fix(rules): trait and racial are non-stacking bonus types (APG Traits, CRB Combining Magic Effects)

src/pf1e_simulator/effects.py

@@ -1,10 +1,14 @@
 """Effect system: bonus types, stat modifiers, and PF1e stacking rules.
 
 PF1e bonus types determine whether multiple bonuses to the same stat stack:
-- Stacking types (dodge, racial, trait, untyped/"") all sum together.
+- Stacking types (dodge, untyped/"") all sum together (UM Table 2-7: "dodge
+  bonuses always stack"; CRB: "A bonus that doesn't have a type stacks with
+  any bonus").
 - Non-stacking types (morale, sacred, profane, competence, insight, luck,
-  enhancement, deflection, natural, shield, armor) keep only the highest
-  value per type.
+  racial, trait, enhancement, deflection, natural, shield, armor,
+  circumstance) keep only the highest value per type (CRB "Combining Magic
+  Effects": "two bonuses of the same type don't stack even if they come from
+  different spells"; APG "Traits": "Trait bonuses do not stack").
 - Penalties follow the same rules: same-type penalties don't stack (the
   least severe — i.e. the max value — is kept).
 
@@ -32,8 +36,8 @@ BonusType = Literal[
     "competence",     # no stack
     "insight",        # no stack
     "luck",           # no stack
-    "racial",         # stacks with itself
-    "trait",          # stacks with itself
+    "racial",         # no stack (CRB "Combining Magic Effects": same type)
+    "trait",          # no stack (APG "Traits": "Trait bonuses do not stack")
     "enhancement",    # no stack
     "deflection",     # no stack
     "natural",        # no stack
@@ -59,7 +63,7 @@ StatTarget = Literal[
     "dex_skills",
 ]
 
-_STACKING_TYPES: frozenset[str] = frozenset({"", "dodge", "racial", "trait"})
+_STACKING_TYPES: frozenset[str] = frozenset({"", "dodge"})
 
 
 @dataclass(frozen=True)
@@ -118,10 +122,11 @@ class ActiveCondition:
 def resolve_modifiers(modifiers: Iterable[StatModifier]) -> int:
     """Sum stat modifiers applying PF1e stacking rules.
 
-    Stacking bonus types (dodge, racial, trait, untyped) all sum together.
-    Non-stacking types keep only the highest value per type (max is used,
-    which correctly handles both bonuses and penalties: for bonuses the
-    highest is most beneficial, for penalties the highest is least severe).
+    Stacking bonus types (dodge, untyped) all sum together. Non-stacking
+    types (including racial and trait) keep only the highest value per type
+    (max is used, which correctly handles both bonuses and penalties: for
+    bonuses the highest is most beneficial, for penalties the highest is
+    least severe).
 
     The caller is responsible for filtering by condition and weapon before
     calling — this function performs pure stacking math.

tests/test_effects.py

@@ -19,20 +19,6 @@ class TestStackingTypes:
         ]
         assert resolve_modifiers(mods) == 3
 
-    def test_two_racial_bonus_stack(self) -> None:
-        mods = [
-            StatModifier("save", 1, "racial"),
-            StatModifier("save", 1, "racial"),
-        ]
-        assert resolve_modifiers(mods) == 2
-
-    def test_two_trait_bonus_stack(self) -> None:
-        mods = [
-            StatModifier("attack", 1, "trait"),
-            StatModifier("attack", 1, "trait"),
-        ]
-        assert resolve_modifiers(mods) == 2
-
     def test_two_untyped_bonus_stack(self) -> None:
         mods = [
             StatModifier("attack", 1),
@@ -43,8 +29,8 @@ class TestStackingTypes:
     def test_mixed_stacking_types_all_sum(self) -> None:
         mods = [
             StatModifier("ac", 1, "dodge"),
-            StatModifier("ac", 2, "racial"),
-            StatModifier("ac", 1, "trait"),
+            StatModifier("ac", 2, "dodge"),
+            StatModifier("ac", 1),
             StatModifier("ac", 1),
         ]
         assert resolve_modifiers(mods) == 5
@@ -101,6 +87,23 @@ class TestNonStackingTypes:
         ]
         assert resolve_modifiers(mods) == 2
 
+    def test_two_racial_bonus_do_not_stack(self) -> None:
+        # CRB "Combining Magic Effects": "two bonuses of the same type don't
+        # stack" — racial is a named type.
+        mods = [
+            StatModifier("save", 1, "racial"),
+            StatModifier("save", 2, "racial"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
+    def test_two_trait_bonus_do_not_stack(self) -> None:
+        # APG "Traits": "Trait bonuses do not stack".
+        mods = [
+            StatModifier("attack", 1, "trait"),
+            StatModifier("attack", 2, "trait"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
 
 # ── Penalties ─────────────────────────────────────────────────────────────────