fix(rules): helpless Dex-0 AC adds exactly -5 on flat-footed base; touch base strips full Dex mod when helpless (no negative-Dex double count)

src/pf1e_simulator/combat.py

@@ -2707,9 +2707,11 @@ class CombatEngine:
         PF1e (CRB): a prone defender takes -4 AC against melee attacks but
         gains +4 AC against ranged attacks; a helpless defender (paralyzed,
         asleep, unconscious, helpless) has an effective Dexterity score of 0,
-        so its whole Dex modifier gives way to the Dex-0 penalty (-5) —
-        flat-footed AC already drops a positive Dex bonus, a native Dex
-        penalty must still be replaced."""
+        so its whole Dex modifier gives way to the Dex-0 penalty (-5). The
+        flat-footed base already excludes ALL Dex (bonus and penalty alike),
+        so the mod is exactly -5 there; on the touch base the stored Dex
+        modifier is stripped at the base (see resolve_attack) before this
+        -5 is added."""
         mods = self._stat_modifiers(target, "ac")
         if self._has_condition_named(target, "prone"):
             mods.append(
@@ -2718,8 +2720,7 @@ class CombatEngine:
                 )
             )
         if self._is_helpless(target):
-            dex_penalty = min(0, target.combatant.abilities.dex_mod)
-            mods.append(StatModifier(target="ac", value=_DEX_ZERO_MOD - dex_penalty))
+            mods.append(StatModifier(target="ac", value=_DEX_ZERO_MOD))
         return mods
 
     def resolve_attack(
@@ -2804,9 +2805,14 @@ class CombatEngine:
             ):
                 base_ac = full_ac_base
             elif denies_dex:
-                base_ac = (
-                    target.combatant.ac.touch - max(0, target.combatant.abilities.dex_mod)
-                )
+                # A flat-footed defender loses only a POSITIVE Dex bonus (a
+                # Dex penalty stays on touch AC); a HELPLESS defender has an
+                # effective Dex score of 0, so its whole stored Dex modifier
+                # is stripped here and the exact -5 Dex-0 penalty is added by
+                # _attack_ac_mods.
+                dex_mod = target.combatant.abilities.dex_mod
+                strip = dex_mod if self._is_helpless(target) else max(0, dex_mod)
+                base_ac = target.combatant.ac.touch - strip
             else:
                 base_ac = target.combatant.ac.touch
         else:

tests/test_conformance_p1.py

@@ -328,8 +328,9 @@ def test_stunned_is_not_helpless() -> None:
 
 
 def test_helpless_dex_zero_replaces_a_native_dex_penalty() -> None:
-    # A Dex 8 (-1) helpless target: the -1 penalty gives way to the Dex-0 -5
-    # (adjustment -5 - (-1) = -4 on top of the flat-footed base).
+    # A Dex 8 (-1) helpless target: the flat-footed base already excludes
+    # ALL Dex (bonus and penalty alike), so effective Dex 0 adds exactly -5.
+    # The old formula added -5 - (-1) = -4, double-counting the Dex penalty.
     eng = _engine(
         [_combatant("atk"), _combatant("def", ac=14, dex=8)],
         [(0, 0), (0, 1)],
@@ -337,7 +338,68 @@ def test_helpless_dex_zero_replaces_a_native_dex_penalty() -> None:
     )
     _add_condition(eng._states[1], PARALYZED)
     result = eng.resolve_attack(eng._states[0], eng._states[1], _melee("atk"))
-    assert result.ac == 10  # 14 - 4
+    assert result.ac == 9  # 14 - 5
+
+
+def _touch(cid: str) -> AttackSpec:
+    return AttackSpec(
+        id=f"{cid}-touch",
+        name="touch",
+        kind="touch",
+        attack_bonus=5,
+        damage=[DamageComponent(formula=parse_dice("1d6"), types=["cold"])],
+    )
+
+
+def test_helpless_positive_dex_melee_ac_is_flat_footed_minus5() -> None:
+    # Dex 16 (+3): flat-footed base drops the +3 already; Dex-0 adds -5.
+    eng = _engine(
+        [_combatant("atk"), _combatant("def", ac=16, dex=16)],
+        [(0, 0), (0, 1)],
+        [10, 4],
+    )
+    _add_condition(eng._states[1], PARALYZED)
+    result = eng.resolve_attack(eng._states[0], eng._states[1], _melee("atk"))
+    assert result.ac == 11  # 16 - 5
+
+
+def test_helpless_positive_dex_touch_ac_strips_dex_then_minus5() -> None:
+    # Dex 16 (+3): touch base 16 includes the +3; effective Dex 0 strips it
+    # (16 - 3) and adds -5 -> 8.
+    eng = _engine(
+        [_combatant("atk"), _combatant("def", ac=16, dex=16)],
+        [(0, 0), (0, 1)],
+        [10, 4],
+    )
+    _add_condition(eng._states[1], PARALYZED)
+    result = eng.resolve_attack(eng._states[0], eng._states[1], _touch("atk"))
+    assert result.ac == 8
+
+
+def test_helpless_negative_dex_touch_ac_strips_penalty_then_minus5() -> None:
+    # Dex 8 (-1): touch base 14 includes the -1; effective Dex 0 replaces it
+    # (14 + 1) and adds -5 -> 10.
+    eng = _engine(
+        [_combatant("atk"), _combatant("def", ac=14, dex=8)],
+        [(0, 0), (0, 1)],
+        [10, 4],
+    )
+    _add_condition(eng._states[1], PARALYZED)
+    result = eng.resolve_attack(eng._states[0], eng._states[1], _touch("atk"))
+    assert result.ac == 10
+
+
+def test_flat_footed_negative_dex_touch_keeps_penalty_without_minus5() -> None:
+    # Control: a merely flat-footed (NOT helpless) Dex 8 target loses only a
+    # positive Dex bonus — the -1 stays on touch AC and no Dex-0 -5 applies.
+    eng = _engine(
+        [_combatant("atk"), _combatant("def", ac=14, dex=8)],
+        [(0, 0), (0, 1)],
+        [10, 4],
+    )
+    _add_condition(eng._states[1], CONDITIONS_BY_NAME["flat_footed"])
+    result = eng.resolve_attack(eng._states[0], eng._states[1], _touch("atk"))
+    assert result.ac == 14
 
 
 # ── B6: nauseated — single move action only, no casting ──────────────────────