diff --git a/src/pf1e_simulator/runner.py b/src/pf1e_simulator/runner.py
index d0f442b..7b3d87c 100644
--- a/src/pf1e_simulator/runner.py
+++ b/src/pf1e_simulator/runner.py
@@ -190,11 +190,16 @@ def run_encounter(
     states = build_states(spec)
     if spell_registry is None:
         spell_registry = load_spell_registry(SPELLS_DIR)
-    ids = [s.combatant.id for s in states]
     wins = {side.name: 0 for side in spec.sides}
     draws = 0
     total_rounds = 0
-    totals = {cid: CombatantStats() for cid in ids}
+    # Accumulate whatever ids the engine reports: summoned creatures act in the
+    # battle and appear in result.stats, so seeding from the initial states
+    # only would KeyError on them. Ids seen for the first time get a fresh
+    # accumulator. Known limitation: two summons of the same creature id share
+    # one engine stats slot (the engine keys _LiveStats by id), so their
+    # attrition is merged, not split.
+    totals: dict[str, CombatantStats] = {}
     transcript: tuple[str, ...] | None = None
     for index in range(runs):
         if index > 0:
@@ -217,7 +222,10 @@ def run_encounter(
             wins[result.winner] = wins.get(result.winner, 0) + 1
         total_rounds += result.rounds
         for cid, stats in result.stats.items():
-            current = totals[cid]
+            current = totals.get(cid)
+            if current is None:
+                totals[cid] = stats
+                continue
             totals[cid] = CombatantStats(
                 hits=current.hits + stats.hits,
                 crits=current.crits + stats.crits,
diff --git a/tests/test_runner.py b/tests/test_runner.py
index 0742119..b943898 100644
--- a/tests/test_runner.py
+++ b/tests/test_runner.py
@@ -10,6 +10,7 @@ from __future__ import annotations
 
 import pytest
 
+from pf1e_simulator.combat import Action, CombatantState, CombatEngine, default_policy
 from pf1e_simulator.dice import parse_dice
 from pf1e_simulator.map import MapSpec, TerrainType
 from pf1e_simulator.metrics import win_rate, win_rate_band, win_rate_sigma
@@ -195,3 +196,76 @@ def test_build_states_rejects_too_many_combatants_for_zone() -> None:
     )
     with pytest.raises(ValueError, match="has 2 cells"):
         build_states(spec)
+
+
+def _make_runner_caster() -> Combatant:
+    attack = AttackSpec(
+        id="caster-w",
+        name="dagger",
+        kind="melee",
+        attack_bonus=2,
+        damage=[DamageComponent(formula=parse_dice("1d4"), types=["piercing"])],
+    )
+    return Combatant(
+        id="caster",
+        name="caster",
+        level=3,
+        size="Medium",
+        abilities=AbilityScores(
+            str_score=10, dex_score=12, con_score=14,
+            int_score=16, wis_score=12, cha_score=10,
+        ),
+        hp_max=40,
+        ac=ACProfile(total=14, touch=13, flat_footed=12),
+        bab=2,
+        initiative_mod=2,
+        speed_land_ft=30,
+        saves=Saves(fort=2, ref=3, will=3),
+        attacks=[attack],
+        casting_ability="int",
+        spells=["Summon Monster I"],
+        spell_slots={1: 3},
+    )
+
+
+def _summoning_policy(
+    engine: CombatEngine, state: CombatantState
+) -> tuple[Action, ...]:
+    caster_id = state.combatant.id
+    if caster_id.startswith("caster") and state.spell_slots_left.get(1, 0) > 0:
+        target = engine.nearest_enemy(state)
+        if target is not None:
+            return (
+                Action(
+                    kind="cast_spell",
+                    target_id=target.combatant.id,
+                    spell_name="Summon Monster I",
+                ),
+            )
+    return default_policy(engine, state)
+
+
+def test_run_encounter_with_summon_aggregates_summoned_ids() -> None:
+    """Regression: summoned creatures act in the battle and appear in
+    result.stats with ids absent from the deployed roster; the aggregation
+    must create their accumulators on demand (it KeyError-crashed before)."""
+    spec = EncounterSpec(
+        map=make_map(),
+        sides=(
+            Side(name="players", combatants=(_make_runner_caster(),), zone="A"),
+            Side(
+                name="monsters",
+                combatants=(make_combatant("gob", hp=30, speed=0),),
+                zone="B",
+            ),
+        ),
+        round_cap=10,
+    )
+    report = run_encounter(
+        spec, runs=5, seed=11, policy=_summoning_policy
+    )
+    assert "celestial_wolf" in report.attrition
+    assert {"caster", "gob"} <= set(report.attrition)
+    wolf = report.attrition["celestial_wolf"]
+    assert wolf.damage_dealt >= 0.0
+    assert report.wins["players"] + report.wins["monsters"] + report.draws == 5