diff --git a/README.md b/README.md
index 282aa4a..ac9cb5e 100644
--- a/README.md
+++ b/README.md
@@ -436,6 +436,16 @@ Règles modélisées :
   manœuvres et invocation sont généralisés aux cellules occupées (distance =
   minimum entre cellules, LoS/couvert = meilleure paire) ; le comportement 1×1
   reste bit-identique.
+- Taille et attaques/CA : les valeurs `ac` (total/touch/flat_footed) et
+  `attack_bonus` des JSON de créatures sont des valeurs FINALES, déjà majorées
+  du modificateur de taille PF1e aux attaques/CA (Small +1, Medium 0, Large −1,
+  Tiny +2…) — vérifié sur toutes les fiches (`data/monsters/*.json`,
+  `data/pcs/*.json`, verrouillé par `tests/test_size_is_baked_in.py`). Le
+  moteur n'applique donc JAMAIS de bonus de taille aux jets d'attaque ni à la
+  CA (ce serait un double comptage) ; la taille n'intervient dans la
+  résolution que pour les manœuvres de combat (CMB/CMD, table
+  `_SIZE_MODIFIERS` Small −1 … Colossal +8 — colonne distincte de la table
+  attaques/CA), l'allonge naturelle, le footprint et l'occupation.
 - Élévation : monter d'un niveau de hauteur coûte +1 case de mouvement par
   niveau (la descente est gratuite) ; une créature située plus haut que sa
   cible gagne +1 à ses jets d'attaque de mêlée et à la confirmation de
diff --git a/tests/test_size_is_baked_in.py b/tests/test_size_is_baked_in.py
new file mode 100644
index 0000000..78ba489
--- /dev/null
+++ b/tests/test_size_is_baked_in.py
@@ -0,0 +1,188 @@
+"""Regression tests: size is BAKED INTO creature attack/AC data, not applied by the engine.
+
+The PF1e size modifier to attack rolls and AC (Small +1, Medium +0, Large -1,
+Tiny +2, ...) is already included in the data-authored ``ac.total/touch/
+flat_footed`` and ``attack_bonus`` values (verified against the source sheets
+in ``fiches_personnages/`` and the Bestiary entries). ``resolve_attack`` must
+therefore NEVER add a size modifier on top — doing so would double-count and
+regress the pinned transcripts and goldens.
+
+The engine's ``_SIZE_MODIFIERS`` table (Fine -8 ... Colossal +8) is the
+COMBAT-MANEUVER size table (CMB/CMD), an entirely different column of the PF1e
+size table, and stays exactly as-is.
+
+Covers:
+- (a) a Small attacker with explicit ``attack_bonus`` resolves at
+  ``d20 + attack_bonus`` — no silent extra +1 size bonus.
+- (b) a Small target with explicit ``ac.total`` is resolved against that value
+  — no silent extra +1 size AC.
+- (c) the CMB/CMD path still applies the combat-maneuver size table via
+  ``_size_mod`` / ``_maneuver_cmb`` / ``_maneuver_cmd``.
+- (d) the shipped goblin data itself proves the bake-in (arithmetic pinned).
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from pf1e_simulator.combat import (
+    _SIZE_MODIFIERS,
+    CombatantState,
+    CombatEngine,
+)
+from pf1e_simulator.dice import parse_dice
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.loaders.combatant import load_combatant
+from pf1e_simulator.map import MapSpec, TerrainType
+from pf1e_simulator.models import (
+    AbilityScores,
+    ACProfile,
+    AttackSpec,
+    Combatant,
+    DamageComponent,
+    Saves,
+)
+from pf1e_simulator.rng import ScriptedRng
+
+_DATA = Path(__file__).resolve().parent.parent / "data"
+
+
+def make_combatant(
+    cid: str,
+    *,
+    size: str = "Medium",
+    ac: int = 13,
+    attack_bonus: int = 2,
+    str_score: int = 10,
+    dex_score: int = 10,
+    bab: int = 1,
+) -> Combatant:
+    """Single-melee-weapon combatant with data-authored FINAL ac/attack_bonus.
+
+    Mirrors ``tests/test_combat.py::make_combatant`` but exposes ``size`` so
+    the tests can pin Small/Large/Tiny statlines whose numbers already bake
+    the attack/AC size modifier.
+    """
+    attack = AttackSpec(
+        id=f"{cid}-w",
+        name="short sword",
+        kind="melee",
+        attack_bonus=attack_bonus,
+        damage=[DamageComponent(formula=parse_dice("1d4"), types=["slashing"])],
+    )
+    return Combatant(
+        id=cid,
+        name=cid,
+        level=1,
+        size=size,
+        abilities=AbilityScores(
+            str_score=str_score,
+            dex_score=dex_score,
+            con_score=12,
+            int_score=10,
+            wis_score=10,
+            cha_score=10,
+        ),
+        hp_max=6,
+        ac=ACProfile(total=ac, touch=ac, flat_footed=ac),
+        bab=bab,
+        initiative_mod=0,
+        speed_land_ft=30,
+        saves=Saves(fort=0, ref=0, will=0),
+        attacks=[attack],
+    )
+
+
+def make_state(combatant: Combatant, side: str, pos: tuple[int, int]) -> CombatantState:
+    return CombatantState(combatant=combatant, side=side, pos=pos, hp=combatant.hp_max)
+
+
+def make_engine(queue: list[int], states: list[CombatantState]) -> CombatEngine:
+    legend = {".": TerrainType(type="floor", move_cost=1)}
+    spec = MapSpec(name="test", terrain=tuple(["." * 8] * 8), legend=legend)
+    return CombatEngine(ScriptedRng(queue), Grid.from_spec(spec), states)
+
+
+def test_small_attacker_gets_no_extra_size_bonus_on_attack() -> None:
+    """(a) Small attacker, data attack_bonus=2 (goblin-like: BAB 1 + Str 0 +
+    size +1 already baked). With roll 10, adjacent, no feats/conditions/cover:
+    total must be exactly 12 — NOT 13 (a +1 size bonus re-applied)."""
