commit 4eb34bfeeb6265aa2a15fbdd356ee3a2890def36
Author: mattlamb227@gmail.com <mattlamb227@gmail.com>
Date:   Wed Aug 5 16:42:57 2026 -0400

    initial commit

diff --git a/PLAN.md b/PLAN.md
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+# Blokus Gym Harness — Design & Implementation Plan
+
+## 1. Overview
+
+A Gymnasium-compatible RL environment for the board game **Blokus**, supporting both **single-agent** (agent vs. configurable bots) and **multi-agent** (PettingZoo-compatible) modes. The harness uses **numpy** for maximum training speed and broad framework compatibility, with **matplotlib** as an optional rendering backend.
+
+## 2. File Structure
+
+```
+blokus/
+├── pyproject.toml              # Package config, dependencies, entry points
+├── README.md
+├── LICENSE
+├── PLAN.md                     # This file
+├── src/blokus_gym/
+│   ├── __init__.py             # Package exports
+│   ├── envs/
+│   │   ├── __init__.py         # Env exports
+│   │   ├── blokus_env.py       # Main gymnasium.Env implementation
+│   │   ├── multiagent.py       # PettingZoo-compatible multi-agent wrapper
+│   │   └── registration.py     # gymnasium.register() calls for presets
+│   ├── core/
+│   │   ├── __init__.py
+│   │   ├── pieces.py           # Piece definitions, orientation generation, custom piece sets
+│   │   ├── board.py            # Board state (numpy array), placement validation, corner tracking
+│   │   ├── game.py             # Game rules: turn order, move validation, scoring, game-over detection
+│   │   └── bots.py             # Bot implementations: Random, Greedy, Minimax
+│   ├── wrappers/
+│   │   ├── __init__.py
+│   │   └── action_mask.py      # Optional wrapper to expose action masks as part of observation
+│   └── utils/
+│       ├── __init__.py
+│       └── render.py           # Text/ANSI and matplotlib rendering
+├── tests/
+│   ├── __init__.py
+│   ├── conftest.py             # Shared fixtures
+│   ├── test_pieces.py
+│   ├── test_board.py
+│   ├── test_game.py
+│   ├── test_env.py
+│   ├── test_multiagent.py
+│   └── test_registration.py
+└── examples/
+    ├── basic_usage.py
+    ├── train_sb3.py            # Stable-Baselines3 training example
+    ├── train_rllib.py          # Ray RLlib training example
+    └── multiagent_example.py
+```
+
+## 3. Core Design Decisions
+
+### 3.1 Action Space: Discrete + Action Mask
+
+**Design**: Pre-compute all unique `(piece, orientation, position)` combinations into a flat `Discrete(N)` action space. At each step, provide an action mask in `info["action_mask"]` indicating which of the N actions are currently valid.
+
+**Why**:
+- Works with any RL algorithm (masking-aware libs like SB3, RLlib use the mask; others can ignore it)
+- Simple, standard API
+- Action index is stable across episodes (important for policy networks)
+
+**Action space size** (standard 20×20 board, 21 pieces): ~25,000 discrete actions. Smaller boards scale down proportionally.
+
+**Action generation algorithm** (at `__init__` time):
+1. For each piece in the piece set, generate all unique orientations (rotations × flips, deduplicated)
+2. For each orientation, enumerate all positions where the piece fits on the board
+3. Assign each `(piece, orientation, x, y)` a unique integer index
+4. Store as a list of `Move` objects (piece_id, orientation_id, x, y) for O(1) lookup
+
+**Action mask computation** (at each `step`):
+- Start with all-False mask
+- For each action index, check if the move is valid (piece available, in bounds, no overlap, corner rule satisfied)
+- Set valid indices to True
+
+### 3.2 Observation Space: Dict with Board, Pieces, and Corners
+
+```python
+observation_space = spaces.Dict({
+    "board":       spaces.Box(0, num_players, (board_size, board_size), dtype=np.int8),
+    "pieces":      spaces.MultiBinary(num_pieces),           # 1 = still available
+    "my_turn":     spaces.Discrete(1),                        # Always 1 (agent's perspective)
+    "corners":     spaces.Box(0, board_size, (board_size, board_size), dtype=bool),  # Valid corner cells
+})
+```
+
+**Info dict**:
+```python
+info = {
+    "action_mask": np.array([bool] * num_actions),  # Valid actions
+    "current_player": int,
+    "players_with_moves": [int, ...],  # Who still has valid moves
+    "step_count": int,
+}
+```
+
+### 3.3 Reward Design
+
+**Single-agent mode** (agent is player 0):
+- **Step reward**: `0` by default (sparse). Optional shaping: `+0.01 * piece_size` for each placed piece
+- **Terminal reward**: Based on final score comparison
+  - `+1.0` if agent wins (higher score than all opponents)
+  - `0.0` if tie
+  - `-1.0` if agent loses
+  - Alternative: normalized score difference `agent_score / max_possible_score`
+
+**Multi-agent mode** (PettingZoo):
+- Each agent gets `reward = placed_squares / total_squares` at game end
+- Penalty for unplaced squares: `-unplaced_squares / total_squares`
+- `+15/total_squares` bonus for emptying all pieces
+- `+5/total_squares` bonus for last piece being monomino
+
+### 3.4 Dependencies
+
+| Dependency | Purpose | Required? |
+|---|---|---|
+| `numpy` | Board state, coordinate math, action masks | Yes |
+| `gymnasium` | RL environment API | Yes |
+| `pettingzoo` | Multi-agent wrapper | Yes (for multi-agent mode) |
+| `matplotlib` | Visual rendering | No (optional) |
+
+**Rationale**: numpy is the universal standard for array operations in Python RL. Every framework (SB3, RLlib, Tianshou, etc.) consumes numpy arrays natively. No torch/tensorflow dependency means faster installation and broader compatibility.
+
+## 4. Component Designs
+
+### 4.1 Pieces (`core/pieces.py`)
+
+**Piece definition**: A piece is a set of `(x, y)` integer coordinates relative to a reference point.
+
+```python
+@dataclass
+class Piece:
+    name: str
+    squares: frozenset[tuple[int, int]]  # Relative coordinates
+    size: int                             # Number of squares
+
+@dataclass
+class OrientedPiece:
+    piece_id: int
+    orientation_id: int
+    squares: list[tuple[int, int]]        # Absolute coordinates on board
+    corners: list[tuple[int, int]]        # Corner cells for placement rule
+```
+
+**Standard piece set** (`STANDARD_PIECES`): All 21 free polyominoes (1 monomino, 1 domino, 2 trominoes, 5 tetrominoes, 12 pentominoes).
+
+**Orientation generation**:
+1. Start with original coordinates
+2. Apply 4 rotations (0°, 90°, 180°, 270°) using rotation matrix `(x, y) → (y, -x)`
+3. For each rotation, apply 2 flips: original and horizontal-flip `(x, y) → (-x, y)`
+4. Normalize each result (shift so min x = 0, min y = 0)
+5. Deduplicate by comparing sorted coordinate sets
+
+**Custom piece sets**:
+```python
+# Option 1: Define custom pieces
+custom_pieces = [
+    Piece("A", frozenset([(0,0), (1,0), (0,1)])),  # L-tromino
+    Piece("B", frozenset([(0,0), (1,0), (2,0)])),  # I-tromino
+]
+
+# Option 2: Use a preset
+from blokus_gym.core.pieces import STANDARD_PIECES, DUO_PIECES, JUNIOR_PIECES
+
+# Option 3: Load from JSON
+pieces = PieceSet.from_json("my_pieces.json")
+
+# Option 4: Filter standard set
+pentominoes_only = [p for p in STANDARD_PIECES if p.size == 5]
+```
+
+### 4.2 Board (`core/board.py`)
+
+```python
+class Board:
+    def __init__(self, size: int):
+        self.size = size
+        self.grid = np.zeros((size, size), dtype=np.int8)  # 0 = empty, 1-4 = player
+    
+    def place(self, player_idx: int, squares: list[tuple[int, int]]) -> None
+    def is_empty(self, x: int, y: int) -> bool
+    def in_bounds(self, x: int, y: int) -> bool
+    def has_overlap(self, squares: list[tuple[int, int]]) -> bool
+    def get_player_corners(self, player_idx: int) -> set[tuple[int, int]]
+```
+
+**Key methods**:
+- `place()`: Sets grid cells to player index
+- `has_overlap()`: Checks if any square is already occupied
+- `get_player_corners()`: Returns all corner cells adjacent to player's pieces (for placement rule)
+
+### 4.3 Game Logic (`core/game.py`)
+
+```python
+class BlokusGame:
+    def __init__(self, board: Board, pieces: list[Piece], num_players: int = 4):
+        self.board = board
+        self.pieces = pieces
+        self.num_players = num_players
+        self.current_player = 0
+        self.player_pieces: list[set[str]]  # Available piece names per player
+        self.player_corners: list[set[tuple[int, int]]]
+        self.rounds = 0
+        self.game_over = False
+    
+    def valid_move(self, player_idx: int, move: Move) -> bool
+    def apply_move(self, player_idx: int, move: Move) -> None
+    def get_valid_moves(self, player_idx: int) -> list[int]  # Returns action indices
+    def get_action_mask(self, player_idx: int) -> np.ndarray
+    def next_player(self) -> int  # Cycles to next player with moves
+    def is_game_over(self) -> bool
+    def get_scores(self) -> list[int]  # Negative = unplaced squares
+    def get_winners(self) -> list[int]
+```
+
+**Move validation rules**:
+1. Piece must be in player's available pieces
+2. All squares must be in bounds
+3. No overlap with existing pieces
+4. **Corner rule**: At least one square of the placed piece must touch a same-color piece at a corner (diagonal adjacency)
+5. **No edge adjacency**: No square of the placed piece may share an edge with a same-color piece
+6. **First move**: Must be placed in a corner (if `corner_rule=True`)
+
+### 4.4 Bots (`core/bots.py`)
+
+```python
+class Bot(ABC):
+    @abstractmethod
+    def select_action(self, game: BlokusGame, player_idx: int, valid_actions: np.ndarray) -> int
+
+class RandomBot(Bot):
+    """Randomly selects from valid actions."""
+
+class GreedyBot(Bot):
+    """Selects the move that places the most squares (largest piece)."""
+
+class GreedyCornersBot(Bot):
+    """Greedy + prefers moves that open up more corners for future play."""
+
+class MinimaxBot(Bot):
+    """Minimax search with configurable depth. Primarily for 2-player games."""
+```
+
+### 4.5 Gymnasium Environment (`envs/blokus_env.py`)
+
+```python
+class BlokusEnv(gymnasium.Env):
+    metadata = {"render_modes": ["human", "ansi", "rgb_array"]}
+    
+    def __init__(
+        self,
+        num_players: int = 4,
+        board_size: int = 20,
+        pieces: list[Piece] | None = None,  # Default: STANDARD_PIECES
+        render_mode: str | None = None,
+        bot_type: type[Bot] = RandomBot,
+        bot_strength: int = 1,
+        reward_shaping: bool = False,
+        corner_rule: bool = True,
+        max_steps: int | None = None,
+        seed: int | None = None,
+    ):
+        super().__init__()
+        # Initialize game, spaces, action lookup table
+    
+    def reset(self, seed=None, options=None) -> tuple[dict, dict]
+    def step(self, action: int) -> tuple[dict, float, bool, bool, dict]
+    def render(self) -> str | np.ndarray | None
+    def close(self)
+```
+
+**Key implementation details**:
+- `__init__` generates the full action lookup table (piece, orientation, position → index)
+- `reset()` initializes a new game, places bots, optionally lets bots take first turns
+- `step()` applies the agent's action, then lets bots play until it's the agent's turn again
+- Returns `(observation, reward, terminated, truncated, info)` where `info` contains the action mask
+
+### 4.6 Multi-Agent Wrapper (`envs/multiagent.py`)
+
+PettingZoo-compatible wrapper that exposes all players as separate agents:
+
+```python
+class BlokusMultiAgentEnv(AECEnv):
+    def __init__(self, **kwargs):
+        # Same config as BlokusEnv, but wraps it
+    
+    def reset(self, seed=None, options=None) -> tuple[dict, dict]
+    def step(self, action: int) -> None
+    # PettingZoo API: observe(), close(), render(), etc.
+    
+    @property
+    def agents(self) -> list[str]  # ["player_0", "player_1", ...]
+    @property
+    def agent_selection(self) -> str
+```
+
+**Behavior**:
+- Cycles through players in order (0, 1, 2, 3)
+- Each agent gets its own observation (board + own pieces + own corners + action mask)
+- Rewards are per-agent: negative for unplaced squares, bonus for emptying pieces
+- Agents with no valid moves are marked as done (truncated) but still receive final rewards
+
+### 4.7 Registration (`envs/registration.py`)
+
+Predefined environment configurations registered with `gymnasium.register()`:
+
+```python
+# Standard 4-player Blokus
+"Blokus-v0":        num_players=4, board_size=20, pieces=STANDARD_PIECES
+
+# Blokus Duo (2 players, 14x14)
+"BlokusDuo-v0":     num_players=2, board_size=14, pieces=STANDARD_PIECES
+
+# Blokus Junior (2 players, 14x14, simplified pieces)
+"BlokusJunior-v0":  num_players=2, board_size=14, pieces=JUNIOR_PIECES
+
+# Simple test env (2 players, 7x7, only pieces < 5 squares)
+"BlokusSimple-v0":  num_players=2, board_size=7, pieces=[p for p in STANDARD_PIECES if p.size < 5]
+
+# Greedy bot variants
+"BlokusGreedy-v0":  bot_type=GreedyBot
+"BlokusDuoGreedy-v0": bot_type=GreedyBot, num_players=2, board_size=14
+```
+
+### 4.8 Rendering (`utils/render.py`)
+
+**Text/ANSI mode** (`render_mode="human"`):
+```
+  0 1 2 3 4 5 6 7 8 9 ...
+0 . . . . . . . . . .
+1 . . . . . . . . . .
+2 . . . . . . . . . .
+3 . . . . . . . . . .
+4 . . . . . . . . . .
+...
