fix(rules): metamagic no longer raises the save DC - only Heighten Spell does (SRD empower-spell: saving throws not affected; heighten-spell: DCs calculated at the heightened level, modeled at its minimum +1 slot adjustment)

src/pf1e_simulator/combat.py

@@ -248,10 +248,14 @@ _IMPROVED_MANEUVER_FEATS: dict[str, str] = {
 }
 # PF1e (CRB, Metamagic Feats): a metamagic spell occupies a slot of its
 # level + the feat's adjustment. B7: applied per cast, never blanket-on.
+# Heighten Spell's RAW adjustment is variable (the caster picks the
+# heightened level); the engine's per-cast API carries feat names only, so
+# it is modeled at its minimum +1 (documented simplification).
 _METAMAGIC_SLOT_ADJUST: dict[str, int] = {
     "Empower Spell": 2,
     "Maximize Spell": 3,
     "Extend Spell": 1,
+    "Heighten Spell": 1,
 }
 _COMBAT_CASTING_FEAT = "Combat Casting"  # PF1e: +4 concentration (defensive/grappled only)
 # PF1e: skill checks that take Dexterity-based penalties (the "dex_skills"
@@ -664,8 +668,10 @@ class PendingCast:
     your turn in the round after you began casting the spell. You then act
     normally after the spell is completed." Everything the delivery still
     needs is frozen at the start of the casting (the slot is already
-    spent): ``slot_level`` carries the metamagic-inflated level (B7) the
-    save DC uses, ``hp_at_cast`` is the caster's hp when the casting began
+    spent): ``slot_level`` carries the metamagic-inflated level (B7) that
+    drives the concentration checks and the Heighten Spell DC (other
+    metamagic does not raise the DC - see _spell_dc), ``hp_at_cast`` is the
+    caster's hp when the casting began
     — the net hp lost before resolution is the interruption damage that
     forces the concentration check (CRB "Concentration": injured while
     casting, DC 10 + damage + spell level).
@@ -5867,15 +5873,30 @@ class CombatEngine:
         return (ability_score - 10) // 2
 
     def _spell_dc(
-        self, caster: CombatantState, spell: SpellSpec, *, slot_level: int | None = None
+        self,
+        caster: CombatantState,
+        spell: SpellSpec,
+        *,
+        slot_level: int | None = None,
+        metamagic: tuple[str, ...] = (),
     ) -> int:
         """Spell save DC: 10 + spell level + casting ability modifier.
 
-        PF1e (CRB, B7): a metamagic spell uses the level of the slot cast
-        from, so ``slot_level`` overrides the spell's base level when given.
+        PF1e (SRD, Metamagic Feats): a metamagic spell occupies a higher
+        slot but its save DC uses the spell's BASE level - "An empowered
+        spell uses up a spell slot two levels higher than the spell's
+        actual level" while "Saving throws and opposed rolls are not
+        affected" (feats/empower-spell.md). The one exception is Heighten
+        Spell: "Unlike other metamagic feats, Heighten Spell actually
+        increases the effective level of the spell [...] All effects
+        dependent on spell level (such as saving throw DCs [...]) are
+        calculated according to the heightened level"
+        (feats/heighten-spell.md) - so ``slot_level`` (the level the spell
+        is cast at) feeds the DC only when Heighten Spell is applied.
         """
-        level = spell.level if slot_level is None else slot_level
-        return 10 + level + self._casting_ability_mod(caster)
+        if "Heighten Spell" in metamagic and slot_level is not None:
+            return 10 + slot_level + self._casting_ability_mod(caster)
+        return 10 + spell.level + self._casting_ability_mod(caster)
 
     def _spell_snapshot(self, state: CombatantState, spell_name: str) -> dict[str, object]:
         """One per-spell dict for the Observation, enriched from the registry."""
@@ -6344,14 +6365,18 @@ class CombatEngine:
 
         ``metamagic`` holds the feats chosen for THIS cast (already filtered
         to known ones, B7); ``slot_level`` is the spell's level in the slot
-        used — the save DC is computed from it (PF1e CRB: a metamagic spell
-        uses the DC of the higher slot level). ``spell_crit`` is a confirmed
+        used - it drives the slot cost and the concentration checks, but NOT
+        the save DC: metamagic other than Heighten Spell does not raise the
+        DC (SRD feats/empower-spell.md: "Saving throws and opposed rolls are
+        not affected"; feats/heighten-spell.md: only Heighten Spell
+        "increases the effective level of the spell", and its DC is computed
+        from the heightened level). ``spell_crit`` is a confirmed
         touch-delivery critical: the spell's damage dice are doubled (CRB
         "Range — Touch": "deals double damage on a successful critical hit").
         ``attack_penalty`` (Spell Combat) folds into each projectile's ranged
         touch attack for multi-projectile ray spells.
         """
-        dc = self._spell_dc(caster, spell, slot_level=slot_level)
+        dc = self._spell_dc(caster, spell, slot_level=slot_level, metamagic=metamagic)
         save_result = (
             self.resolve_save(
                 target, spell.save, dc, save_descriptors=spell_save_descriptors(spell)