fix(rules): allow a Will save to halve channeled energy damage

PF1e CRB (Channel Energy): creatures damaged by channeled energy get a Will
save (DC 10 + half the channeler's level + her Charisma modifier) for half
damage (floor). Harm mode only; heal mode unchanged. Charisma is used RAW
even for casters whose spellcasting ability differs (channeling is a
Charisma class feature).

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

README.md

@@ -949,7 +949,9 @@ Règles modélisées :
   si un allié est < 50 % de ses PV → soigne tous les êtres vivants à 30 ft ; sinon, si au
   moins 2 morts-vivants ennemis sont groupés à 30 ft → les blesse en 2d6 ; le camp de la
   cible n'est pas pris en compte — seul `creature_type` compte, `selective_channeling` non
-  modélisé), sortilège `evil_eye` (nairda), deed `up_close_and_deadly`
+  modélisé ; les créatures blessées obtiennent un jet de Volonté DD `10 +
+  demi-niveau du canaliseur + modificateur de Charisme` pour demi-dégâts
+  — CRB Channel Energy ; le mode soin n'accorde pas de sauvegarde), sortilège `evil_eye` (nairda), deed `up_close_and_deadly`
   (tammara, action bonus +1d6 à ≤ 30 ft — consommé sur la première attaque
   résolue : si elle est à distance ET à ≤ 30 ft, +1d6 s'ajoute aux dégâts ;
   sinon le bonus est perdu ; le drapeau est réinitialisé dans tous les cas),

src/pf1e_simulator/combat.py

@@ -4195,7 +4195,9 @@ class CombatEngine:
         Heal mode heals all living creatures in the burst; harm mode damages
         all undead creatures in the burst — the target's side does not matter
         (PF1e Channel Energy: positive energy heals living creatures and
-        damages undead creatures within 30 feet).
+        damages undead creatures within 30 feet). Damaged creatures get a
+        Will save (DC 10 + half the channeler's level + her Cha modifier)
+        for half damage (CRB Channel Energy).
         """
         if not state.spend_resource("channel_energy", 1):
             self._log(
@@ -4223,11 +4225,20 @@ class CombatEngine:
         # 2d6 damage/healing
         dmg = 1 + self._rng.roll_die(11)  # 2d6
 
+        # PF1e (CRB, Channel Energy): creatures damaged by the channel get a
+        # Will save (DC 10 + half the channeler's level + her Charisma
+        # modifier) for half damage. Charisma is used RAW even when the
+        # caster's spellcasting ability is another one (channeling is a
+        # Charisma class feature). Heal mode grants no save.
+        channel_dc = 10 + state.combatant.level // 2 + (
+            (state.combatant.abilities.cha_score - 10) // 2
+        )
+
         for affected_state in hit:
             if channel_mode == "heal":
                 self._channel_heal(state, affected_state, dmg)
             else:
-                self._channel_damage(state, affected_state, dmg)
+                self._channel_damage(state, affected_state, dmg, channel_dc)
 
         self._log(
             f"round {self._current_round} {state.combatant.id}: "
@@ -4248,15 +4259,20 @@ class CombatEngine:
         )
 
     def _channel_damage(
-        self, state: CombatantState, affected_state: CombatantState, dmg: int
+        self, state: CombatantState, affected_state: CombatantState, dmg: int, dc: int
     ) -> None:
+        """Apply channel energy damage: Will save (DC from the channeler) for half."""
+        save = self.resolve_save(affected_state, "will", dc)
+        dealt = dmg // 2 if save.success else dmg
         hp_before = affected_state.hp
-        affected_state.hp -= dmg
+        affected_state.hp -= dealt
-        self._stats[state.combatant.id].damage_dealt += dmg
+        self._stats[state.combatant.id].damage_dealt += dealt
-        self._stats[affected_state.combatant.id].damage_taken += dmg
+        self._stats[affected_state.combatant.id].damage_taken += dealt
         self._log(
-            f"round {self._current_round} {state.combatant.id}: "
+            f"round {self._current_round} {state.combatant.id}: channel energy "
-            f"channel energy damages {affected_state.combatant.id} {dmg} "
+            f"damages {affected_state.combatant.id} {dealt} "
+            f"(Will DC{dc} d20={save.roll}+{save.total - save.roll}={save.total} "
+            f"{'SAVED half' if save.success else 'FAILED full'}) "
             f"({hp_before}->{affected_state.hp})"
         )
         if affected_state.dead:
@@ -4269,7 +4285,7 @@ class CombatEngine:
                 kind="damage_taken",
                 actor=state,
                 target=affected_state,
-                damage=dmg,
+                damage=dealt,
             )
         )
 

tests/test_class_abilities.py

@@ -155,15 +155,32 @@ def test_oni_channel_harm_does_not_damage_living_enemy() -> None:
 
 
 def test_oni_channel_harm_damages_undead() -> None:
-    """Harm mode damages undead creatures in the burst regardless of side."""
+    """Harm mode damages undead creatures in the burst regardless of side;
+    a failed Will save takes full damage (CRB Channel Energy)."""
     oni = _combatant("oni", resources={"channel_energy": 6}, hp_max=30)
     lich = _combatant("lich", hp_max=100, creature_type="undead")
-    engine, me, enemy = _engine(oni, [lich], rng_queue=[11])
+    engine, me, enemy = _engine(oni, [lich], rng_queue=[11, 5])
 
     action = Action(kind="channel_energy", target_id=lich.id, channel_mode="harm")
     engine._execute(me, action)
 
     assert enemy[0].hp == 100 - (1 + 11)  # 2d6 = 1 + roll_die(11)
+    # Will save fails: d20 5 + will 4 = 9 < DC 14 (10 + lvl//2 2 + cha 2)
+    assert any("Will DC14" in line and "FAILED full" in line for line in engine._transcript)
+
+
+def test_oni_channel_harm_will_save_halves_damage() -> None:
+    """CRB Channel Energy: an undead that passes the Will save takes half
+    damage (floor)."""
+    oni = _combatant("oni", resources={"channel_energy": 6}, hp_max=30)
+    lich = _combatant("lich", hp_max=100, creature_type="undead")
+    engine, me, enemy = _engine(oni, [lich], rng_queue=[11, 20])
+
+    action = Action(kind="channel_energy", target_id=lich.id, channel_mode="harm")
+    engine._execute(me, action)
+
+    assert enemy[0].hp == 100 - (1 + 11) // 2  # 2d6 = 12, saved for half = 6
+    assert any("Will DC14" in line and "SAVED half" in line for line in engine._transcript)
 
 
 def test_oni_channel_heal_heals_living_enemies_in_range() -> None: