diff --git a/src/core/actions.gd b/src/core/actions.gd
new file mode 100644
index 0000000..8cc220c
--- /dev/null
+++ b/src/core/actions.gd
@@ -0,0 +1,40 @@
+# src/core/actions.gd
+class_name Actions
+extends RefCounted
+
+class Action:
+    extends RefCounted
+    var player_idx: int
+    var action_type: String
+    var cards: Array = []
+    var seq_id: int = 0
+    var nonce: int = 0
+    var timestamp: int = 0
+
+    func _to_string() -> String:
+        return "Action(seq=%d, player=%d, type=%s, cards=%d)" % [seq_id, player_idx, action_type, cards.size()]
+
+class GameStateSnapshot:
+    extends RefCounted
+    var game_state: Dictionary
+
+class Team:
+    extends RefCounted
+    var team_id: int
+    var player_indices: Array[int] = []
+    var score: int = 0
+    var current_level: int = 2
+
+    static func create_team(team_id: int, p1: int, p2: int) -> Team:
+        var t := Team.new()
+        t.team_id = team_id
+        t.player_indices = [p1, p2]
+        return t
+
+    func teammate_of(player_idx: int) -> int:
+        if player_indices[0] == player_idx:
+            return player_indices[1]
+        return player_indices[0]
+
+    func contains(player_idx: int) -> bool:
+        return player_idx in player_indices
diff --git a/src/core/game_state.gd b/src/core/game_state.gd
new file mode 100644
index 0000000..ff2b29c
--- /dev/null
+++ b/src/core/game_state.gd
@@ -0,0 +1,114 @@
+# src/core/game_state.gd
+class_name GameState
+extends RefCounted
+
+enum Phase { INIT, DEAL, TRIBUTE, PLAY, LEVEL_UP, GAME_OVER }
+
+const _Actions = preload("res://src/core/actions.gd")
+
+var phase: int = Phase.INIT
+var round_seq: int = 0
+var current_rank: int = 2
+var rule_config: RuleConfig
+var teams: Array = []
+var player_hands: Array = [[], [], [], []]
+var player_names: Array[String] = ["Player", "AI-1", "AI-2", "AI-3"]
+var player_human: Array[bool] = [true, false, false, false]
+var round: Round
+var action_log: Array = []
+var seed: int = 0
+var finished_players: Array[int] = []
+var current_winner_team: int = -1
+var game_end_reason: String = ""
+
+static func create(config: RuleConfig, seed_: int = -1) -> GameState:
+    var gs := GameState.new()
+    gs.rule_config = config
+    if seed_ >= 0:
+        gs.seed = seed_
+    else:
+        gs.seed = Time.get_unix_time_from_system() as int
+    gs.teams = [_Actions.Team.create_team(0, 0, 2), _Actions.Team.create_team(1, 1, 3)]
+    gs.round = Round.new()
+    return gs
+
+func get_team(player_idx: int) -> Actions.Team:
+    for t in teams:
+        if t.contains(player_idx):
+            return t
+    return null
+
+func get_partner(player_idx: int) -> int:
+    var t := get_team(player_idx)
+    if t != null:
+        return t.teammate_of(player_idx)
+    return -1
+
+func get_hand(player_idx: int) -> Array:
+    return player_hands[player_idx]
+
+func remove_cards_from_hand(player_idx: int, cards: Array) -> void:
+    var hand: Array = player_hands[player_idx]
+    var to_remove := {}
+    for c in cards:
+        to_remove[c.card_id] = true
+    var new_hand: Array = []
+    for c in hand:
+        if not to_remove.has(c.card_id):
+            new_hand.append(c)
+    player_hands[player_idx] = new_hand
+
+func add_cards_to_hand(player_idx: int, cards: Array) -> void:
+    for c in cards:
+        player_hands[player_idx].append(c)
+
+func deal_cards(deck: Deck) -> void:
+    for i in range(4):
+        player_hands[i] = deck.deal(27)
+
+func is_player_finished(player_idx: int) -> bool:
+    return player_idx in finished_players
+
+func add_finished_player(player_idx: int) -> void:
+    if not is_player_finished(player_idx):
+        finished_players.append(player_idx)
+
+func get_rank_for_player(player_idx: int) -> int:
+    var idx := finished_players.find(player_idx)
+    if idx >= 0:
+        return idx + 1
+    return 0
+
+func all_hands_empty() -> bool:
+    var empty_count := 0
+    for i in range(4):
+        if player_hands[i].is_empty():
+            empty_count += 1
+    return empty_count >= 3
+
+func to_packed_snapshot(for_player: int) -> Dictionary:
+    return {
+        "phase": phase,
+        "current_rank": current_rank,
+        "own_hand": _pack_hand(player_hands[for_player]),
+        "table": _pack_table(round.table),
+        "finished": finished_players.duplicate(),
+        "team_scores": [teams[0].score, teams[1].score]
+    }
+
+func _pack_hand(hand: Array) -> Array:
+    var result := []
+    for c in hand:
+        result.append(c.to_packed())
+    return result
+
+func _pack_table(table: Array) -> Array:
+    var result := []
+    for ep in table:
+        var d := {"type": ep.type, "primary_rank": ep.primary_rank, "is_pure": ep.is_pure_bomb}
+        var cards_packed := []
+        for c in ep.cards:
+            cards_packed.append(c.to_packed())
+        d["cards"] = cards_packed
+        result.append(d)
+    return result