+    attacker = make_state(
+        make_combatant("gob", size="Small", attack_bonus=2), "players", (1, 1)
+    )
+    target = make_state(make_combatant("orc", ac=20), "monsters", (1, 2))
+    engine = make_engine([10], [attacker, target])
+    result = engine.resolve_attack(attacker, target, attacker.combatant.attacks[0])
+    assert result.roll == 10
+    assert result.base_bonus == 2
+    assert result.total == 12
+    assert result.hit is False  # 12 < 20 — and would still miss even at 13
+
+
+def test_small_target_ac_is_used_verbatim() -> None:
+    """(b) Small target, data ac.total=16 (goblin: 10 + 1 size + 2 Dex + 3
+    armour already baked). resolve_attack must compare against exactly 16 —
+    NOT 17 (a +1 size AC re-applied)."""
+    attacker = make_state(make_combatant("a", attack_bonus=2), "players", (1, 1))
+    target = make_state(make_combatant("gob", size="Small", ac=16), "monsters", (1, 2))
+    engine = make_engine([14, 1], [attacker, target])
+    result = engine.resolve_attack(attacker, target, attacker.combatant.attacks[0])
+    assert result.ac == 16
+    assert result.total == 16  # roll 14 + attack_bonus 2
+    assert result.hit is True  # hits at exactly 16, not vs a 17-inflated AC
+    # Fresh states (the first hit damaged the target): a total of 15 must MISS
+    # 16 — pinning AC to exactly 16 (not 15, not 17).
+    attacker2 = make_state(make_combatant("a2", attack_bonus=2), "players", (1, 1))
+    target2 = make_state(make_combatant("gob2", size="Small", ac=16), "monsters", (1, 2))
+    engine2 = make_engine([13], [attacker2, target2])
+    miss = engine2.resolve_attack(attacker2, target2, attacker2.combatant.attacks[0])
+    assert miss.ac == 16
+    assert miss.total == 15
+    assert miss.hit is False
+
+
+def test_combat_maneuver_size_table_still_applied() -> None:
+    """(c) _SIZE_MODIFIERS is the CMB/CMD table (opposite sign to the
+    attack/AC table) and _size_mod / _maneuver_cmb / _maneuver_cmd still use
+    it: Small -1, Medium 0, Large +1, Tiny -2, Colossal +8, Fine -8."""
+    assert _SIZE_MODIFIERS["small"] == -1
+    assert _SIZE_MODIFIERS["medium"] == 0
+    assert _SIZE_MODIFIERS["large"] == 1
+    assert _SIZE_MODIFIERS["tiny"] == -2
+    assert _SIZE_MODIFIERS["colossal"] == 8
+    assert _SIZE_MODIFIERS["fine"] == -8
+
+    engine = make_engine([], [])
+    small = make_state(make_combatant("s", size="Small", bab=1, str_score=10), "p", (0, 0))
+    large = make_state(make_combatant("l", size="Large", bab=1, str_score=10), "m", (0, 1))
+    colossal = make_state(
+        make_combatant("c", size="Colossal", bab=1, str_score=10), "m", (0, 2)
+    )
+    assert engine._size_mod(small) == -1
+    assert engine._size_mod(large) == 1
+    assert engine._size_mod(colossal) == 8
+    # CMB = BAB + Str mod + size mod (no explicit cmb on the model):
+    # bab 1 + str 0 + size.
+    assert engine._maneuver_cmb(small) == 0  # 1 + 0 - 1
+    assert engine._maneuver_cmb(large) == 2  # 1 + 0 + 1
+    assert engine._maneuver_cmb(colossal) == 9  # 1 + 0 + 8
+    # CMD = 10 + CMB + Dex mod (dex 0 here): the size modifier propagates.
+    assert engine._maneuver_cmd(small) == 10  # 10 + 0 + 0
+    assert engine._maneuver_cmd(large) == 12  # 10 + 2 + 0
+    assert engine._maneuver_cmd(colossal) == 19  # 10 + 9 + 0
+
+
+def test_goblin_json_size_is_baked_in() -> None:
+    """(d) The shipped Small goblin proves the bake-in arithmetically:
+    short sword attack_bonus 2 = BAB 1 + Str 0 + size +1;
+    touch AC 13 = 10 + Dex 2 + size +1. If the engine added size on top,
+    these data values would be wrong by +1."""
+    goblin = load_combatant(_DATA / "monsters" / "goblin.json")
+    assert goblin.size == "Small"
+    assert goblin.bab == 1
+    assert goblin.abilities.str_mod == 0
+    assert goblin.abilities.dex_mod == 2
+    sword = next(a for a in goblin.attacks if a.name == "short sword")
+    assert sword.attack_bonus == goblin.bab + goblin.abilities.str_mod + 1  # +1 = Small
+    assert goblin.ac.touch == 10 + goblin.abilities.dex_mod + 1  # +1 = Small
+    assert goblin.ac.total == 16  # 10 + 1 size + 2 Dex + 3 armour (subtle/leather)