fix(rules): CMD never adds Dex on top of a Tiny Dex-substituted pre-baked CMB

src/pf1e_simulator/combat.py

@@ -4782,6 +4782,18 @@ class CombatEngine:
         # Strength modifier + Dexterity modifier + special size modifier" —
         # the Tiny-or-smaller Dex-for-Str substitution is CMB-only, so the
         # ability component is computed here instead of via _maneuver_cmb.
+        # A pre-baked CMB on a Tiny-or-smaller creature ALREADY carries that
+        # substitution (SRD: they "use their Dexterity modifier in place of
+        # their Strength modifier to determine their CMB"), so it must never
+        # seed the CMD base — the extra Dexterity below would be double-
+        # counted and Strength lost. Use the raw CMD formula for those sizes.
+        if target.combatant.size.lower() in _TINY_OR_SMALLER_SIZES:
+            target_base = (
+                target.combatant.bab
+                + target.combatant.abilities.str_mod
+                + self._size_mod(target)
+            )
+        else:
             target_base = target.combatant.cmb or (
                 target.combatant.bab
                 + target.combatant.abilities.str_mod

tests/test_maneuvers.py

@@ -622,6 +622,27 @@ def test_tiny_or_smaller_cmd_uses_strength_and_dexterity() -> None:
         assert engine._maneuver_cmd(tiny) == expected
 
 
+def test_cmd_never_double_counts_tiny_dex_substituted_cmb() -> None:
+    # Latent bug guard: a Tiny creature with a PRE-BAKED cmb (already
+    # Dex-substituted per SRD CMB) must not get the Dexterity modifier added
+    # on top of it. Tiny: str -1, dex +4, bab 2, size -2, cmb pre-baked to
+    # 2 + 4 - 2 = 4. Correct CMD = 10 + 2 - 1 + 4 - 2 = 13; the buggy path
+    # (10 + cmb + dex) would give 10 + 4 + 4 = 18.
+    engine = make_engine([], [])
+    tiny = make_state(
+        _resized(make_combatant("t"), "Tiny", 8, 18).model_copy(update={"cmb": 4}),
+        "p",
+        (0, 0),
+    )
+    assert engine._maneuver_cmd(tiny) == 13
+    # A Medium creature's pre-baked cmb (Str-based) keeps the old behavior:
+    # CMD = 10 + cmb + dex = 10 + 8 + 0 = 18 (thrune_agent-like shape).
+    medium = make_state(
+        make_combatant("m").model_copy(update={"cmb": 8}), "p", (0, 1)
+    )
+    assert engine._maneuver_cmd(medium) == 18
+
+
 def _buffed(spec: Combatant, mods: tuple[StatModifier, ...]) -> CombatantState:
     state = make_state(spec, "p", (0, 0))
     state.active_buffs.append(