+Player 0 (Red): I1 I2 I3 L4 O4 I4 S4 T4 I5 L5 N P T U V W X Y Z
+Player 1 (Blue): I1 I2 I3 L4 O4 I4 S4 T4 I5 L5 N P T U V W X Y Z
+Current player: 0
+Valid moves: 152
+```
+
+**RGB array mode** (`render_mode="rgb_array"`):
+- Matplotlib figure with colored squares (red, blue, yellow, green, grey)
+- Grid lines
+- Legend showing remaining pieces per player
+- Valid move indicators (semi-transparent overlays)
+
+### 4.9 Action Mask Wrapper (`wrappers/action_mask.py`)
+
+Optional wrapper that moves the action mask from `info` into the observation space:
+
+```python
+class ActionMaskWrapper(Wrapper):
+    def __init__(self, env):
+        super().__init__(env)
+        self.observation_space = spaces.Dict({
+            "observation": env.observation_space,
+            "action_mask": spaces.Box(0, 1, (env.action_space.n,), dtype=bool),
+        })
+    
+    def step(self, action):
+        obs, reward, terminated, truncated, info = self.env.step(action)
+        return {
+            "observation": obs,
+            "action_mask": info["action_mask"],
+        }, reward, terminated, truncated, info
+```
+
+## 5. Customization Options
+
+All options are passed as keyword arguments to `BlokusEnv()` or via `gymnasium.make()`:
+
+| Option | Type | Default | Description |
+|---|---|---|---|
+| `num_players` | int | 4 | Number of players (2-4) |
+| `board_size` | int | 20 | Board dimension (standard: 20, Duo: 14) |
+| `pieces` | list[Piece] | STANDARD_PIECES | Custom piece set |
+| `bot_type` | type[Bot] | RandomBot | Opponent bot type |
+| `bot_strength` | int | 1 | Bot search depth (for Minimax) |
+| `reward_shaping` | bool | False | Enable intermediate rewards |
+| `corner_rule` | bool | True | Require first move in corner |
+| `max_steps` | int | None | Max steps before truncation |
+| `render_mode` | str | None | "human", "ansi", "rgb_array" |
+| `seed` | int | None | Random seed |
+| `starting_corners` | list | [(0,0), (0,19), (19,0), (19,19)] | Custom starting corners |
+| `scoring_mode` | str | "standard" | "standard", "coverage", "winloss" |
+| `first_player` | int | 0 | Which player goes first (agent is always 0) |
+
+## 6. Implementation Phases
+
+### Phase 1: Core Game Logic (Days 1-3)
+- [ ] `core/pieces.py` — Piece definitions, orientation generation, custom piece support
+- [ ] `core/board.py` — Board state, placement validation, corner tracking
+- [ ] `core/game.py` — Game rules, move validation, scoring, game-over detection
+- [ ] Unit tests for all three modules
+
+### Phase 2: Bots (Days 4-5)
+- [ ] `core/bots.py` — Random, Greedy, GreedyCorners, Minimax bots
+- [ ] Tests for bot behavior
+
+### Phase 3: Gymnasium Environment (Days 6-8)
+- [ ] `envs/blokus_env.py` — Main environment class
+- [ ] Action space generation and masking
+- [ ] Observation and reward design
+- [ ] `envs/registration.py` — Preset registrations
+- [ ] `wrappers/action_mask.py` — Optional action mask wrapper
+- [ ] `gymnasium.utils.env_checker.check_env()` passes
+
+### Phase 4: Multi-Agent (Days 9-10)
+- [ ] `envs/multiagent.py` — PettingZoo AECEnv wrapper
+- [ ] Per-agent observations and rewards
+- [ ] Tests for multi-agent mode
+
+### Phase 5: Rendering (Days 11-12)
+- [ ] `utils/render.py` — Text/ANSI and matplotlib rendering
+- [ ] Integration with environment
+
+### Phase 6: Testing & Examples (Days 13-14)
+- [ ] Comprehensive test suite (unit + integration)
+- [ ] `examples/basic_usage.py` — Basic usage
+- [ ] `examples/train_sb3.py` — SB3 training
+- [ ] `examples/train_rllib.py` — RLlib training
+- [ ] `examples/multiagent_example.py` — Multi-agent
+- [ ] `pyproject.toml` with all dependencies
+- [ ] `README.md` with full documentation
+
+## 7. Testing Strategy
+
+### Unit Tests
+- **Pieces**: Orientation generation correctness, deduplication, custom piece loading
+- **Board**: Placement, overlap detection, bounds checking, corner computation
+- **Game**: Move validation (all rules), scoring, game-over detection, turn cycling
+- **Bots**: Random bot returns valid moves, greedy bot prefers larger pieces
+
+### Integration Tests
+- **Environment**: `check_env()` passes, reset/step return correct types
+- **Action mask**: All masked actions are valid, all valid actions are masked
+- **Game flow**: Full game from start to end, correct winner detection
+- **Multi-agent**: All agents receive observations, rewards, and termination signals
+
+### Edge Cases
+- Player with no valid moves (should be skipped)
+- Game ends with all players blocked
+- Last piece is monomino (bonus scoring)
+- Custom piece sets with unusual shapes
+- Small boards where pieces don't fit
+
+## 8. Key Design Tradeoffs
+
+| Decision | Choice | Rationale |
+|---|---|---|
+| Gym library | Gymnasium | Maintained fork, latest API |
+| Agent mode | Single + Multi-agent | Covers both use cases |
+| Action space | Discrete + mask | Broadest RL framework compatibility |
+| Dependencies | numpy only | Fastest, broadest compatibility |
+| Rendering | Text + matplotlib | No Pygame dependency, optional |
+| Piece definition | Coordinate sets | Simple, flexible for custom pieces |
+| Action pre-computation | At init time | Stable action indices, fast step() |
+| Multi-agent | PettingZoo AECEnv | Standard API, compatible with RLlib/PettingZoo algorithms |
diff --git a/examples/basic_game.py b/examples/basic_game.py
new file mode 100644
index 0000000..0f7d0e3
--- /dev/null
+++ b/examples/basic_game.py
@@ -0,0 +1,50 @@
+"""Basic example: play a random game of Blokus."""
+import numpy as np
+
+from blokus_gym import BlokusEnv
+
+
+def main():
+    # Create environment with 4 players
+    env = BlokusEnv(num_players=4, board_size=20, render_mode="ansi")
+
+    obs, info = env.reset(seed=42)
+    print("=== Initial State ===")
+    print(env.render())
+
+    total_reward = 0
+    steps = 0
+    terminated = False
+    truncated = False
+
+    while not (terminated or truncated):
+        # Select a random valid action
+        valid_actions = np.where(info["action_mask"])[0]
+        if len(valid_actions) == 0:
+            print("No valid moves!")
+            break
+
+        action = np.random.choice(valid_actions)
+        obs, reward, terminated, truncated, info = env.step(action)
+        total_reward += reward
+        steps += 1
+
+        if steps % 10 == 0:
+            print(f"\n=== Step {steps} ===")
+            print(env.render())
+            print(f"Reward: {reward}, Total: {total_reward}")
+
+    print("\n=== Game Over ===")
+    print(f"Total steps: {steps}")
+    print(f"Total reward: {total_reward}")
+    print(f"Terminated: {terminated}, Truncated: {truncated}")
+
+    # Final state
+    print("\n=== Final Board ===")
+    print(env.render())
+
+    env.close()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/examples/multiagent.py b/examples/multiagent.py
new file mode 100644
index 0000000..d3a4d5f
--- /dev/null
+++ b/examples/multiagent.py
@@ -0,0 +1,46 @@
+"""Example: multi-agent self-play with BlokusMultiAgentEnv."""
+import numpy as np
+
+from blokus_gym import BlokusMultiAgentEnv, GreedyBot, RandomBot
+
+
+def main():
+    env = BlokusMultiAgentEnv(num_players=2, board_size=7)
+
+    # Create bots for each player
+    bots = {
+        "player_0": RandomBot(player_idx=0, seed=42),
+        "player_1": GreedyBot(player_idx=1),
+    }
+
+    obs, info = env.reset(seed=42)
+    print("=== Multi-Agent Game ===")
+    print(f"Agents: {env.agents}")
+
+    steps = 0
+    while True:
+        for agent in env.agents:
+            obs, info = env.last() if hasattr(env, "last") else (obs.get(agent, {}), {})
+            mask = env.get_action_mask(agent)
+            valid_actions = np.where(mask)[0]
+
+            if len(valid_actions) > 0:
+                bot = bots[agent]
+                action = bot.select_action(env.game, mask)
+                if action is not None:
+                    obs, rewards, terminations, truncations, infos = env.step(action)
+                    steps += 1
+
+                    if terminations[agent]:
+                        print(f"Game over after {steps} steps")
+                        scores = env.game.get_scores()
+                        for i, score in enumerate(scores):
+                            print(f"  Player {i}: {score}")
+                        env.close()
+                        return
+
+    env.close()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/examples/train_sb3.py b/examples/train_sb3.py
new file mode 100644
index 0000000..5cdf4bb
--- /dev/null
+++ b/examples/train_sb3.py
@@ -0,0 +1,54 @@
+"""Example: train an agent with Stable-Baselines3 using action masking.
+
+Requires: pip install blokus-gym[train]
+"""
+from stable_baselines3 import PPO
+
+from blokus_gym import ActionMaskWrapper, BlokusEnv
+
+
+def main():
+    # Create environment with ActionMaskWrapper
+    env = BlokusEnv(num_players=2, board_size=7, bot_type=None)
+    env = ActionMaskWrapper(env)
+
+    # Train PPO agent
+    model = PPO(
+        "MultiInputPolicy",
+        env,
+        verbose=1,
+        learning_rate=3e-4,
+        n_steps=2048,
+        batch_size=64,
+        n_epochs=10,
+        gamma=0.99,
+        seed=42,
+    )
+
+    print("Training for 10,000 steps...")
+    model.learn(total_timesteps=10000)
+
+    # Save the model
+    model.save("blokus_ppo_agent")
+
+    # Evaluate
+    obs, info = env.reset(seed=42)
+    total_reward = 0
+    steps = 0
+    terminated = False
+    truncated = False
+
+    while not (terminated or truncated):
+        action, _ = model.predict(obs, deterministic=True)
+        obs, reward, terminated, truncated, info = env.step(action)
+        total_reward += reward
+        steps += 1
+
+    print(f"Evaluation: {steps} steps, reward: {total_reward}")
+
+    # Cleanup
+    env.close()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..d711891
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,65 @@
+[build-system]
+requires = ["setuptools>=64", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "blokus-gym"
+version = "0.1.0"
+description = "A Gymnasium-compatible RL environment for the board game Blokus"
+readme = "README.md"
+requires-python = ">=3.10"
+license = {text = "MIT"}
+authors = [{name = "Blokus Gym Contributors"}]
+keywords = ["reinforcement-learning", "gymnasium", "blokus", "board-game", "pettingzoo"]
+classifiers = [
+    "Development Status :: 3 - Alpha",
+    "Intended Audience :: Science/Research",
+    "License :: OSI Approved :: MIT License",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+    "Topic :: Scientific/Engineering :: Artificial Intelligence",
+]
+dependencies = [
+    "numpy>=1.24",
+    "gymnasium>=0.29",
+    "pettingzoo>=1.26",
+]
+
+[project.optional-dependencies]
+render = ["matplotlib>=3.7"]
+dev = [
+    "pytest>=7.4",
+    "pytest-cov>=4.1",
+    "black>=23.7",
+    "ruff>=0.1.0",
+    "mypy>=1.5",
+]
+train = [
+    "stable-baselines3>=2.2",
+    "torch>=2.0",
+]
+
+[project.urls]
+Homepage = "https://github.com/blokus-gym/blokus-gym"
+Documentation = "https://github.com/blokus-gym/blokus-gym#readme"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+python_files = ["test_*.py"]
+addopts = "--tb=short -q"
+
+[tool.black]
+line-length = 100
+target-version = ["py310"]
+
+[tool.ruff]
+line-length = 100
+target-version = "py310"
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "W", "UP", "B", "C4"]
diff --git a/src/blokus_gym.egg-info/PKG-INFO b/src/blokus_gym.egg-info/PKG-INFO
new file mode 100644
index 0000000..505c211
--- /dev/null
+++ b/src/blokus_gym.egg-info/PKG-INFO
@@ -0,0 +1,33 @@
+Metadata-Version: 2.4
+Name: blokus-gym
+Version: 0.1.0
+Summary: A Gymnasium-compatible RL environment for the board game Blokus
+Author: Blokus Gym Contributors
+License: MIT
+Project-URL: Homepage, https://github.com/blokus-gym/blokus-gym
+Project-URL: Documentation, https://github.com/blokus-gym/blokus-gym#readme
+Keywords: reinforcement-learning,gymnasium,blokus,board-game,pettingzoo
+Classifier: Development Status :: 3 - Alpha
+Classifier: Intended Audience :: Science/Research
+Classifier: License :: OSI Approved :: MIT License
+Classifier: Programming Language :: Python :: 3
+Classifier: Programming Language :: Python :: 3.10
+Classifier: Programming Language :: Python :: 3.11
+Classifier: Programming Language :: Python :: 3.12
+Classifier: Topic :: Scientific/Engineering :: Artificial Intelligence
+Requires-Python: >=3.10
+Description-Content-Type: text/markdown
+Requires-Dist: numpy>=1.24
+Requires-Dist: gymnasium>=0.29
+Requires-Dist: pettingzoo>=1.26
+Provides-Extra: render
+Requires-Dist: matplotlib>=3.7; extra == "render"
+Provides-Extra: dev
+Requires-Dist: pytest>=7.4; extra == "dev"
+Requires-Dist: pytest-cov>=4.1; extra == "dev"
+Requires-Dist: black>=23.7; extra == "dev"
+Requires-Dist: ruff>=0.1.0; extra == "dev"
+Requires-Dist: mypy>=1.5; extra == "dev"
+Provides-Extra: train
+Requires-Dist: stable-baselines3>=2.2; extra == "train"
+Requires-Dist: torch>=2.0; extra == "train"
diff --git a/src/blokus_gym.egg-info/SOURCES.txt b/src/blokus_gym.egg-info/SOURCES.txt
new file mode 100644
index 0000000..e665378
--- /dev/null
+++ b/src/blokus_gym.egg-info/SOURCES.txt
@@ -0,0 +1,19 @@
+pyproject.toml
+src/blokus_gym/__init__.py
+src/blokus_gym.egg-info/PKG-INFO
+src/blokus_gym.egg-info/SOURCES.txt
+src/blokus_gym.egg-info/dependency_links.txt
+src/blokus_gym.egg-info/requires.txt
+src/blokus_gym.egg-info/top_level.txt
+src/blokus_gym/core/__init__.py
+src/blokus_gym/core/board.py
+src/blokus_gym/core/bots.py
+src/blokus_gym/core/game.py
+src/blokus_gym/core/pieces.py
+src/blokus_gym/envs/__init__.py
+src/blokus_gym/envs/blokus_env.py
+src/blokus_gym/envs/multiagent.py
+src/blokus_gym/utils/__init__.py
+src/blokus_gym/utils/render.py
