kind="damage_taken", actor=caster, target=target, damage=dmg
+            )
+        )
         return f"{dmg} damage"
 
     def _apply_condition_effect(
@@ -2576,6 +2672,14 @@ class CombatEngine:
                 f"{target.combatant.id} down"
             )
             return True
+        self._check_reactions(
+            ReactionEvent(
+                kind="attack_resolved",
+                actor=attacker,
+                target=target,
+                damage=result.damage,
+            )
+        )
         return False
 
     def _attack(
@@ -2652,6 +2756,9 @@ class CombatEngine:
                 break
             state.pos = step
             actual_path.append(step)
+            self._check_reactions(ReactionEvent(kind="movement_step", actor=state))
+            if not state.active:
+                break
         if actual_path:
             state.moved_this_turn = True
             coords = "->".join(f"({p[0]},{p[1]})" for p in (start, *actual_path))
@@ -2791,6 +2898,9 @@ class CombatEngine:
                 break
             state.pos = step
             actual_path.append(step)
+            self._check_reactions(ReactionEvent(kind="movement_step", actor=state))
+            if not state.active:
+                break
         if actual_path:
             state.moved_this_turn = True
             coords = "->".join(f"({p[0]},{p[1]})" for p in (start, *actual_path))
@@ -2926,6 +3036,16 @@ _FULL_ROUND_KINDS: frozenset[str] = frozenset({"full_attack", "charge", "withdra
 _STANDARD_KINDS: frozenset[str] = frozenset({
     "attack", "cast_spell", "maneuver", "hex", "deed", "spell_combat", "channel_energy",
 })
+_SWIFT_BONUS_KINDS: frozenset[str] = frozenset({"deed", "inspire_courage", "swift"})
+
+
+def _without_swift_bonus(actions: tuple[Action, ...]) -> tuple[Action, ...]:
+    """Drop swift/free bonus actions (deed, inspire_courage, swift).
+
+    An immediate action consumes the combatant's next swift action; the
+    policy-returned sequence is filtered so no swift bonus fires that turn.
+    """
+    return tuple(a for a in actions if a.kind not in _SWIFT_BONUS_KINDS)
 
 
 def _staggered_filter(actions: tuple[Action, ...]) -> tuple[Action, ...]:
@@ -2953,6 +3073,10 @@ def _nauseated_filter(actions: tuple[Action, ...]) -> tuple[Action, ...]:
 
 Policy = Callable[[CombatEngine, CombatantState], tuple[Action, ...]]
 
+ReactionPolicy = Callable[
+    [CombatEngine, CombatantState, ReactionEvent], Action | None
+]
+
 