diff --git a/src/core/round.gd b/src/core/round.gd
new file mode 100644
index 0000000..1518f4d
--- /dev/null
+++ b/src/core/round.gd
@@ -0,0 +1,41 @@
+# src/core/round.gd
+class_name Round
+extends RefCounted
+
+var active_player_idx: int = 0
+var pass_count: int = 0
+var table: Array = []
+var action_seq: int = 0
+var is_cleared: bool = false
+
+func can_pass() -> bool:
+    return not table.is_empty()
+
+func add_play(play: HandEvaluator.EvaluatedPlay, player_idx: int) -> void:
+    table.append(play)
+    if play.type == -1:
+        pass_count += 1
+    else:
+        pass_count = 0
+    if pass_count >= 3:
+        is_cleared = true
+    else:
+        is_cleared = false
+
+func next_player() -> int:
+    return (active_player_idx + 1) % 4
+
+func last_non_pass_player() -> int:
+    for i in range(table.size() - 1, -1, -1):
+        if table[i].type != -1:
+            return i % 4
+    return -1
+
+func next_seq() -> int:
+    action_seq += 1
+    return action_seq
+
+func reset_for_new_round() -> void:
+    table.clear()
+    pass_count = 0
+    is_cleared = false
diff --git a/src/core/rule_engine.gd b/src/core/rule_engine.gd
new file mode 100644
index 0000000..efe2453
--- /dev/null
+++ b/src/core/rule_engine.gd
@@ -0,0 +1,108 @@
+# src/core/rule_engine.gd
+# Pure functions — zero Godot dependency
+# Validate play legality and compare plays
+
+class_name RuleEngine
+extends RefCounted
+
+const _C = preload("res://src/core/constants.gd")
+
+static func can_play(
+    hand: Array,
+    play: HandEvaluator.EvaluatedPlay,
+    table_history: Array,
+    last_player_idx: int,
+    current_rank: int,
+    config: RuleConfig
+) -> Dictionary:
+    if play.type == -1:
+        var last_non_pass := _last_non_pass(table_history)
+        if table_history.is_empty() or last_non_pass == null:
+            return _err(_C.ERR_CANNOT_PASS)
+        if last_player_idx == _last_player_idx(table_history):
+            return _err(_C.ERR_CANNOT_PASS)
+        return _ok()
+    for pc in play.cards:
+        if not _card_in_hand(hand, pc):
+            return _err(_C.ERR_CARD_NOT_FOUND)
+    var last_play := _last_non_pass(table_history)
+    if last_play == null:
+        return _ok()
+    if last_player_idx < 0:
+        return _ok()
+    if _last_player_idx(table_history) == last_player_idx:
+        return _ok()
+    var cmp := compare(play, last_play, config)
+    if cmp <= 0:
+        return _err(_C.ERR_INVALID_CARDS)
+    if play.type != last_play.type:
+        if play.type != _C.TYPE_BOMB and play.type != _C.TYPE_ROCKET:
+            return _err(_C.ERR_INVALID_CARDS)
+    return _ok()
+
+static func compare(a: HandEvaluator.EvaluatedPlay, b: HandEvaluator.EvaluatedPlay, config: RuleConfig) -> int:
+    if a.type == -1 or b.type == -1:
+        return 0
+    if a.type == _C.TYPE_ROCKET and b.type == _C.TYPE_ROCKET:
+        return 0
+    if a.type == _C.TYPE_ROCKET:
+        return 1
+    if b.type == _C.TYPE_ROCKET:
+        return -1
+    if a.type == _C.TYPE_BOMB and b.type != _C.TYPE_BOMB:
+        return 1
+    if b.type == _C.TYPE_BOMB and a.type != _C.TYPE_BOMB:
+        return -1
+    if a.type == _C.TYPE_BOMB and b.type == _C.TYPE_BOMB:
+        return compare_bombs(a, b, config)
+    if a.type == b.type:
+        if a.primary_rank > b.primary_rank:
+            return 1
+        if a.primary_rank < b.primary_rank:
+            return -1
+        return 0
+    return 0
+
+static func compare_bombs(a: HandEvaluator.EvaluatedPlay, b: HandEvaluator.EvaluatedPlay, config: RuleConfig) -> int:
+    if a.is_pure_bomb and not b.is_pure_bomb:
+        return 1
+    if b.is_pure_bomb and not a.is_pure_bomb:
+        return -1
+    if config.bomb_compare_priority == RuleConfig.BOMB_BY_COUNT:
+        if a.cards.size() > b.cards.size():
+            return 1
+        if b.cards.size() > a.cards.size():
+            return -1
+    if a.primary_rank > b.primary_rank:
+        return 1
+    if b.primary_rank > a.primary_rank:
+        return -1
+    if a.cards.size() > b.cards.size():
+        return 1
+    if b.cards.size() > a.cards.size():
+        return -1
+    return 0
+
+static func _card_in_hand(hand: Array, card: Card) -> bool:
+    for c in hand:
+        if c.card_id == card.card_id:
+            return true
+    return false
+
+static func _last_non_pass(table: Array) -> HandEvaluator.EvaluatedPlay:
+    for i in range(table.size() - 1, -1, -1):
+        if table[i].type != -1:
+            return table[i]
+    return null
+
+static func _last_player_idx(table: Array) -> int:
+    for i in range(table.size() - 1, -1, -1):
+        if table[i].type != -1:
+            return i % 4
+    return -1
+
+static func _ok() -> Dictionary:
+    return {"ok": true, "error_code": _C.RESULT_OK, "data": null}
+
+static func _err(code: int) -> Dictionary:
+    return {"ok": false, "error_code": code, "data": null}