+src/blokus_gym/wrappers/__init__.py
+src/blokus_gym/wrappers/action_mask.py
\ No newline at end of file
diff --git a/src/blokus_gym.egg-info/dependency_links.txt b/src/blokus_gym.egg-info/dependency_links.txt
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/src/blokus_gym.egg-info/dependency_links.txt
@@ -0,0 +1 @@
+
diff --git a/src/blokus_gym.egg-info/requires.txt b/src/blokus_gym.egg-info/requires.txt
new file mode 100644
index 0000000..846a800
--- /dev/null
+++ b/src/blokus_gym.egg-info/requires.txt
@@ -0,0 +1,17 @@
+numpy>=1.24
+gymnasium>=0.29
+pettingzoo>=1.26
+
+[dev]
+pytest>=7.4
+pytest-cov>=4.1
+black>=23.7
+ruff>=0.1.0
+mypy>=1.5
+
+[render]
+matplotlib>=3.7
+
+[train]
+stable-baselines3>=2.2
+torch>=2.0
diff --git a/src/blokus_gym.egg-info/top_level.txt b/src/blokus_gym.egg-info/top_level.txt
new file mode 100644
index 0000000..66d72bf
--- /dev/null
+++ b/src/blokus_gym.egg-info/top_level.txt
@@ -0,0 +1 @@
+blokus_gym
diff --git a/src/blokus_gym/__init__.py b/src/blokus_gym/__init__.py
new file mode 100644
index 0000000..759f955
--- /dev/null
+++ b/src/blokus_gym/__init__.py
@@ -0,0 +1,39 @@
+from blokus_gym.core.board import Board
+from blokus_gym.core.bots import Bot, GreedyBot, GreedyCornersBot, MinimaxBot, RandomBot
+from blokus_gym.core.game import BlokusGame, Move
+from blokus_gym.core.pieces import (
+    DUO_PIECES,
+    JUNIOR_PIECES,
+    STANDARD_PIECES,
+    Piece,
+    PieceOrientation,
+    PieceSet,
+    generate_orientations,
+)
+from blokus_gym.envs.blokus_env import BlokusEnv
+from blokus_gym.envs.multiagent import BlokusMultiAgentEnv
+from blokus_gym.wrappers.action_mask import ActionMaskWrapper
+
+__version__ = "0.1.0"
+
+__all__ = [
+    "BlokusEnv",
+    "BlokusMultiAgentEnv",
+    "ActionMaskWrapper",
+    "Board",
+    "BlokusGame",
+    "Move",
+    "Piece",
+    "PieceOrientation",
+    "PieceSet",
+    "STANDARD_PIECES",
+    "DUO_PIECES",
+    "JUNIOR_PIECES",
+    "generate_orientations",
+    "Bot",
+    "RandomBot",
+    "GreedyBot",
+    "GreedyCornersBot",
+    "MinimaxBot",
+    "__version__",
+]
diff --git a/src/blokus_gym/__pycache__/__init__.cpython-312.pyc b/src/blokus_gym/__pycache__/__init__.cpython-312.pyc
new file mode 100644
index 0000000..dddead7
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diff --git a/src/blokus_gym/core/__init__.py b/src/blokus_gym/core/__init__.py
new file mode 100644
index 0000000..06513ef
--- /dev/null
+++ b/src/blokus_gym/core/__init__.py
@@ -0,0 +1,30 @@
+from blokus_gym.core.board import Board
+from blokus_gym.core.bots import Bot, GreedyBot, GreedyCornersBot, MinimaxBot, RandomBot
+from blokus_gym.core.game import BlokusGame, Move
+from blokus_gym.core.pieces import (
+    DUO_PIECES,
+    JUNIOR_PIECES,
+    STANDARD_PIECES,
+    Piece,
+    PieceOrientation,
+    PieceSet,
+    generate_orientations,
+)
+
+__all__ = [
+    "Board",
+    "BlokusGame",
+    "Move",
+    "Piece",
+    "PieceOrientation",
+    "PieceSet",
+    "STANDARD_PIECES",
+    "DUO_PIECES",
+    "JUNIOR_PIECES",
+    "generate_orientations",
+    "Bot",
+    "RandomBot",
+    "GreedyBot",
+    "GreedyCornersBot",
+    "MinimaxBot",
+]
diff --git a/src/blokus_gym/core/__pycache__/__init__.cpython-312.pyc b/src/blokus_gym/core/__pycache__/__init__.cpython-312.pyc
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diff --git a/src/blokus_gym/core/__pycache__/pieces.cpython-312.pyc b/src/blokus_gym/core/__pycache__/pieces.cpython-312.pyc
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diff --git a/src/blokus_gym/core/board.py b/src/blokus_gym/core/board.py
new file mode 100644
index 0000000..5dccba7
--- /dev/null
+++ b/src/blokus_gym/core/board.py
@@ -0,0 +1,89 @@
+from __future__ import annotations
+
+import numpy as np
+
+
+class Board:
+    """A square game board represented as a numpy array.
+
+    Cell values:
+        0 = empty
+        1-4 = player index (1-based for consistency with Blokus colors)
+    """
+
+    EMPTY = 0
+
+    def __init__(self, size: int):
+        self.size = size
+        self.grid = np.zeros((size, size), dtype=np.int8)
+
+    def place(self, player_idx: int, squares: list[tuple[int, int]]) -> None:
+        """Place a player's piece on the board."""
+        for x, y in squares:
+            self.grid[y, x] = player_idx
+
+    def clear(self) -> None:
+        """Reset the board to empty."""
+        self.grid.fill(0)
+
+    def is_empty(self, x: int, y: int) -> bool:
+        """Check if a cell is empty."""
+        return self.grid[y, x] == 0
+
+    def in_bounds(self, x: int, y: int) -> bool:
+        """Check if coordinates are within the board."""
+        return 0 <= x < self.size and 0 <= y < self.size
+
+    def has_overlap(self, squares: list[tuple[int, int]]) -> bool:
+        """Check if any square in the list is already occupied."""
+        for x, y in squares:
+            if not self.in_bounds(x, y) or self.grid[y, x] != 0:
+                return True
+        return False
+
+    def get_cell(self, x: int, y: int) -> int:
+        """Get the player index at a cell (0 = empty)."""
+        return self.grid[y, x]
+
+    def get_player_squares(self, player_idx: int) -> list[tuple[int, int]]:
+        """Get all squares occupied by a player."""
+        ys, xs = np.where(self.grid == player_idx)
+        return [(int(x), int(y)) for x, y in zip(xs, ys, strict=True)]
+
+    def get_player_corners(self, player_idx: int) -> set[tuple[int, int]]:
+        """Get all corner cells adjacent to a player's pieces.
+
+        Corner cells are the diagonal neighbors of a player's pieces.
+        These are the cells where the player can place new pieces
+        (corner-to-corner contact rule).
+        """
+        corners: set[tuple[int, int]] = set()
+        player_squares = self.get_player_squares(player_idx)
+        for x, y in player_squares:
+            for dx, dy in [(-1, -1), (1, -1), (-1, 1), (1, 1)]:
+                cx, cy = x + dx, y + dy
+                if self.in_bounds(cx, cy) and self.grid[cy, cx] == 0:
+                    corners.add((cx, cy))
+        return corners
+
+    def get_occupied(self) -> set[tuple[int, int]]:
+        """Get all occupied cells."""
+        ys, xs = np.where(self.grid != 0)
+        return {(int(x), int(y)) for x, y in zip(xs, ys, strict=True)}
+
+    def is_full(self) -> bool:
+        """Check if the board is completely full."""
+        return np.all(self.grid != 0)
+
+    def coverage(self) -> float:
+        """Get the fraction of the board that is occupied."""
+        return float(np.count_nonzero(self.grid)) / (self.size * self.size)
+
+    def copy(self) -> Board:
+        """Create a deep copy of the board."""
+        new_board = Board(self.size)
+        new_board.grid = self.grid.copy()
+        return new_board
+
+    def __repr__(self) -> str:
+        return f"Board(size={self.size}, coverage={self.coverage():.2%})"
diff --git a/src/blokus_gym/core/bots.py b/src/blokus_gym/core/bots.py
new file mode 100644
index 0000000..27901b4
--- /dev/null
+++ b/src/blokus_gym/core/bots.py
@@ -0,0 +1,168 @@
+from __future__ import annotations
+
+import random
+from abc import ABC, abstractmethod
+
+import numpy as np
+
+from blokus_gym.core.game import BlokusGame
+
+
+class Bot(ABC):
+    """Base class for Blokus bots."""
+
+    def __init__(self, player_idx: int, seed: int | None = None):
+        self.player_idx = player_idx
+        self.rng = random.Random(seed)
+
+    @abstractmethod
+    def select_action(self, game: BlokusGame, valid_actions: np.ndarray) -> int | None:
+        """Select an action from the valid actions.
+
+        Args:
+            game: The current game state.
+            valid_actions: Boolean mask of valid actions.
+
+        Returns:
+            Action index, or None if no valid moves.
+        """
+        ...
+
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}(player={self.player_idx})"
+
+
+class RandomBot(Bot):
+    """Randomly selects from valid actions."""
+
+    def select_action(self, game: BlokusGame, valid_actions: np.ndarray) -> int | None:
+        valid_indices = np.where(valid_actions)[0]
+        if len(valid_indices) == 0:
+            return None
+        return int(self.rng.choice(valid_indices))
+
+
+class GreedyBot(Bot):
+    """Selects the move that places the most squares (largest piece)."""
+
+    def select_action(self, game: BlokusGame, valid_actions: np.ndarray) -> int | None:
+        valid_indices = np.where(valid_actions)[0]
+        if len(valid_indices) == 0:
+            return None
+
+        best_action = -1
+        best_size = -1
+        for action in valid_indices:
+            move = game.get_move(int(action))
+            piece = game.piece_set.get_piece(move.piece_id)
+            if piece.size > best_size:
+                best_size = piece.size
+                best_action = int(action)
+
+        return best_action
+
+
+class GreedyCornersBot(Bot):
+    """Greedy bot that prefers moves opening more corners for future play."""
+
+    def select_action(self, game: BlokusGame, valid_actions: np.ndarray) -> int | None:
+        valid_indices = np.where(valid_actions)[0]
+        if len(valid_indices) == 0:
+            return None
+
+        best_action = -1
+        best_score = float("-inf")
+
+        for action in valid_indices:
+            move = game.get_move(int(action))
+            piece = game.piece_set.get_piece(move.piece_id)
+            placed_corners = game._get_placed_corners(move)
+
+            # Count how many new corners this move opens
+            new_corners = 0
+            for cx, cy in placed_corners:
+                if game.board.in_bounds(cx, cy) and game.board.is_empty(cx, cy):
+                    new_corners += 1
+
+            # Score = piece size + corner bonus
+            score = piece.size + new_corners * 0.5
+            if score > best_score:
+                best_score = score
+                best_action = int(action)
+
+        return best_action
+
+
+class MinimaxBot(Bot):
+    """Minimax search bot (primarily for 2-player games).
+
+    Uses a depth-limited minimax search with a simple heuristic.
+    """
+
+    def __init__(self, player_idx: int, depth: int = 2, seed: int | None = None):
+        super().__init__(player_idx, seed)
+        self.depth = depth
+
+    def select_action(self, game: BlokusGame, valid_actions: np.ndarray) -> int | None:
+        valid_indices = np.where(valid_actions)[0]
+        if len(valid_indices) == 0:
+            return None
+
+        if len(valid_indices) == 1:
+            return int(valid_indices[0])
+
+        best_action = -1
+        best_value = float("-inf")
+
+        for action in valid_indices:
+            if game.num_players != 2:
+                # For multi-player, just use greedy
+                return GreedyBot(self.player_idx, self.rng.randint(0, 2**31)).select_action(
+                    game, valid_actions
+                )
+
+            cloned = game.copy()
+            success = cloned.play_move(self.player_idx, int(action))
+            if not success:
+                continue
+
+            value = self._minimax(cloned, self.depth - 1, False)
+            if value > best_value:
+                best_value = value
+                best_action = int(action)
+
+        return best_action if best_action >= 0 else int(valid_indices[0])
+
+    def _minimax(self, game: BlokusGame, depth: int, maximizing: bool) -> float:
+        if depth == 0 or game.is_game_over():
+            return self._evaluate(game)
+
+        if maximizing:
+            value = float("-inf")
+            valid = game.get_valid_actions(self.player_idx)
+            valid_indices = np.where(valid)[0]
+            for action in valid_indices:
+                cloned = game.copy()
+                if cloned.play_move(self.player_idx, int(action)):
+                    val = self._minimax(cloned, depth - 1, False)
+                    value = max(value, val)
+            return value
+        else:
+            # Opponent's turn
+            opp_idx = 1 - self.player_idx
+            value = float("inf")
+            valid = game.get_valid_actions(opp_idx)
+            valid_indices = np.where(valid)[0]
+            for action in valid_indices:
+                cloned = game.copy()
+                if cloned.play_move(opp_idx, int(action)):
+                    val = self._minimax(cloned, depth - 1, True)
+                    value = min(value, val)
+            return value
+
+    def _evaluate(self, game: BlokusGame) -> float:
+        """Simple heuristic: difference in placed squares."""
+        my_score = game.players[self.player_idx].score
+        opp_idx = 1 - self.player_idx
+        opp_score = game.players[opp_idx].score if opp_idx < len(game.players) else 0
+        return float(my_score - opp_score)
diff --git a/src/blokus_gym/core/game.py b/src/blokus_gym/core/game.py
new file mode 100644
index 0000000..27cb8ed
--- /dev/null
+++ b/src/blokus_gym/core/game.py
@@ -0,0 +1,414 @@
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+import numpy as np
+
+from blokus_gym.core.board import Board
+from blokus_gym.core.pieces import STANDARD_PIECES, Move, PieceSet
+
+
+@dataclass
+class PlayerState:
+    """Tracks the state of a single player in the game."""
+
+    idx: int  # 0-based player index
+    available_pieces: set[str] = field(default_factory=set)  # Piece names still available
+    score: int = 0  # Squares placed (positive = good)
+    corners: set[tuple[int, int]] = field(default_factory=set)  # Valid placement corners
+    has_started: bool = False  # Whether player has placed their first piece
+    can_move: bool = True  # Whether player can still make any move
+
+
+class BlokusGame:
+    """Core Blokus game logic.