 def default_policy(engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
     """Full-attack the nearest enemy; cast spells; charge, 5ft-step, or approach if out of range.
diff --git a/tests/test_immediate_actions.py b/tests/test_immediate_actions.py
new file mode 100644
index 0000000..eeeb5b7
--- /dev/null
+++ b/tests/test_immediate_actions.py
@@ -0,0 +1,353 @@
+"""Tests for off-turn immediate actions (B5.4).
+
+A ``reaction_policy`` hook lets a combatant act outside its own turn with an
+``Action(kind="immediate")``. Per PF1e, an immediate action consumes the swift
+action of the combatant's next turn. The hook is opt-in: without a
+``reaction_policy`` the engine behaves exactly as before.
+"""
+
+from __future__ import annotations
+
+from typing import Literal
+
+from pf1e_simulator.combat import (
+    Action,
+    CombatantState,
+    CombatEngine,
+    CombatResult,
+    ReactionEvent,
+    ReactionPolicy,
+    default_policy,
+)
+from pf1e_simulator.dice import parse_dice
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.map import MapSpec, TerrainType
+from pf1e_simulator.models import (
+    AbilityScores,
+    ACProfile,
+    AttackSpec,
+    Combatant,
+    DamageComponent,
+    Saves,
+)
+from pf1e_simulator.rng import SeededRng
+from pf1e_simulator.spells import DamageEffect, SpellSpec
+
+
+def make_combatant(
+    cid: str,
+    *,
+    hp: int = 30,
+    ac: int = 15,
+    attack_bonus: int = 5,
+    damage: str = "1d6",
+    initiative_mod: int = 0,
+    speed: int = 30,
+    kind: Literal["melee", "ranged"] = "melee",
+) -> Combatant:
+    attack = AttackSpec(
+        id=f"{cid}-w",
+        name="short bow" if kind == "ranged" else "short sword",
+        kind=kind,
+        attack_bonus=attack_bonus,
+        damage=[
+            DamageComponent(
+                formula=parse_dice(damage),
+                types=["piercing"] if kind == "ranged" else ["slashing"],
+            )
+        ],
+        range_increment_ft=60 if kind == "ranged" else None,
+    )
+    return Combatant(
+        id=cid,
+        name=cid,
+        level=1,
+        size="Medium",
+        abilities=AbilityScores(
+            str_score=10,
+            dex_score=10,
+            con_score=12,
+            int_score=10,
+            wis_score=10,
+            cha_score=10,
+        ),
+        hp_max=hp,
+        ac=ACProfile(total=ac, touch=ac, flat_footed=ac),
+        bab=attack_bonus,
+        initiative_mod=initiative_mod,
+        speed_land_ft=speed,
+        saves=Saves(fort=0, ref=0, will=0),
+        attacks=[attack],
+    )
+
+
+def make_state(combatant: Combatant, side: str, pos: tuple[int, int]) -> CombatantState:
+    return CombatantState(combatant=combatant, side=side, pos=pos, hp=combatant.hp_max)
+
+
+def make_grid() -> Grid:
+    legend = {".": TerrainType(type="floor", move_cost=1)}
+    spec = MapSpec(name="test", terrain=tuple(["." * 8] * 8), legend=legend)
+    return Grid.from_spec(spec)
+
+
+def test_reaction_policy_records_events() -> None:
+    """A reaction policy is invoked for movement and attack events."""
+    hero = make_combatant("hero", initiative_mod=5)
+    goblin = make_combatant("goblin")
+    states = [make_state(hero, "players", (1, 1)), make_state(goblin, "monsters", (1, 6))]
+    events: list[ReactionEvent] = []
+
+    def approach_policy(
+        _engine: CombatEngine, state: CombatantState
+    ) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (
+                Action(kind="move", target_id="goblin"),
+                Action(kind="attack", target_id="goblin"),
+            )
+        return default_policy(_engine, state)
+
+    def recording_policy(
+        _engine: CombatEngine,
+        _state: CombatantState,
+        event: ReactionEvent,
+    ) -> Action | None:
+        events.append(event)
+        return None
+
+    engine = CombatEngine(
+        SeededRng(3),
+        make_grid(),
+        states,
+        policy=approach_policy,
+        reaction_policy=recording_policy,
+    )
+    engine.run()
+    assert any(e.kind == "movement_step" for e in events)
+    assert any(e.kind == "attack_resolved" for e in events)
+
+
+def test_no_reaction_policy_runs_unchanged() -> None:
+    """A no-op reaction policy leaves the transcript bit-identical."""
+
+    def make_run(reaction_policy: ReactionPolicy | None = None) -> CombatResult:
+        states = [
+            make_state(make_combatant("hero", initiative_mod=5), "players", (1, 1)),
+            make_state(make_combatant("goblin"), "monsters", (1, 2)),
+        ]
+        return CombatEngine(
+            SeededRng(9), make_grid(), states, reaction_policy=reaction_policy
+        ).run()
+
+    def noop_policy(
+        _engine: CombatEngine,
+        _state: CombatantState,
+        _event: ReactionEvent,
+    ) -> Action | None:
+        return None
+
+    assert make_run(reaction_policy=noop_policy).transcript == make_run().transcript
+
+
+def test_immediate_attack_reaction_executes() -> None:
+    """A waiting ally can react to an enemy attack with an immediate attack."""
+    hero = make_combatant("hero", initiative_mod=5)
+    goblin = make_combatant("goblin")
+    hero_state = make_state(hero, "players", (1, 1))
+    goblin_state = make_state(goblin, "monsters", (1, 2))
+    states = [hero_state, goblin_state]
+
+    def wait_policy(
+        _engine: CombatEngine, state: CombatantState
+    ) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="wait"),)
+        return default_policy(_engine, state)
+
+    def reaction_policy(
+        _engine: CombatEngine, state: CombatantState, event: ReactionEvent
+    ) -> Action | None:
+        if (
+            state.combatant.id == "hero"
+            and event.kind == "attack_resolved"
+            and event.target is not None
+            and event.target.combatant.id == "hero"
+        ):
+            return Action(kind="immediate", target_id="goblin")
+        return None
+
+    engine = CombatEngine(
+        SeededRng(4),
+        make_grid(),
+        states,
+        policy=wait_policy,
