diff --git a/src/pf1e_simulator/combat.py b/src/pf1e_simulator/combat.py
index 18274f7..ac62c59 100644
--- a/src/pf1e_simulator/combat.py
+++ b/src/pf1e_simulator/combat.py
@@ -7165,8 +7165,6 @@ class CombatEngine:
         self, effect: SpellEffect, *, has_extend: bool
     ) -> SpellEffect:
         """Apply Extend Spell to one effect (recursing into AreaEffect sub-effects)."""
-        if isinstance(effect, (DamageEffect, HealEffect)):
-            return effect
         if isinstance(effect, ConditionEffect):
             duration = effect.duration_rounds * 2 if has_extend else effect.duration_rounds
             per_level = (
@@ -7210,6 +7208,23 @@ class CombatEngine:
                 duration_rounds_per_level=duration,
                 half_on_save=effect.half_on_save,
             )
+        if isinstance(effect, SummonEffect):
+            # PF1e (SRD feats/extend-spell.md): "An extended spell lasts
+            # twice as long as normal. A spell with a duration of
+            # concentration, instantaneous, or permanent is not affected" -
+            # summon monster's "1 round/level (D)" is none of those, so
+            # Extend doubles it (both the fixed and the per-level form).
+            duration = effect.duration_rounds * 2 if has_extend else effect.duration_rounds
+            per_level = (
+                effect.duration_rounds_per_level * 2
+                if has_extend
+                else effect.duration_rounds_per_level
+            )
+            return SummonEffect(
+                creature=effect.creature,
+                duration_rounds=duration,
+                duration_rounds_per_level=per_level,
+            )
         if isinstance(effect, AreaEffect):
             return AreaEffect(
                 shape=effect.shape,
diff --git a/tests/test_summoning.py b/tests/test_summoning.py
index c77ccac..4b5e6d8 100644
--- a/tests/test_summoning.py
+++ b/tests/test_summoning.py
@@ -335,3 +335,70 @@ class TestSummonDurationScalesWithCasterLevel:
             effect = spell.effects[0]
             assert isinstance(effect, SummonEffect)
             assert effect.duration_rounds_per_level == 1
+
+
+class TestExtendSpellOnSummon:
+    """SRD feats/extend-spell.md: "An extended spell lasts twice as long as
+    normal. A spell with a duration of concentration, instantaneous, or
+    permanent is not affected" - summon monster's "1 round/level (D)" is
+    none of those, so Extend must double it."""
+
+    def test_extend_doubles_the_per_level_summon_duration(self) -> None:
+        # Given: a level 5 caster who knows Extend Spell
+        # When: summoning with Extend Spell (1 round/level -> 2 rounds/level)
+        # Then: the wolf is bound for 10 rounds, and the level-3 slot burns.
+        caster = _make_caster().model_copy(
+            update={
+                "metamagic_feats": ["Extend Spell"],
+                "spell_slots": {1: 3, 2: 3, 3: 3},
+            }
+        )
+        enemy = _make_enemy()
+        c = CombatantState(combatant=caster, side="players", pos=(0, 0), hp=caster.hp_max)
+        e = CombatantState(combatant=enemy, side="monsters", pos=(5, 0), hp=enemy.hp_max)
+        spell = SpellSpec(
+            name="Summon Monster II",
+            level=2,
+            school="conjuration",
+            range="close",
+            effects=(
+                SummonEffect(
+                    creature="data/monsters/celestial_wolf.json",
+                    duration_rounds=1,
+                    duration_rounds_per_level=1,
+                ),
+            ),
+        )
+        legend = {".": TerrainType(type="floor", move_cost=1)}
+        spec = MapSpec(name="test", terrain=tuple(["." * 12] * 12), legend=legend)
+        engine = CombatEngine(
+            SeededRng(1), Grid.from_spec(spec), [c, e],
+            spell_registry={"Summon Monster II": spell},
+        )
+        engine._cast_spell(c, e, "Summon Monster II", metamagic=("Extend Spell",))
+        wolf = next(s for s in engine._states if s.combatant.id == "celestial_wolf")
+        assert wolf.summon_duration == 10
+        assert c.spell_slots_left[3] == 2  # extended: level-3 slot
+
+    def test_extend_doubles_a_fixed_summon_duration(self) -> None:
+        # Given: a summon with only a fixed duration (per_level = 0)
+        # When: the summon is cast extended (feat applied to this cast)
+        # Then: the fixed rounds double (2 -> 4).
+        caster = _make_caster().model_copy(
+            update={
+                "metamagic_feats": ["Extend Spell"],
+                "spell_slots": {1: 3, 2: 3, 3: 3},
+            }
+        )
+        enemy = _make_enemy()
+        c = CombatantState(combatant=caster, side="players", pos=(0, 0), hp=caster.hp_max)
+        e = CombatantState(combatant=enemy, side="monsters", pos=(5, 0), hp=enemy.hp_max)
+        reg = {"Summon Monster II": _make_summon_spell(duration_rounds=2)}
+        legend = {".": TerrainType(type="floor", move_cost=1)}
+        spec = MapSpec(name="test", terrain=tuple(["." * 12] * 12), legend=legend)
+        engine = CombatEngine(
+            SeededRng(1), Grid.from_spec(spec), [c, e], spell_registry=reg
+        )
+        engine._cast_spell(c, e, "Summon Monster II", metamagic=("Extend Spell",))
+        wolf = next(s for s in engine._states if s.combatant.id == "celestial_wolf")
+        assert wolf.summon_duration == 4