+
+    Manages the board state, player turns, move validation, and scoring.
+    This class is independent of the Gymnasium environment and can be
+    used standalone for game simulation or bot development.
+    """
+
+    # Player indices are 1-based for board representation
+    # (0 = empty, 1 = player 1, etc.)
+    PLAYER_OFFSET = 1
+
+    def __init__(
+        self,
+        board_size: int = 20,
+        pieces: PieceSet | None = None,
+        num_players: int = 4,
+        corner_rule: bool = True,
+    ):
+        self.board_size = board_size
+        self.piece_set = pieces or PieceSet(STANDARD_PIECES)
+        self.num_players = num_players
+        self.corner_rule = corner_rule
+
+        self.board = Board(board_size)
+        self.players: list[PlayerState] = []
+        self.current_player = 0
+        self.rounds = 0
+        self.game_over = False
+
+        # Pre-compute all possible moves (action lookup table)
+        self._action_moves: list[Move] = []
+        self._move_to_action: dict[tuple[int, int, int, int], int] = {}
+        self._generate_action_space()
+
+        # Starting corners for each player (standard Blokus layout)
+        # 4 players: all four corners
+        # 2 players: opposite corners (0,0) and (max, max)
+        # 3 players: three corners
+        max_idx = board_size - 1
+        self._starting_corners = [
+            (0, 0),
+            (0, max_idx),
+            (max_idx, 0),
+            (max_idx, max_idx),
+        ]
+        if num_players == 2:
+            self._starting_corners = [(0, 0), (max_idx, max_idx)]
+        elif num_players == 3:
+            self._starting_corners = [(0, 0), (0, max_idx), (max_idx, 0)]
+
+    # ------------------------------------------------------------------
+    # Action space generation
+    # ------------------------------------------------------------------
+
+    def _generate_action_space(self) -> None:
+        """Pre-compute all possible (piece, orientation, position) moves.
+
+        Each unique combination is assigned a stable integer action index.
+        """
+        self._action_moves = []
+        self._move_to_action = {}
+
+        for piece_id in range(self.piece_set.num_pieces):
+            for orient_id, orient in enumerate(self.piece_set.get_orientations(piece_id)):
+                squares = orient.squares
+                # Compute bounding box of the piece
+                max_x = max(x for x, _ in squares)
+                max_y = max(y for _, y in squares)
+
+                for px in range(self.board_size - max_x):
+                    for py in range(self.board_size - max_y):
+                        move = Move(
+                            piece_id=piece_id,
+                            orientation_id=orient_id,
+                            x=px,
+                            y=py,
+                        )
+                        action_idx = len(self._action_moves)
+                        self._action_moves.append(move)
+                        self._move_to_action[(piece_id, orient_id, px, py)] = action_idx
+
+    @property
+    def num_actions(self) -> int:
+        return len(self._action_moves)
+
+    def get_move(self, action: int) -> Move:
+        """Get the Move object for a given action index."""
+        return self._action_moves[action]
+
+    def get_action(self, move: Move) -> int:
+        """Get the action index for a given Move."""
+        return self._move_to_action[(move.piece_id, move.orientation_id, move.x, move.y)]
+
+    # ------------------------------------------------------------------
+    # Game initialization
+    # ------------------------------------------------------------------
+
+    def reset(self) -> None:
+        """Reset the game to its initial state."""
+        self.board = Board(self.board_size)
+        self.players = []
+        self.current_player = 0
+        self.rounds = 0
+        self.game_over = False
+
+        for i in range(self.num_players):
+            player = PlayerState(idx=i)
+            player.available_pieces = set(self.piece_set.piece_names)
+            if self.corner_rule and i < len(self._starting_corners):
+                player.corners = {self._starting_corners[i]}
+            else:
+                player.corners = set()
+            self.players.append(player)
+
+    # ------------------------------------------------------------------
+    # Move validation
+    # ------------------------------------------------------------------
+
+    def _get_placed_squares(self, move: Move) -> list[tuple[int, int]]:
+        """Get the absolute board coordinates for a move."""
+        orient = self.piece_set.get_orientations(move.piece_id)[move.orientation_id]
+        return [(move.x + dx, move.y + dy) for dx, dy in orient.squares]
+
+    def _get_placed_corners(self, move: Move) -> list[tuple[int, int]]:
+        """Get the absolute corner coordinates for a move."""
+        orient = self.piece_set.get_orientations(move.piece_id)[move.orientation_id]
+        return [(move.x + dx, move.y + dy) for dx, dy in orient.corners]
+
+    def valid_move(self, player_idx: int, move: Move) -> bool:
+        """Check if a move is valid for the given player.
+
+        Validation rules:
+        1. Player must have the piece available
+        2. All squares must be in bounds
+        3. No overlap with existing pieces
+        4. Corner rule: piece must touch same-color piece at a corner
+        5. No edge adjacency with same-color pieces
+        6. First move must be in a corner (if corner_rule is enabled)
+        """
+        player = self.players[player_idx]
+        piece = self.piece_set.get_piece(move.piece_id)
+
+        # Rule 1: Check if player has the piece
+        if piece.name not in player.available_pieces:
+            return False
+
+        placed_squares = self._get_placed_squares(move)
+
+        # Rule 2: Check bounds
+        for x, y in placed_squares:
+            if not self.board.in_bounds(x, y):
+                return False
+
+        # Rule 3: Check overlap
+        if self.board.has_overlap(placed_squares):
+            return False
+
+        player_board_idx = player_idx + self.PLAYER_OFFSET
+
+        # Rule 6: First move must be in a corner
+        if self.corner_rule and not player.has_started:
+            if len(self.players) <= len(self._starting_corners):
+                start_corner = self._starting_corners[player_idx]
+                if (move.x, move.y) != start_corner:
+                    return False
+
+        # Rule 5: No edge adjacency with same-color pieces
+        for x, y in placed_squares:
+            for dx, dy in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
+                nx, ny = x + dx, y + dy
+                if self.board.in_bounds(nx, ny):
+                    if self.board.get_cell(nx, ny) == player_board_idx:
+                        return False
+
+        # Rule 4: Corner rule (must touch same-color at a corner)
+        if player.has_started:
+            # Must touch at least one same-color piece at a corner
+            placed_corners = self._get_placed_corners(move)
+            touches_corner = False
+            for cx, cy in placed_corners:
+                if self.board.in_bounds(cx, cy):
+                    if self.board.get_cell(cx, cy) == player_board_idx:
+                        touches_corner = True
+                        break
+            if not touches_corner:
+                return False
+        # If not started yet, first move doesn't need to touch (it's the first piece)
+
+        return True
+
+    def get_valid_actions(self, player_idx: int) -> np.ndarray:
+        """Get a boolean mask of valid actions for the given player."""
+        mask = np.zeros(self.num_actions, dtype=bool)
+        player = self.players[player_idx]
+
+        if not player.can_move:
+            return mask
+
+        for action_idx in range(self.num_actions):
+            move = self._action_moves[action_idx]
+            piece = self.piece_set.get_piece(move.piece_id)
+            if piece.name not in player.available_pieces:
+                continue
+            if self.valid_move(player_idx, move):
+                mask[action_idx] = True
+
+        return mask
+
+    def get_valid_action_indices(self, player_idx: int) -> list[int]:
+        """Get a list of valid action indices for the given player."""
+        mask = self.get_valid_actions(player_idx)
+        return [int(i) for i in np.where(mask)[0]]
+
+    def has_valid_moves(self, player_idx: int) -> bool:
+        """Check if a player has any valid moves."""
+        player = self.players[player_idx]
+        if not player.can_move:
+            return False
+
+        for action_idx in range(self.num_actions):
+            move = self._action_moves[action_idx]
+            piece = self.piece_set.get_piece(move.piece_id)
+            if piece.name in player.available_pieces:
+                if self.valid_move(player_idx, move):
+                    return True
+        return False
+
+    # ------------------------------------------------------------------
+    # Move execution
+    # ------------------------------------------------------------------
+
+    def apply_move(self, player_idx: int, move: Move) -> None:
+        """Apply a move to the game state. Does not validate."""
+        player = self.players[player_idx]
+        piece = self.piece_set.get_piece(move.piece_id)
+        placed_squares = self._get_placed_squares(move)
+        placed_corners = self._get_placed_corners(move)
+
+        # Place on board
+        player_board_idx = player_idx + self.PLAYER_OFFSET
+        self.board.place(player_board_idx, placed_squares)
+
+        # Update player state
+        player.score += piece.size
+        player.available_pieces.discard(piece.name)
+        player.has_started = True
+
+        # Update corners: remove covered squares, add new corners
+        for cx, cy in placed_corners:
+            if self.board.in_bounds(cx, cy) and self.board.is_empty(cx, cy):
+                player.corners.add((cx, cy))
+
+        # Remove corners that are now occupied
+        player.corners = {
+            (x, y) for x, y in player.corners if self.board.is_empty(x, y)
+        }
+
+    def play_move(self, player_idx: int, action: int) -> bool:
+        """Validate and apply a move. Returns True if successful."""
+        move = self.get_move(action)
+        if not self.valid_move(player_idx, move):
+            return False
+        self.apply_move(player_idx, move)
+        return True
+
+    # ------------------------------------------------------------------
+    # Turn management
+    # ------------------------------------------------------------------
+
+    def next_player(self) -> int:
+        """Advance to the next player who can still move.
+
+        If no players can move, the game is over.
+        """
+        if self.game_over:
+            return self.current_player
+
+        for offset in range(1, self.num_players + 1):
+            next_idx = (self.current_player + offset) % self.num_players
+            next_player = self.players[next_idx]
+            if next_player.can_move and self.has_valid_moves(next_idx):
+                self.current_player = next_idx
+                return next_idx
+
+        # No one can move — game over
+        self.game_over = True
+        return self.current_player
+
+    def advance_turn(self) -> None:
+        """Apply a move for the current player, then advance.
+
+        This is a convenience method for bot-driven games.
+        """
+        self.next_player()
+
+    # ------------------------------------------------------------------
+    # Game state queries
+    # ------------------------------------------------------------------
+
+    def is_game_over(self) -> bool:
+        """Check if the game is over (no player can make a move)."""
+        if self.game_over:
+            return True
+        # Check if any player can still move
+        for i in range(self.num_players):
+            if self.players[i].can_move and self.has_valid_moves(i):
+                return False
+        self.game_over = True
+        return True
+
+    def get_scores(self) -> list[int]:
+        """Get the score for each player.
+
+        Score = squares placed (positive) - unplaced squares (negative).
+        A player who placed all pieces gets a +15 bonus.
+        If the last piece was the monomino, an additional +5 bonus.
+        """
+        scores = []
+        for player in self.players:
+            # Sum sizes of remaining pieces
+            unplaced = sum(
+                self.piece_set.pieces[pid].size
+                for pid, piece in enumerate(self.piece_set.pieces)
+                if piece.name in player.available_pieces
+            )
+            score = player.score - unplaced
+
+            # Bonus for placing all pieces
+            if len(player.available_pieces) == 0:
+                score += 15
+
+            scores.append(score)
+        return scores
+
+    def get_winners(self) -> list[int] | None:
+        """Get the winning player(s). Returns None if game is not over."""
+        if not self.is_game_over():
+            return None
+
+        scores = self.get_scores()
+        max_score = max(scores)
+        winners = [i for i, s in enumerate(scores) if s == max_score]
+        return winners
+
+    def get_current_observation(self) -> dict:
+        """Get the current game state as an observation dict."""
+        player_idx = self.current_player
+        player = self.players[player_idx]
+
+        # Board as player indices (1-based)
+        board_obs = self.board.grid.copy()
+
+        # Available pieces as binary vector
+        pieces_obs = np.zeros(self.piece_set.num_pieces, dtype=bool)
+        for name in player.available_pieces:
+            pieces_obs[self.piece_set.get_piece_id(name)] = True
+
+        # Corners as boolean grid
+        corners_obs = np.zeros((self.board_size, self.board_size), dtype=bool)
+        for x, y in player.corners:
+            if self.board.in_bounds(x, y):
+                corners_obs[y, x] = True
+
+        return {
+            "board": board_obs.astype(np.int8),
+            "pieces": pieces_obs,
+            "corners": corners_obs,
+        }
+
+    def get_action_mask(self, player_idx: int) -> np.ndarray:
+        """Get the action mask for a player."""
+        return self.get_valid_actions(player_idx)
+
+    def copy(self) -> BlokusGame:
+        """Create a deep copy of the game state."""
+        import copy
+
+        new_game = BlokusGame.__new__(BlokusGame)
+        new_game.board_size = self.board_size
+        new_game.piece_set = self.piece_set
+        new_game.num_players = self.num_players
+        new_game.corner_rule = self.corner_rule
+        new_game.board = self.board.copy()
+        new_game.players = copy.deepcopy(self.players)
+        new_game.current_player = self.current_player
+        new_game.rounds = self.rounds
+        new_game.game_over = self.game_over
+        new_game._action_moves = self._action_moves
+        new_game._move_to_action = self._move_to_action
+        new_game._starting_corners = self._starting_corners
+        return new_game
diff --git a/src/blokus_gym/core/pieces.py b/src/blokus_gym/core/pieces.py
new file mode 100644
index 0000000..51895c6
--- /dev/null
+++ b/src/blokus_gym/core/pieces.py
@@ -0,0 +1,265 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Piece:
+    """A polyomino piece defined by a set of (x, y) coordinates.
+
+    Coordinates are relative to a reference point (typically the
+    top-left corner of the bounding box after normalization).
+    """
+
+    name: str
+    squares: frozenset[tuple[int, int]]
+
+    @property
+    def size(self) -> int:
+        return len(self.squares)
+
+    def to_dict(self) -> dict:
+        return {
+            "name": self.name,
+            "squares": sorted(self.squares),
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> Piece:
+        return cls(
+            name=data["name"],
+            squares=frozenset(tuple(s) for s in data["squares"]),
+        )
+
+
+@dataclass
+class OrientedPiece:
+    """A piece in a specific orientation placed at a specific position."""
+
+    piece_id: int
+    orientation_id: int
+    squares: list[tuple[int, int]]
+    corners: list[tuple[int, int]]
+
+
+@dataclass
+class Move:
+    """A complete move: which piece, which orientation, where placed."""
+
+    piece_id: int
+    orientation_id: int
+    x: int
+    y: int
+
+
+@dataclass
+class PieceOrientation:
+    """A piece in a specific orientation (before placement on board)."""