+        reaction_policy=reaction_policy,
+    )
+    result = engine.run()
+    assert any("hero: short sword vs goblin" in line for line in result.transcript)
+    assert goblin_state.hp < 30
+
+
+def test_immediate_consumes_next_swift() -> None:
+    """An immediate action consumes the swift action of the next turn."""
+    hero = make_combatant("hero", initiative_mod=5)
+    goblin = make_combatant("goblin")
+    hero_state = make_state(hero, "players", (1, 1))
+    hero_state.resources_left = {"grit": 2}
+    states = [hero_state, make_state(goblin, "monsters", (1, 2))]
+    reacted = False
+
+    def deed_policy(
+        _engine: CombatEngine, state: CombatantState
+    ) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            if state.resources_left.get("grit", 0) > 0:
+                return (
+                    Action(
+                        kind="deed",
+                        target_id="goblin",
+                        deed_name="up_close_and_deadly",
+                    ),
+                    Action(kind="attack", target_id="goblin"),
+                )
+            return (Action(kind="attack", target_id="goblin"),)
+        return default_policy(_engine, state)
+
+    def reaction_policy(
+        _engine: CombatEngine, state: CombatantState, event: ReactionEvent
+    ) -> Action | None:
+        nonlocal reacted
+        if state.combatant.id == "hero" and event.kind == "attack_resolved" and not reacted:
+            reacted = True
+            return Action(kind="immediate", target_id="goblin")
+        return None
+
+    engine = CombatEngine(
+        SeededRng(4),
+        make_grid(),
+        states,
+        policy=deed_policy,
+        reaction_policy=reaction_policy,
+    )
+    result = engine.run()
+    deed_rounds = [
+        int(line.split()[1])
+        for line in result.transcript
+        if "hero: deed Up Close and Deadly activated" in line
+    ]
+    assert deed_rounds == [1, 3]
+
+
+def test_immediate_in_turn_sequence_executes() -> None:
+    """An immediate action in the turn sequence executes like an attack."""
+    hero = make_combatant("hero", initiative_mod=5)
+    goblin = make_combatant("goblin")
+    hero_state = make_state(hero, "players", (1, 1))
+    goblin_state = make_state(goblin, "monsters", (1, 2))
+    states = [hero_state, goblin_state]
+
+    def immediate_policy(
+        _engine: CombatEngine, state: CombatantState
+    ) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="immediate", target_id="goblin"),)
+        return default_policy(_engine, state)
+
+    engine = CombatEngine(
+        SeededRng(4),
+        make_grid(),
+        states,
+        policy=immediate_policy,
+    )
+    result = engine.run()
+    assert any("hero: short sword vs goblin" in line for line in result.transcript)
+    assert goblin_state.hp < 30
+
+
+def test_reaction_determinism_same_seed_identical_transcripts() -> None:
+    """Reactions draw from the engine RNG: same seed, identical transcripts."""
+
+    def make_run() -> CombatResult:
+        hero = make_combatant("hero", initiative_mod=5)
+        goblin = make_combatant("goblin")
+        states = [make_state(hero, "players", (1, 1)), make_state(goblin, "monsters", (1, 2))]
+
+        def wait_policy(
+            _engine: CombatEngine, state: CombatantState
+        ) -> tuple[Action, ...]:
+            if state.combatant.id == "hero":
+                return (Action(kind="wait"),)
+            return default_policy(_engine, state)
+
+        def reaction_policy(
+            _engine: CombatEngine, state: CombatantState, event: ReactionEvent
+        ) -> Action | None:
+            if (
+                state.combatant.id == "hero"
+                and event.kind == "attack_resolved"
+                and event.target is not None
+                and event.target.combatant.id == "hero"
+            ):
+                return Action(kind="immediate", target_id="goblin")
+            return None
+
+        return CombatEngine(
+            SeededRng(7),
+            make_grid(),
+            states,
+            policy=wait_policy,
+            reaction_policy=reaction_policy,
+        ).run()
+
+    result_one = make_run()
+    result_two = make_run()
+    assert result_one.transcript == result_two.transcript
+    assert result_one.transcript
+    assert any("hero: short sword vs goblin" in line for line in result_one.transcript)
+
+
+def test_damage_taken_reaction_fires() -> None:
+    """A reaction policy can react to spell damage taken."""
+    caster = make_combatant("caster", hp=50)
+    reactor = make_combatant("reactor", hp=50, kind="ranged")
+    caster_state = make_state(caster, "players", (1, 1))
+    reactor_state = make_state(reactor, "monsters", (1, 3))
+    states = [caster_state, reactor_state]
+
+    bolt = SpellSpec(
+        name="Bolt",
+        level=1,
+        school="evocation",
+        range="close",
+        save="none",
+        effects=(DamageEffect(formula=parse_dice("1d6"), types=("force",)),),
+    )
+
+    def cast_policy(
+        _engine: CombatEngine, state: CombatantState
+    ) -> tuple[Action, ...]:
+        if state.combatant.id == "caster":
+            return (Action(kind="cast_spell", target_id="reactor", spell_name="Bolt"),)
+        if state.combatant.id == "reactor":
+            return (Action(kind="wait"),)
+        return default_policy(_engine, state)
+
+    def reaction_policy(
+        _engine: CombatEngine, state: CombatantState, event: ReactionEvent
+    ) -> Action | None:
+        if state.combatant.id == "reactor" and event.kind == "damage_taken":
+            return Action(kind="immediate", target_id="caster")
+        return None
+
+    engine = CombatEngine(
+        SeededRng(4),
+        make_grid(),
+        states,
+        policy=cast_policy,
+        spell_registry={"Bolt": bolt},
+        reaction_policy=reaction_policy,
+    )
+    result = engine.run()
+    assert any("reactor: short bow vs caster" in line for line in result.transcript)
+    assert caster_state.hp < 50