+
+    piece_id: int
+    orientation_id: int
+    squares: list[tuple[int, int]]  # normalized relative coordinates
+    corners: list[tuple[int, int]]  # corner cells relative to squares
+
+
+def _normalize(squares: list[tuple[int, int]]) -> list[tuple[int, int]]:
+    """Shift coordinates so the minimum x and y are 0."""
+    min_x = min(x for x, _ in squares)
+    min_y = min(y for _, y in squares)
+    return [(x - min_x, y - min_y) for x, y in squares]
+
+
+def _rotate(squares: list[tuple[int, int]]) -> list[tuple[int, int]]:
+    """Rotate 90 degrees clockwise: (x, y) -> (y, -x)."""
+    return [(y, -x) for x, y in squares]
+
+
+def _flip(squares: list[tuple[int, int]]) -> list[tuple[int, int]]:
+    """Flip horizontally: (x, y) -> (-x, y)."""
+    return [(-x, y) for x, y in squares]
+
+
+def _compute_corners(squares: list[tuple[int, int]]) -> list[tuple[int, int]]:
+    """Compute the corner cells for a piece.
+
+    Corner cells are the diagonal neighbors of each square that are not
+    edge-adjacent to any other square in the piece. These are the cells
+    where a same-color piece must touch (corner-to-corner).
+    """
+    square_set = set(squares)
+    corners = set()
+    for x, y in squares:
+        for dx, dy in [(-1, -1), (1, -1), (-1, 1), (1, 1)]:
+            cx, cy = x + dx, y + dy
+            if (cx, cy) not in square_set:
+                # Check that this corner is not edge-adjacent to another square
+                edge_neighbors = [
+                    (cx + 1, cy), (cx - 1, cy), (cx, cy + 1), (cx, cy - 1)
+                ]
+                if not any(en in square_set for en in edge_neighbors):
+                    corners.add((cx, cy))
+    return sorted(corners)
+
+
+def generate_orientations(piece: Piece) -> list[PieceOrientation]:
+    """Generate all unique orientations of a piece.
+
+    Applies 4 rotations and 2 flips, then deduplicates by comparing
+    normalized coordinate sets.
+    """
+    orientations: list[PieceOrientation] = []
+    seen: set[tuple[tuple[int, int], ...]] = set()
+
+    squares = sorted(piece.squares)
+    for flip_count in range(2):
+        for rotation_count in range(4):
+            current = list(squares)
+            for _ in range(rotation_count):
+                current = _rotate(current)
+            for _ in range(flip_count):
+                current = _flip(current)
+            normalized = tuple(sorted(_normalize(current)))
+            if normalized not in seen:
+                seen.add(normalized)
+                corners = _compute_corners(list(normalized))
+                orientations.append(
+                    PieceOrientation(
+                        piece_id=-1,  # Set by PieceSet
+                        orientation_id=len(orientations),
+                        squares=list(normalized),
+                        corners=corners,
+                    )
+                )
+    return orientations
+
+
+# ---------------------------------------------------------------------------
+# Standard Blokus piece sets
+# ---------------------------------------------------------------------------
+
+# The 21 free polyominoes of size 1-5
+# Naming follows standard pentomino/tetromino conventions
+
+_I1 = Piece("I1", frozenset([(0, 0)]))
+
+_I2 = Piece("I2", frozenset([(0, 0), (0, 1)]))
+
+_I3 = Piece("I3", frozenset([(0, 0), (0, 1), (0, 2)]))
+
+_V3 = Piece("V3", frozenset([(0, 0), (1, 0), (0, 1)]))
+
+_I4 = Piece("I4", frozenset([(0, 0), (0, 1), (0, 2), (0, 3)]))
+
+_L4 = Piece("L4", frozenset([(0, 0), (0, 1), (0, 2), (1, 0)]))
+
+_T4 = Piece("T4", frozenset([(0, 0), (1, 0), (2, 0), (1, 1)]))
+
+_S4 = Piece("S4", frozenset([(0, 0), (1, 0), (1, 1), (2, 1)]))
+
+_O4 = Piece("O4", frozenset([(0, 0), (1, 0), (0, 1), (1, 1)]))
+
+_I5 = Piece("I5", frozenset([(0, 0), (0, 1), (0, 2), (0, 3), (0, 4)]))
+
+_L5 = Piece("L5", frozenset([(0, 0), (0, 1), (0, 2), (0, 3), (1, 0)]))
+
+_Y5 = Piece("Y5", frozenset([(0, 0), (0, 1), (0, 2), (0, 3), (1, 1)]))
+
+_N5 = Piece("N5", frozenset([(0, 0), (1, 0), (2, 0), (2, 1), (3, 1)]))
+
+_T5 = Piece("T5", frozenset([(0, 0), (1, 0), (2, 0), (1, 1), (1, 2)]))
+
+_U5 = Piece("U5", frozenset([(0, 0), (2, 0), (0, 1), (1, 1), (2, 1)]))
+
+_V5 = Piece("V5", frozenset([(0, 0), (0, 1), (0, 2), (1, 0), (2, 0)]))
+
+_W5 = Piece("W5", frozenset([(0, 0), (0, 1), (1, 0), (1, 1), (2, 0)]))
+
+_Z5 = Piece("Z5", frozenset([(0, 0), (1, 0), (1, 1), (1, 2), (2, 2)]))
+
+_F5 = Piece("F5", frozenset([(0, 0), (1, 0), (1, 1), (2, 1), (1, 2)]))
+
+_X5 = Piece("X5", frozenset([(0, 0), (1, 0), (2, 0), (1, 1), (1, -1)]))
+
+_P5 = Piece("P5", frozenset([(0, 0), (1, 0), (0, 1), (1, 1), (0, 2)]))
+
+STANDARD_PIECES: list[Piece] = [
+    _I1, _I2, _I3, _V3, _I4, _L4, _T4, _S4, _O4,
+    _I5, _L5, _Y5, _N5, _T5, _U5, _V5, _W5, _Z5, _F5, _X5, _P5,
+]
+
+# Blokus Duo uses the same pieces but on a 14x14 board
+DUO_PIECES: list[Piece] = STANDARD_PIECES
+
+# Blokus Junior: simplified pieces (only 12 unique pieces, 2 copies each)
+# Uses only pieces with size <= 4 for simplicity
+JUNIOR_PIECES: list[Piece] = [
+    _I1, _I2, _I3, _V3, _I4, _L4, _T4, _S4, _O4,
+]
+
+
+class PieceSet:
+    """A collection of pieces with pre-computed orientations.
+
+    This class manages the piece set used in a Blokus game, including
+    all unique orientations for each piece and a lookup table for
+    generating moves.
+    """
+
+    def __init__(self, pieces: list[Piece]):
+        self.pieces: list[Piece] = pieces
+        self.piece_names: list[str] = [p.name for p in pieces]
+        self.piece_id_map: dict[str, int] = {name: i for i, name in enumerate(self.piece_names)}
+
+        # Pre-compute all orientations for each piece
+        self.orientations: list[list[PieceOrientation]] = []
+        for piece_id, piece in enumerate(pieces):
+            orients = generate_orientations(piece)
+            for orient in orients:
+                orient.piece_id = piece_id
+            self.orientations.append(orients)
+
+        # Total number of (piece, orientation) combinations
+        self.num_orientations: int = sum(len(o) for o in self.orientations)
+
+    @property
+    def num_pieces(self) -> int:
+        return len(self.pieces)
+
+    def get_orientations(self, piece_id: int) -> list[PieceOrientation]:
+        return self.orientations[piece_id]
+
+    def get_piece(self, piece_id: int) -> Piece:
+        return self.pieces[piece_id]
+
+    def get_piece_id(self, name: str) -> int:
+        return self.piece_id_map[name]
+
+    def to_dict(self) -> dict:
+        return {
+            "pieces": [p.to_dict() for p in self.pieces],
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> PieceSet:
+        pieces = [Piece.from_dict(p) for p in data["pieces"]]
+        return cls(pieces)
+
+    @classmethod
+    def from_json(cls, path: str) -> PieceSet:
+        import json
+
+        with open(path) as f:
+            data = json.load(f)
+        return cls.from_dict(data)
+
+    def save_json(self, path: str) -> None:
+        import json
+
+        with open(path, "w") as f:
+            json.dump(self.to_dict(), f, indent=2)
+
+    def __len__(self) -> int:
+        return len(self.pieces)
+
+    def __repr__(self) -> str:
+        return f"PieceSet(num_pieces={self.num_pieces}, num_orientations={self.num_orientations})"
diff --git a/src/blokus_gym/envs/__init__.py b/src/blokus_gym/envs/__init__.py
new file mode 100644
index 0000000..e7fd5e2
--- /dev/null
+++ b/src/blokus_gym/envs/__init__.py
@@ -0,0 +1,97 @@
+from blokus_gym.core.bots import GreedyBot, GreedyCornersBot, MinimaxBot, RandomBot
+from blokus_gym.core.pieces import DUO_PIECES, JUNIOR_PIECES, STANDARD_PIECES
+from blokus_gym.envs.blokus_env import BlokusEnv
+from blokus_gym.envs.multiagent import BlokusMultiAgentEnv
+
+__all__ = [
+    "BlokusEnv",
+    "BlokusMultiAgentEnv",
+    "STANDARD_PIECES",
+    "DUO_PIECES",
+    "JUNIOR_PIECES",
+    "RandomBot",
+    "GreedyBot",
+    "GreedyCornersBot",
+    "MinimaxBot",
+]
+
+
+def _register_envs():
+    """Register all predefined environment configurations."""
+    import gymnasium as gym
+
+    # Standard 4-player Blokus (20x20)
+    gym.register(
+        id="Blokus-v0",
+        entry_point="blokus_gym.envs:BlokusEnv",
+        kwargs={
+            "num_players": 4,
+            "board_size": 20,
+            "pieces": STANDARD_PIECES,
+        },
+        max_episode_steps=500,
+    )
+
+    # Blokus Duo (2 players, 14x14)
+    gym.register(
+        id="BlokusDuo-v0",
+        entry_point="blokus_gym.envs:BlokusEnv",
+        kwargs={
+            "num_players": 2,
+            "board_size": 14,
+            "pieces": DUO_PIECES,
+        },
+        max_episode_steps=300,
+    )
+
+    # Blokus Junior (2 players, 14x14, simplified pieces)
+    gym.register(
+        id="BlokusJunior-v0",
+        entry_point="blokus_gym.envs:BlokusEnv",
+        kwargs={
+            "num_players": 2,
+            "board_size": 14,
+            "pieces": JUNIOR_PIECES,
+        },
+        max_episode_steps=200,
+    )
+
+    # Simple test env (2 players, 7x7, only small pieces)
+    gym.register(
+        id="BlokusSimple-v0",
+        entry_point="blokus_gym.envs:BlokusEnv",
+        kwargs={
+            "num_players": 2,
+            "board_size": 7,
+            "pieces": [p for p in STANDARD_PIECES if p.size < 5],
+        },
+        max_episode_steps=100,
+    )
+
+    # Greedy bot variants
+    gym.register(
+        id="BlokusGreedy-v0",
+        entry_point="blokus_gym.envs:BlokusEnv",
+        kwargs={
+            "num_players": 4,
+            "board_size": 20,
+            "pieces": STANDARD_PIECES,
+            "bot_type": GreedyBot,
+        },
+        max_episode_steps=500,
+    )
+
+    gym.register(
+        id="BlokusDuoGreedy-v0",
+        entry_point="blokus_gym.envs:BlokusEnv",
+        kwargs={
+            "num_players": 2,
+            "board_size": 14,
+            "pieces": DUO_PIECES,
+            "bot_type": GreedyBot,
+        },
+        max_episode_steps=300,
+    )
+
+
+_register_envs()
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diff --git a/src/blokus_gym/envs/blokus_env.py b/src/blokus_gym/envs/blokus_env.py
new file mode 100644
index 0000000..f6209f8
--- /dev/null
+++ b/src/blokus_gym/envs/blokus_env.py
@@ -0,0 +1,366 @@
+from __future__ import annotations
+
+from typing import Any
+
+import gymnasium as gym
+import numpy as np
+from gymnasium import spaces
+
+from blokus_gym.core.bots import Bot, RandomBot
+from blokus_gym.core.game import BlokusGame
+from blokus_gym.core.pieces import STANDARD_PIECES, Piece, PieceSet
+from blokus_gym.utils.render import render_ansi_board, render_text_board
+
+
+class BlokusEnv(gym.Env):
+    """Gymnasium environment for the board game Blokus.
+
+    Supports 2-4 players on customizable board sizes with custom piece sets.
+    In single-agent mode, the agent controls player 0 and opponents are bots.
+
+    Action Space: Discrete(N) where N is the total number of possible
+    (piece, orientation, position) combinations. An action mask is provided
+    in the info dict to indicate valid actions.
+
+    Observation Space: Dict with:
+        - "board": (board_size, board_size) int8 array (0=empty, 1-4=player)
+        - "pieces": MultiBinary(num_pieces) - 1 if piece still available
+        - "corners": (board_size, board_size) bool array - valid corner cells
+
+    Example:
+        >>> import gymnasium as gym
+        >>> from blokus_gym import BlokusEnv
+        >>> env = BlokusEnv(num_players=4, board_size=20)
+        >>> obs, info = env.reset(seed=42)
+        >>> action = env.action_space.sample()  # Use info["action_mask"] instead
+        >>> obs, reward, terminated, truncated, info = env.step(action)
+    """
+
+    metadata = {"render_modes": ["human", "ansi", "rgb_array"]}
+
+    # Player colors for rendering (index 0 = empty)
+    PLAYER_COLORS = ["empty", "red", "blue", "yellow", "green"]
+
+    def __init__(
+        self,
+        num_players: int = 4,
+        board_size: int = 20,
+        pieces: list[Piece] | None = None,
+        render_mode: str | None = None,
+        bot_type: type[Bot] = RandomBot,
+        bot_strength: int = 1,
+        reward_shaping: bool = False,
+        corner_rule: bool = True,
+        max_steps: int | None = None,
+        seed: int | None = None,
+        **kwargs: Any,
+    ):
+        super().__init__()
+
+        # Validate parameters
+        assert 2 <= num_players <= 4, f"num_players must be 2-4, got {num_players}"
+        assert board_size >= 5, f"board_size must be >= 5, got {board_size}"
+
+        self.num_players = num_players
+        self.board_size = board_size
+        self.render_mode = render_mode
+        self.bot_type = bot_type
+        self.bot_strength = bot_strength
+        self.reward_shaping = reward_shaping
+        self.corner_rule = corner_rule
+        self.max_steps = max_steps
+        self._seed = seed
+
+        # Initialize piece set
+        self.piece_set = PieceSet(pieces or STANDARD_PIECES)
+
+        # Initialize game
+        self.game = BlokusGame(
+            board_size=board_size,
+            pieces=self.piece_set,
+            num_players=num_players,
+            corner_rule=corner_rule,
+        )
+
+        # Define spaces
+        self.observation_space = spaces.Dict({
+            "board": spaces.Box(0, num_players, (board_size, board_size), dtype=np.int8),
+            "pieces": spaces.MultiBinary(self.piece_set.num_pieces),
+            "corners": spaces.Box(0, 1, (board_size, board_size), dtype=bool),
+        })
+
+        self.action_space = spaces.Discrete(self.game.num_actions)
+
+        # Bot instances (created during reset)
+        self.bots: list[Bot | None] = [None] * num_players
+
+        # Episode tracking
+        self.current_step = 0
+        self._last_observation: dict | None = None
+
+    def _get_obs(self) -> dict:
+        """Get the current observation from the agent's perspective (player 0)."""
+        obs = self.game.get_current_observation()
+        return {
+            "board": obs["board"],
+            "pieces": obs["pieces"],
+            "corners": obs["corners"],
+        }
+
+    def _get_info(self) -> dict:
+        """Get auxiliary information."""
+        return {
+            "action_mask": self.game.get_action_mask(0),
+            "current_player": self.game.current_player,
+            "players_with_moves": [
+                i for i in range(self.num_players)
+                if self.game.players[i].can_move and self.game.has_valid_moves(i)
+            ],
+            "step_count": self.current_step,
+            "scores": self.game.get_scores(),
+        }
+
+    def reset(
+        self,
+        seed: int | None = None,
+        options: dict | None = None,
+    ) -> tuple[dict, dict]:
+        """Reset the environment to a new episode.
+
+        Args:
+            seed: Random seed for reproducibility.
+            options: Additional options (unused).
+
+        Returns:
+            Tuple of (observation, info).
+        """
+        super().reset(seed=seed)
+
+        # Re-initialize game
+        self.game = BlokusGame(
+            board_size=self.board_size,
+            pieces=self.piece_set,
+            num_players=self.num_players,
+            corner_rule=self.corner_rule,
+        )
+        self.game.reset()
+
+        # Create bots for opponents
+        bot_seed = seed or 0
+        for i in range(1, self.num_players):
+            self.bots[i] = self.bot_type(
+                player_idx=i,
+                seed=bot_seed + i * 1000,
+            )
+
+        self.current_step = 0
+
+        # Let bots play until it's the agent's turn
+        # The agent (player 0) starts first if corner_rule is enabled
+        self._play_bots_until_agent()
+
+        obs = self._get_obs()
+        info = self._get_info()
+        self._last_observation = obs
+
+        return obs, info
+
+    def _play_bots_until_agent(self) -> None:
+        """Let bot players take their turns until it's the agent's turn."""
+        max_bot_turns = self.num_players * 10  # Safety limit
+        turns = 0
+
+        while self.game.current_player != 0 and not self.game.is_game_over():
+            if turns > max_bot_turns:
+                break
+            bot = self.bots[self.game.current_player]
+            if bot is None:
+                # Shouldn't happen, but safety
+                self.game.next_player()
+                turns += 1
+                continue
+
+            valid_actions = self.game.get_valid_actions(self.game.current_player)
+            if not np.any(valid_actions):
+                self.game.players[self.game.current_player].can_move = False
+                self.game.next_player()
+                turns += 1
+                continue
+
+            action = bot.select_action(self.game, valid_actions)
+            if action is not None:
+                self.game.play_move(self.game.current_player, action)
+            else:
+                self.game.players[self.game.current_player].can_move = False
+
+            self.game.next_player()
+            turns += 1
+
+    def step(self, action: int) -> tuple[dict, float, bool, bool, dict]:
+        """Execute one step in the environment.
+
+        The agent (player 0) takes an action, then all bots play until
+        it's the agent's turn again.
+
+        Args:
+            action: Action index (piece, orientation, position).
+
+        Returns:
+            Tuple of (observation, reward, terminated, truncated, info).
+        """
+        self.current_step += 1
+
+        # Apply agent's action
+        success = self.game.play_move(0, action)
+
+        if not success:
+            # Invalid action - penalize and end episode
+            reward = -10.0
+            obs = self._get_obs()
+            info = self._get_info()
+            info["invalid_action"] = True
+            self._last_observation = obs
+            return obs, reward, True, False, info
+
+        # Advance to next player
+        self.game.next_player()
+
+        # Let bots play until it's the agent's turn or game is over
+        self._play_bots_until_agent()
+
+        # Check if game is over
+        terminated = self.game.is_game_over()
+        truncated = False
+        if self.max_steps is not None and self.current_step >= self.max_steps:
+            truncated = True
+
+        # Compute reward
+        if terminated or truncated:
+            reward = self._compute_terminal_reward()
+        else:
+            reward = self._compute_step_reward()
+
+        obs = self._get_obs()
+        info = self._get_info()
+        self._last_observation = obs
+
+        return obs, reward, terminated, truncated, info
+
+    def _compute_step_reward(self) -> float:
+        """Compute reward for a non-terminal step."""
+        if self.reward_shaping:
+            # Small reward for each square placed
+            return 0.01 * self.game.players[0].score
+        return 0.0
+
+    def _compute_terminal_reward(self) -> float:
+        """Compute reward when the game ends."""
+        scores = self.game.get_scores()
+        agent_score = scores[0]
+
+        if self.reward_shaping:
+            # Normalized score: agent score / max possible score
+            max_possible = sum(p.size for p in self.piece_set.pieces)
+            return agent_score / max_possible
+
+        # Win/loss reward
+        max_score = max(scores)
+        if agent_score == max_score:
+            # Check if it's a tie
+            winners = [i for i, s in enumerate(scores) if s == max_score]
+            if len(winners) == 1:
+                return 1.0  # Win
+            else:
+                return 0.0  # Tie
+        else:
+            return -1.0  # Loss
+
+    def render(self) -> str | np.ndarray | None:
+        """Render the environment.
+
+        Args:
+            No arguments - uses self.render_mode.
+
+        Returns:
+            For "human": None (prints to stdout)
+            For "ansi": string representation
+            For "rgb_array": numpy array of shape (H, W, 3)
+        """
+        if self.render_mode is None:
+            return None
+
+        if self.render_mode == "human":
+            render_text_board(self.game)
+            return None
+
+        elif self.render_mode == "ansi":
+            return render_ansi_board(self.game)
+
+        elif self.render_mode == "rgb_array":
+            return self._render_rgb_array()
+
+        else:
+            raise ValueError(f"Unknown render_mode: {self.render_mode}")
+
+    def _render_rgb_array(self) -> np.ndarray:
+        """Render the board as an RGB array using matplotlib."""
+        try:
+            import matplotlib
+            matplotlib.use("Agg")
+            import matplotlib.pyplot as plt
+        except ImportError as exc:
+            raise ImportError(
+                "matplotlib is required for rgb_array rendering. "
+                "Install with: pip install matplotlib"
+            ) from exc
+
+        fig, ax = plt.subplots(figsize=(8, 8))
+
+        # Color map: 0=empty(light grey), 1=red, 2=blue, 3=yellow, 4=green
+        colors = {
+            0: "#d3d3d3",  # light grey
+            1: "#ff6b6b",  # red
+            2: "#4dabf7",  # blue
+            3: "#ffd43b",  # yellow
+            4: "#51cf66",  # green
+        }
+
+        grid = self.game.board.grid.astype(float)
+        colored = np.zeros((self.board_size, self.board_size, 3))
+        for i in range(self.board_size):
+            for j in range(self.board_size):
+                val = int(grid[i, j])
+                hex_color = colors.get(val, "#d3d3d3")
+                # Parse hex color
+                r = int(hex_color[1:3], 16) / 255
+                g = int(hex_color[3:5], 16) / 255
+                b = int(hex_color[5:7], 16) / 255
+                colored[i, j] = [r, g, b]
+
+        ax.imshow(colored, interpolation="nearest")
+        ax.set_xticks(np.arange(-0.5, self.board_size, 1), minor=True)
+        ax.set_yticks(np.arange(-0.5, self.board_size, 1), minor=True)
+        ax.grid(True, which="minor", color="black", linewidth=0.5)
+        ax.set_xticks([])
+        ax.set_yticks([])
+
+        # Title with current player and scores
+        scores = self.game.get_scores()
+        title = f"Current player: {self.game.current_player + 1}\n"
+        title += "Scores: " + " | ".join(f"P{i+1}: {s}" for i, s in enumerate(scores))
+        ax.set_title(title, fontsize=10)
+
+        plt.tight_layout()
+        fig.canvas.draw()
+        image = np.frombuffer(fig.canvas.tostring_rgb(), dtype=np.uint8)
+        image = image.reshape(fig.canvas.get_width_height()[::-1] + (3,))
+        plt.close(fig)
+        return image
+
+    def close(self) -> None:
+        """Clean up resources."""
+        pass
+
+    def get_action_mask(self) -> np.ndarray:
+        """Get the action mask for the current player (player 0)."""
+        return self.game.get_action_mask(0)
diff --git a/src/blokus_gym/envs/multiagent.py b/src/blokus_gym/envs/multiagent.py
new file mode 100644
index 0000000..670f811
--- /dev/null
+++ b/src/blokus_gym/envs/multiagent.py
@@ -0,0 +1,254 @@
+from __future__ import annotations
+
+import numpy as np
+from gymnasium import spaces
+
+from blokus_gym.core.game import BlokusGame
+from blokus_gym.core.pieces import STANDARD_PIECES, PieceSet
+
+
+class BlokusMultiAgentEnv:
+    """PettingZoo-style AEC (Actor-Environment-Cycle) environment for Blokus.
+
+    This wraps the BlokusGame core to provide a multi-agent interface where
+    each player acts independently. Compatible with PettingZoo's API and
+    RLlib's multi-agent RL.
+
+    Unlike the single-agent BlokusEnv, all players are controlled by the
+    caller (no built-in bots). This allows for self-play training and
+    evaluation of multi-agent policies.
+
+    Attributes:
+        num_players: Number of players (2-4).
+        board_size: Size of the square board.
+        agents: List of agent names (e.g., ["player_0", "player_1", ...]).
+        possible_agents: Same as agents (never changes).
+        agent_order: Order in which agents take turns.
+
+    Example:
+        >>> from blokus_gym import BlokusMultiAgentEnv
+        >>> env = BlokusMultiAgentEnv(num_players=4)
+        >>> obs, info = env.reset()
+        >>> for agent in env.agent_iter():
+        ...     obs, reward, terminated, truncated, info = env.last()
+        ...     action = env.action_space(agent).sample()  # Use action mask
+        ...     env.step(action)
+    """
+
+    metadata = {"render_modes": ["human", "ansi"]}
+
+    def __init__(
+        self,
+        num_players: int = 4,
+        board_size: int = 20,
+        pieces: list | None = None,
+        render_mode: str | None = None,
+        corner_rule: bool = True,
+        seed: int | None = None,
+    ):
+        assert 2 <= num_players <= 4, f"num_players must be 2-4, got {num_players}"
+        assert board_size >= 5, f"board_size must be >= 5, got {board_size}"
+
+        self.num_players = num_players
+        self.board_size = board_size
+        self.render_mode = render_mode
+        self.corner_rule = corner_rule
+        self._seed = seed
+
+        self.piece_set = PieceSet(pieces or STANDARD_PIECES)
+        self.game: BlokusGame | None = None
+
+        self.agents = [f"player_{i}" for i in range(num_players)]
+        self.possible_agents = list(self.agents)
+        self.agent_order = list(self.agents)
+
+        # Observation and action spaces (same for all agents)
+        self.observation_space = spaces.Dict({
+            "board": spaces.Box(0, num_players, (board_size, board_size), dtype=np.int8),
+            "pieces": spaces.MultiBinary(self.piece_set.num_pieces),
+            "corners": spaces.Box(0, 1, (board_size, board_size), dtype=bool),
+        })
+
+        self._cur_step = 0
+        self._current_agent_idx = 0
+        self._agent_dones: dict[str, bool] = {}
+
+    def reset(self, seed: int | None = None, options: dict | None = None) -> tuple[dict, dict]:
+        """Reset the environment.
+
+        Returns:
+            obs: Dict mapping agent name to observation.
+            info: Dict with auxiliary information.
+        """
+        self.game = BlokusGame(
+            board_size=self.board_size,
+            pieces=self.piece_set,
+            num_players=self.num_players,
+            corner_rule=self.corner_rule,
+        )
+        self.game.reset()
+
+        self._cur_step = 0
+        self._current_agent_idx = 0
+        self._agent_dones = dict.fromkeys(self.agents, False)
+
+        # Get observation for the first agent
+        first_agent = self.agent_order[0]
+        obs = self._get_obs(first_agent)
+        info = self._get_info(first_agent)
+
+        return {first_agent: obs}, info
+
+    def step(self, action: int) -> tuple[dict, dict, dict, dict, dict]:
+        """Execute one step. Must be called for the current agent in agent_iter().
+
+        Returns:
+            observations: Dict mapping agent -> observation
+            rewards: Dict mapping agent -> reward
+            terminations: Dict mapping agent -> bool
+            truncations: Dict mapping agent -> bool
+            infos: Dict mapping agent -> info
+        """
+        self._cur_step += 1
+        agent = self.agent_order[self._current_agent_idx]
+        player_idx = int(agent.split("_")[1])
+
+        # Apply the action
+        success = self.game.play_move(player_idx, action)
+
+        # Compute rewards for this player
+        reward = 0.0
+        if not success:
+            reward = -10.0  # Invalid action penalty
+            self._agent_dones[agent] = True
+
+        # Check game over
+        game_over = self.game.is_game_over()
+
+        # Advance to next player
+        if not game_over:
+            self.game.next_player()
+            # Skip players who can't move
+            while not self.game.is_game_over():
+                player = self.game.current_player
+                agent_name = self.agents[player]
+                if self._agent_dones.get(agent_name, False):
+                    self.game.next_player()
+                    continue
+                if not self.game.has_valid_moves(player):
+                    self.game.players[player].can_move = False
+                    self.game.next_player()
+                    continue
+                break
+
+        # Determine next agent
+        next_player = self.game.current_player
+        next_agent = self.agents[next_player]
+
+        # Check if game is over
+        if game_over:
+            # Final rewards based on scores
+            scores = self.game.get_scores()
+            max_score = max(scores)
+            terminations = {}
+            rewards = {}
+            for i, agent_name in enumerate(self.agents):
+                if scores[i] == max_score:
+                    winners = [j for j, s in enumerate(scores) if s == max_score]
+                    if len(winners) == 1:
+                        rewards[agent_name] = 1.0
+                    else:
+                        rewards[agent_name] = 0.0
+                else:
+                    rewards[agent_name] = -1.0
+                terminations[agent_name] = True
+                self._agent_dones[agent_name] = True
+
+            infos = {agent_name: {"step_count": self._cur_step} for agent_name in self.agents}
+            observations = {agent_name: self._get_obs(agent_name) for agent_name in self.agents}
+
+            return observations, rewards, terminations, dict.fromkeys(self.agents, False), infos
+
+        # Normal step
+        rewards = dict.fromkeys(self.agents, 0.0)
+        rewards[agent] = reward
+        terminations = {
+            agent_name: self._agent_dones.get(agent_name, False)
+            for agent_name in self.agents
+        }
+        truncations = dict.fromkeys(self.agents, False)
+
+        observations = {next_agent: self._get_obs(next_agent)}
+        infos = {next_agent: self._get_info(next_agent)}
+
+        # Advance current agent index
+        self._current_agent_idx = self.agents.index(next_agent)
+
+        return observations, rewards, terminations, truncations, infos
+
+    def observe(self, agent: str) -> dict:
+        """Get observation for a specific agent."""
+        return self._get_obs(agent)
+
+    def get_action_mask(self, agent: str) -> np.ndarray:
+        """Get valid action mask for a specific agent."""
+        player_idx = int(agent.split("_")[1])
+        return self.game.get_action_mask(player_idx)
+
+    def action_space(self, agent: str) -> spaces.Discrete:
+        """Get action space for a specific agent."""
+        return spaces.Discrete(self.game.num_actions)
+
+    def observation_space(self, agent: str) -> spaces.Dict:
+        """Get observation space for a specific agent."""
+        return self.observation_space
+
+    def render(self) -> str | None:
+        """Render the current state."""
+        if self.render_mode is None:
+            return None
+
+        from blokus_gym.utils.render import render_ansi_board, render_text_board
+
+        if self.render_mode == "human":
+            print(render_text_board(self.game))
+            return None
+        elif self.render_mode == "ansi":
+            return render_ansi_board(self.game)
+        else:
+            raise ValueError(f"Unknown render_mode: {self.render_mode}")
+
+    def close(self) -> None:
+        """Clean up resources."""
+        pass
+
+    def _get_obs(self, agent: str) -> dict:
+        """Get observation from an agent's perspective."""
+        obs = self.game.get_current_observation()
+        return {
+            "board": obs["board"],
+            "pieces": obs["pieces"],
+            "corners": obs["corners"],
+        }
+
+    def _get_info(self, agent: str) -> dict:
+        """Get auxiliary info for an agent."""
+        player_idx = int(agent.split("_")[1])
+        return {
+            "action_mask": self.game.get_action_mask(player_idx),
+            "current_player": self.game.current_player,
+            "step_count": self._cur_step,
+            "scores": self.game.get_scores(),
+        }
+
+    def agent_iter(self):
+        """Iterate over agents in turn order."""
+        # This is a generator that yields the current agent
+        while True:
+            agent = self.agents[self.game.current_player]
+            if self._agent_dones.get(agent, False):
+                # Skip done agents
+                if all(self._agent_dones.values()):
+                    break
+                continue
+            yield agent
diff --git a/src/blokus_gym/utils/__init__.py b/src/blokus_gym/utils/__init__.py
new file mode 100644
index 0000000..0d8ff89
--- /dev/null
+++ b/src/blokus_gym/utils/__init__.py
@@ -0,0 +1,17 @@
+from blokus_gym.utils.render import (
+    ANSI_COLORS,
+    ANSI_RESET,
+    PLAYER_NAMES,
+    print_board,
+    render_ansi_board,
+    render_text_board,
+)
+
+__all__ = [
+    "ANSI_COLORS",
+    "ANSI_RESET",
+    "PLAYER_NAMES",
+    "render_ansi_board",
+    "render_text_board",
+    "print_board",
+]
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diff --git a/src/blokus_gym/utils/render.py b/src/blokus_gym/utils/render.py
new file mode 100644
index 0000000..ff8ccb6
--- /dev/null
+++ b/src/blokus_gym/utils/render.py
@@ -0,0 +1,105 @@
+from __future__ import annotations
+
+from blokus_gym.core.game import BlokusGame
+
+# ANSI color codes for terminal rendering
+ANSI_COLORS = {
+    0: "\033[90m",  # grey (empty)
+    1: "\033[91m",  # red
+    2: "\033[94m",  # blue
+    3: "\033[93m",  # yellow
+    4: "\033[92m",  # green
+}
+ANSI_RESET = "\033[0m"
+
+# Player names
+PLAYER_NAMES = {1: "Red", 2: "Blue", 3: "Yellow", 4: "Green"}
+
+
+def render_text_board(game: BlokusGame) -> str:
+    """Render the board as a text string (no ANSI colors).
+
+    Returns a string representation of the board.
+    """
+    lines = []
+    size = game.board_size
+
+    # Column headers
+    header = "  " + " ".join(str(i % 10) for i in range(size))
+    lines.append(header)
+
+    for y in range(size):
+        row = f"{y % 10} "
+        for x in range(size):
+            cell = game.board.get_cell(x, y)
+            if cell == 0:
+                row += ". "
+            else:
+                row += f"{cell} "
+        lines.append(row)
+
+    # Player info
+    lines.append("")
+    for i, player in enumerate(game.players):
+        remaining = len(player.available_pieces)
+        total = len(game.piece_set.pieces)
+        name = PLAYER_NAMES.get(i + 1, f"Player {i + 1}")
+        lines.append(f"{name}: {remaining}/{total} pieces remaining, score: {player.score}")
+
+    cur_name = PLAYER_NAMES.get(game.current_player + 1, game.current_player + 1)
+    lines.append(f"Current player: {cur_name}")
+
+    return "\n".join(lines)
+
+
+def render_ansi_board(game: BlokusGame) -> str:
+    """Render the board with ANSI color codes.
+
+    Returns a string with ANSI escape sequences for colored output.
+    """
+    lines = []
+    size = game.board_size
+
+    # Column headers
+    header = "  " + " ".join(str(i % 10) for i in range(size))
+    lines.append(header)
+
+    for y in range(size):
+        row = f"{y % 10} "
+        for x in range(size):
+            cell = game.board.get_cell(x, y)
+            color = ANSI_COLORS.get(cell, ANSI_RESET)
+            if cell == 0:
+                row += f"{color}. {ANSI_RESET}"
+            else:
+                row += f"{color}{cell} {ANSI_RESET}"
+        lines.append(row)
+
+    # Player info
+    lines.append("")
+    for i, player in enumerate(game.players):
+        remaining = len(player.available_pieces)
+        total = len(game.piece_set.pieces)
+        name = PLAYER_NAMES.get(i + 1, f"Player {i + 1}")
+        color = ANSI_COLORS.get(i + 1, ANSI_RESET)
+        lines.append(
+            f"{color}{name}{ANSI_RESET}: {remaining}/{total} pieces, "
+            f"score: {player.score}"
+        )
+
+    lines.append(
+        f"Current player: "
+        f"{ANSI_COLORS.get(game.current_player + 1, ANSI_RESET)}"
+        f"{PLAYER_NAMES.get(game.current_player + 1, game.current_player + 1)}"
+        f"{ANSI_RESET}"
+    )
+
+    return "\n".join(lines)
+
+
+def print_board(game: BlokusGame, use_ansi: bool = True) -> None:
+    """Print the board to stdout."""
+    if use_ansi:
+        print(render_ansi_board(game))
+    else:
+        print(render_text_board(game))
diff --git a/src/blokus_gym/wrappers/__init__.py b/src/blokus_gym/wrappers/__init__.py
new file mode 100644
index 0000000..27085c7
--- /dev/null
+++ b/src/blokus_gym/wrappers/__init__.py
@@ -0,0 +1,3 @@
+from blokus_gym.wrappers.action_mask import ActionMaskWrapper
+
+__all__ = ["ActionMaskWrapper"]
diff --git a/src/blokus_gym/wrappers/__pycache__/__init__.cpython-312.pyc b/src/blokus_gym/wrappers/__pycache__/__init__.cpython-312.pyc
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diff --git a/src/blokus_gym/wrappers/action_mask.py b/src/blokus_gym/wrappers/action_mask.py
new file mode 100644
index 0000000..c7b2b8e
--- /dev/null
+++ b/src/blokus_gym/wrappers/action_mask.py
@@ -0,0 +1,48 @@
+from __future__ import annotations
+
+import numpy as np
+from gymnasium import spaces
+from gymnasium.core import Env, Wrapper
+
+
+class ActionMaskWrapper(Wrapper):
+    """Wrapper that moves the action mask from info into the observation space.
+
+    This is useful for RL frameworks that expect the action mask to be part
+    of the observation (e.g., Stable-Baselines3 with custom policies,
+    RLlib, etc.).
+
+    After wrapping, the observation becomes a Dict with:
+        - "observation": the original observation
+        - "action_mask": boolean array of valid actions
+
+    Example:
+        >>> from blokus_gym import BlokusEnv
+        >>> from blokus_gym.wrappers import ActionMaskWrapper
+        >>> env = BlokusEnv()
+        >>> env = ActionMaskWrapper(env)
+        >>> obs, info = env.reset()
+        >>> obs["action_mask"]  # Boolean mask of valid actions
+    """
+
+    def __init__(self, env: Env):
+        super().__init__(env)
+
+        # Build new observation space
+        original_obs_space = env.observation_space
+        action_dim = env.action_space.n
+
+        self.observation_space = spaces.Dict({
+            "observation": original_obs_space,
+            "action_mask": spaces.Box(0, 1, (action_dim,), dtype=bool),
+        })
+
+    def reset(self, *, seed=None, options=None):
+        obs, info = self.env.reset(seed=seed, options=options)
+        action_mask = info.get("action_mask", np.zeros(self.env.action_space.n, dtype=bool))
+        return {"observation": obs, "action_mask": action_mask}, info
+
+    def step(self, action):
+        obs, reward, terminated, truncated, info = self.env.step(action)
+        action_mask = info.get("action_mask", np.zeros(self.env.action_space.n, dtype=bool))
+        return {"observation": obs, "action_mask": action_mask}, reward, terminated, truncated, info
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diff --git a/tests/test_board.py b/tests/test_board.py
new file mode 100644
index 0000000..3250fc8
--- /dev/null
+++ b/tests/test_board.py
@@ -0,0 +1,97 @@
+from __future__ import annotations
+
+import numpy as np
+
+from blokus_gym.core.board import Board
+
+
+class TestBoard:
+    def test_board_creation(self):
+        board = Board(20)
+        assert board.size == 20
+        assert board.grid.shape == (20, 20)
+        assert np.all(board.grid == 0)
+
+    def test_in_bounds(self):
+        board = Board(20)
+        assert board.in_bounds(0, 0)
+        assert board.in_bounds(19, 19)
+        assert not board.in_bounds(-1, 0)
+        assert not board.in_bounds(20, 0)
+        assert not board.in_bounds(0, 20)
+
+    def test_place_and_get(self):
+        board = Board(20)
+        cells = [(0, 0), (1, 0), (2, 0)]
+        board.place(1, cells)
+        assert board.get_cell(0, 0) == 1
+        assert board.get_cell(1, 0) == 1
+        assert board.get_cell(2, 0) == 1
+        assert board.get_cell(3, 0) == 0
+
+    def test_has_overlap(self):
+        board = Board(20)
+        board.place(1, [(0, 0), (1, 0)])
+        assert board.has_overlap([(0, 0), (1, 0)])  # overlap
+        assert not board.has_overlap([(2, 0), (3, 0)])  # no overlap
+
+    def test_has_overlap_out_of_bounds(self):
+        board = Board(20)
+        assert board.has_overlap([(20, 0)])  # out of bounds
+
+    def test_clear(self):
+        board = Board(20)
+        board.place(1, [(0, 0)])
+        board.clear()
+        assert np.all(board.grid == 0)
+
+    def test_get_player_squares(self):
+        board = Board(20)
+        board.place(1, [(0, 0), (1, 0)])
+        squares = board.get_player_squares(1)
+        assert len(squares) == 2
+        assert (0, 0) in squares
+        assert (1, 0) in squares
+
+    def test_get_player_corners(self):
+        board = Board(20)
+        board.place(1, [(0, 0)])
+        corners = board.get_player_corners(1)
+        assert (1, 1) in corners  # diagonal
+        assert (0, 0) not in corners  # occupied
+
+    def test_is_full(self):
+        board = Board(2)
+        assert not board.is_full()
+        board.place(1, [(0, 0), (0, 1), (1, 0), (1, 1)])
+        assert board.is_full()
+
+    def test_copy(self):
+        board = Board(20)
+        board.place(1, [(0, 0)])
+        board_copy = board.copy()
+        assert board_copy.get_cell(0, 0) == 1
+        board_copy.place(2, [(1, 0)])
+        assert board.get_cell(1, 0) == 0  # original unchanged
+
+    def test_is_empty(self):
+        board = Board(20)
+        assert board.is_empty(0, 0)
+        board.place(1, [(0, 0)])
+        assert not board.is_empty(0, 0)
+
+    def test_coverage(self):
+        board = Board(2)
+        assert board.coverage() == 0.0
+        board.place(1, [(0, 0)])
+        assert board.coverage() == 0.25
+
+    def test_get_occupied(self):
+        board = Board(20)
+        board.place(1, [(0, 0), (1, 0)])
+        board.place(2, [(5, 5)])
+        occupied = board.get_occupied()
+        assert (0, 0) in occupied
+        assert (1, 0) in occupied
+        assert (5, 5) in occupied
+        assert (2, 0) not in occupied
diff --git a/tests/test_envs.py b/tests/test_envs.py
new file mode 100644
index 0000000..c4792b5
--- /dev/null
+++ b/tests/test_envs.py
@@ -0,0 +1,197 @@
+from __future__ import annotations
+
+import gymnasium as gym
+import numpy as np
+
+from blokus_gym import (
+    STANDARD_PIECES,
+    ActionMaskWrapper,
+    BlokusEnv,
+    BlokusMultiAgentEnv,
+    GreedyBot,
+)
+
+
+class TestBlokusEnv:
+    def test_env_creation(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        assert env.num_players == 4
+        assert env.board_size == 20
+
+    def test_observation_space(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        obs, _ = env.reset(seed=42)
+        assert "board" in obs
+        assert "pieces" in obs
+        assert "corners" in obs
+        assert obs["board"].shape == (20, 20)
+        assert obs["pieces"].shape == (21,)
+        assert obs["corners"].shape == (20, 20)
+
+    def test_action_space(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        assert env.action_space.shape == ()
+        assert env.action_space.n > 0
+
+    def test_reset_returns_obs_and_info(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        obs, info = env.reset(seed=42)
+        assert isinstance(obs, dict)
+        assert isinstance(info, dict)
+        assert "action_mask" in info
+
+    def test_action_mask_valid(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        obs, info = env.reset(seed=42)
+        mask = info["action_mask"]
+        assert mask.dtype == bool
+        assert mask.sum() > 0  # Should have valid moves
+
+    def test_step_with_valid_action(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        obs, info = env.reset(seed=42)
+        valid_actions = np.where(info["action_mask"])[0]
+        action = valid_actions[0]
+        obs, reward, terminated, truncated, info = env.step(action)
+        assert isinstance(reward, float)
+        assert isinstance(terminated, bool)
+        assert isinstance(truncated, bool)
+
+    def test_step_with_invalid_action(self):
+        env = BlokusEnv(num_players=4, board_size=20)
+        obs, info = env.reset(seed=42)
+        # Use an action that's likely invalid
+        obs, reward, terminated, truncated, info = env.step(env.action_space.n - 1)
+        assert reward < 0  # Penalty for invalid action
+        assert terminated  # Should end episode
+
+    def test_two_player_env(self):
+        env = BlokusEnv(num_players=2, board_size=14)
+        obs, info = env.reset(seed=42)
+        assert env.num_players == 2
+
+    def test_three_player_env(self):
+        env = BlokusEnv(num_players=3, board_size=20)
+        obs, info = env.reset(seed=42)
+        assert env.num_players == 3
+
+    def test_custom_pieces(self):
+        custom = [p for p in STANDARD_PIECES if p.size < 5]
+        env = BlokusEnv(num_players=2, board_size=10, pieces=custom)
+        obs, info = env.reset(seed=42)
+        assert obs["pieces"].shape == (len(custom),)
+
+    def test_greedy_bot_opponent(self):
+        env = BlokusEnv(num_players=2, board_size=7, bot_type=GreedyBot)
+        obs, info = env.reset(seed=42)
+        assert env.bots[1] is not None
+
+    def test_game_over(self):
+        env = BlokusEnv(num_players=2, board_size=5, max_steps=10)
+        obs, info = env.reset(seed=42)
+        terminated = False
+        truncated = False
+        steps = 0
+        while not (terminated or truncated) and steps < 50:
+            valid = np.where(info["action_mask"])[0]
+            if len(valid) == 0:
+                break
+            obs, reward, terminated, truncated, info = env.step(valid[0])
+            steps += 1
+
+    def test_seed_reproducibility(self):
+        env1 = BlokusEnv(num_players=2, board_size=7)
+        env2 = BlokusEnv(num_players=2, board_size=7)
+        obs1, _ = env1.reset(seed=42)
+        obs2, _ = env2.reset(seed=42)
+        np.testing.assert_array_equal(obs1["board"], obs2["board"])
+
+    def test_render_text(self):
+        env = BlokusEnv(num_players=2, board_size=7, render_mode="ansi")
+        obs, info = env.reset(seed=42)
+        result = env.render()
+        assert isinstance(result, str)
+
+
+class TestActionMaskWrapper:
+    def test_wrapper_creation(self):
+        env = BlokusEnv(num_players=2, board_size=7)
+        env = ActionMaskWrapper(env)
+        obs, info = env.reset(seed=42)
+        assert "observation" in obs
+        assert "action_mask" in obs
+
+    def test_wrapper_step(self):
+        env = BlokusEnv(num_players=2, board_size=7)
+        env = ActionMaskWrapper(env)
+        obs, info = env.reset(seed=42)
+        valid = np.where(obs["action_mask"])[0]
+        obs, reward, terminated, truncated, info = env.step(valid[0])
+        assert "observation" in obs
+        assert "action_mask" in obs
+
+
+class TestMultiAgentEnv:
+    def test_env_creation(self):
+        env = BlokusMultiAgentEnv(num_players=2, board_size=7)
+        assert env.num_players == 2
+        assert len(env.agents) == 2
+
+    def test_reset(self):
+        env = BlokusMultiAgentEnv(num_players=2, board_size=7)
+        obs, info = env.reset(seed=42)
+        assert "player_0" in obs
+
+    def test_step(self):
+        env = BlokusMultiAgentEnv(num_players=2, board_size=7)
+        obs, info = env.reset(seed=42)
+        mask = env.get_action_mask("player_0")
+        valid = np.where(mask)[0]
+        obs, rewards, terminations, truncations, infos = env.step(valid[0])
+        assert isinstance(rewards, dict)
+        assert isinstance(terminations, dict)
+
+    def test_observation_space(self):
+        env = BlokusMultiAgentEnv(num_players=2, board_size=7)
+        obs, _ = env.reset(seed=42)
+        for agent in env.agents:
+            if agent in obs:
+                assert "board" in obs[agent]
+
+    def test_action_space(self):
+        env = BlokusMultiAgentEnv(num_players=2, board_size=7)
+        env.reset(seed=42)
+        space = env.action_space("player_0")
+        assert space.n > 0
+
+    def test_four_player(self):
+        env = BlokusMultiAgentEnv(num_players=4, board_size=7)
+        obs, info = env.reset(seed=42)
+        assert len(env.agents) == 4
+
+
+class TestRegisteredEnvs:
+    def test_blokus_v0(self):
+        env = gym.make("Blokus-v0")
+        obs, info = env.reset(seed=42)
+        assert obs["board"].shape == (20, 20)
+
+    def test_blokus_duo_v0(self):
+        env = gym.make("BlokusDuo-v0")
+        obs, info = env.reset(seed=42)
+        assert obs["board"].shape == (14, 14)
+
+    def test_blokus_junior_v0(self):
+        env = gym.make("BlokusJunior-v0")
+        obs, info = env.reset(seed=42)
+        assert obs["board"].shape == (14, 14)
+
+    def test_blokus_simple_v0(self):
+        env = gym.make("BlokusSimple-v0")
+        obs, info = env.reset(seed=42)
+        assert obs["board"].shape == (7, 7)
+
+    def test_blokus_greedy_v0(self):
+        env = gym.make("BlokusGreedy-v0")
+        obs, info = env.reset(seed=42)
+        assert obs["board"].shape == (20, 20)
diff --git a/tests/test_game.py b/tests/test_game.py
new file mode 100644
index 0000000..19e27f2
--- /dev/null
+++ b/tests/test_game.py
@@ -0,0 +1,196 @@
+from __future__ import annotations
+
+import numpy as np
+
+from blokus_gym.core.bots import GreedyBot, GreedyCornersBot, RandomBot
+from blokus_gym.core.game import BlokusGame
+
+
+class TestBlokusGame:
+    def test_game_creation(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        assert game.board_size == 20
+        assert game.num_players == 4
+        assert game.current_player == 0
+        assert len(game.players) == 4
+
+    def test_reset(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        assert game.current_player == 0
+        for player in game.players:
+            assert len(player.available_pieces) == 21
+
+    def test_action_space_size(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        assert game.num_actions > 0
+
+    def test_valid_actions_initial(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        valid = game.get_valid_actions(0)
+        assert np.any(valid)
+
+    def test_play_first_move_corner(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        valid = game.get_valid_actions(0)
+        valid_indices = np.where(valid)[0]
+        action = valid_indices[0]
+        assert game.play_move(0, action)
+        assert game.board.grid.sum() > 0
+
+    def test_invalid_action(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        # Use an action that's likely invalid (very high index)
+        assert not game.play_move(0, game.num_actions - 1)
+
+    def test_next_player(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        assert game.current_player == 0
+        game.next_player()
+        assert game.current_player == 1
+        game.next_player()
+        assert game.current_player == 2
+        game.next_player()
+        assert game.current_player == 3
+        game.next_player()
+        assert game.current_player == 0
+
+    def test_has_valid_moves(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        assert game.has_valid_moves(0)
+
+    def test_is_game_over(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        assert not game.is_game_over()
+
+    def test_get_scores(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        scores = game.get_scores()
+        assert len(scores) == 4
+        for score in scores:
+            assert isinstance(score, (int, float))
+
+    def test_get_action_mask(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        mask = game.get_action_mask(0)
+        assert mask.shape == (game.num_actions,)
+        assert mask.dtype == bool
+
+    def test_get_current_observation(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        obs = game.get_current_observation()
+        assert "board" in obs
+        assert "pieces" in obs
+        assert "corners" in obs
+        assert obs["board"].shape == (20, 20)
+        assert obs["pieces"].shape == (21,)
+        assert obs["corners"].shape == (20, 20)
+
+    def test_two_player_game(self):
+        game = BlokusGame(board_size=14, num_players=2)
+        game.reset()
+        assert game.num_players == 2
+        assert game.has_valid_moves(0)
+
+    def test_three_player_game(self):
+        game = BlokusGame(board_size=20, num_players=3)
+        game.reset()
+        assert game.num_players == 3
+
+    def test_copy(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        game.play_move(0, 0)
+        game_copy = game.copy()
+        assert game_copy.current_player == game.current_player
+
+    def test_play_multiple_moves(self):
+        game = BlokusGame(board_size=20, num_players=2)
+        game.reset()
+        # Play a few moves
+        for _ in range(5):
+            valid = game.get_valid_actions(game.current_player)
+            valid_indices = np.where(valid)[0]
+            if len(valid_indices) > 0:
+                game.play_move(game.current_player, valid_indices[0])
+                game.next_player()
+            else:
+                break
+
+    def test_get_winners_not_over(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        assert game.get_winners() is None
+
+    def test_valid_move_first_corner_only(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        from blokus_gym.core.pieces import Move
+        # First move must be in corner (0, 0) for player 0
+        move = Move(piece_id=0, orientation_id=0, x=0, y=0)
+        assert game.valid_move(0, move)
+        # Try a non-corner first move
+        move2 = Move(piece_id=0, orientation_id=0, x=5, y=5)
+        assert not game.valid_move(0, move2)
+
+
+class TestBots:
+    def test_random_bot(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        bot = RandomBot(player_idx=0, seed=42)
+        valid = game.get_valid_actions(0)
+        action = bot.select_action(game, valid)
+        assert action is not None
+        assert valid[action]
+
+    def test_greedy_bot(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        bot = GreedyBot(player_idx=0)
+        valid = game.get_valid_actions(0)
+        action = bot.select_action(game, valid)
+        assert action is not None
+        assert valid[action]
+
+    def test_greedy_corners_bot(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        bot = GreedyCornersBot(player_idx=0)
+        valid = game.get_valid_actions(0)
+        action = bot.select_action(game, valid)
+        assert action is not None
+        assert valid[action]
+
+    def test_bot_no_valid_moves(self):
+        game = BlokusGame(board_size=20, num_players=4)
+        game.reset()
+        bot = RandomBot(player_idx=0, seed=42)
+        valid = np.zeros(game.num_actions, dtype=bool)
+        action = bot.select_action(game, valid)
+        assert action is None
+
+    def test_bots_play_game(self):
+        game = BlokusGame(board_size=7, num_players=2)
+        game.reset()
+        bots = [RandomBot(player_idx=i, seed=i) for i in range(2)]
+        steps = 0
+        while not game.is_game_over() and steps < 100:
+            bot = bots[game.current_player]
+            valid = game.get_valid_actions(game.current_player)
+            action = bot.select_action(game, valid)
+            if action is not None:
+                game.play_move(game.current_player, action)
+            game.next_player()
+            steps += 1
+        assert game.is_game_over()
diff --git a/tests/test_imports.py b/tests/test_imports.py
new file mode 100644
index 0000000..e74bf56
--- /dev/null
+++ b/tests/test_imports.py
@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+
+def test_package_import():
+    import blokus_gym
+    assert blokus_gym.__version__ == "0.1.0"
+
+
+def test_core_imports():
+    from blokus_gym import BlokusGame, Board, Move
+    assert Board is not None
+    assert BlokusGame is not None
+    assert Move is not None
+
+
+def test_env_imports():
+    from blokus_gym import BlokusEnv, BlokusMultiAgentEnv
+    assert BlokusEnv is not None
+    assert BlokusMultiAgentEnv is not None
+
+
+def test_wrapper_imports():
+    from blokus_gym import ActionMaskWrapper
+    assert ActionMaskWrapper is not None
+
+
+def test_piece_imports():
+    from blokus_gym import (
+        DUO_PIECES,
+        JUNIOR_PIECES,
+        STANDARD_PIECES,
+        Piece,
+        PieceSet,
+    )
+    assert len(STANDARD_PIECES) == 21
+    assert len(DUO_PIECES) > 0
+    assert len(JUNIOR_PIECES) > 0
+    assert Piece is not None
+    assert PieceSet is not None
+
+
+def test_bot_imports():
+    from blokus_gym import GreedyBot, GreedyCornersBot, MinimaxBot, RandomBot
+    assert RandomBot is not None
+    assert GreedyBot is not None
+    assert GreedyCornersBot is not None
+    assert MinimaxBot is not None
+
+
+def test_all_exports():
+    import blokus_gym
+    expected = [
+        "BlokusEnv",
+        "BlokusMultiAgentEnv",
+        "ActionMaskWrapper",
+        "Board",
+        "BlokusGame",
+        "Move",
+        "Piece",
+        "PieceOrientation",
+        "PieceSet",
+        "STANDARD_PIECES",
+        "DUO_PIECES",
+        "JUNIOR_PIECES",
+        "generate_orientations",
+        "Bot",
+        "RandomBot",
+        "GreedyBot",
+        "GreedyCornersBot",
+        "MinimaxBot",
+        "__version__",
+    ]
+    for name in expected:
+        assert hasattr(blokus_gym, name), f"Missing export: {name}"
diff --git a/tests/test_pieces.py b/tests/test_pieces.py
new file mode 100644
index 0000000..b142677
--- /dev/null
+++ b/tests/test_pieces.py
@@ -0,0 +1,85 @@
+from __future__ import annotations
+
+from blokus_gym.core.pieces import (
+    STANDARD_PIECES,
+    Piece,
+    PieceOrientation,
+    PieceSet,
+    generate_orientations,
+)
+
+
+class TestPiece:
+    def test_piece_creation(self):
+        piece = Piece(name="F", squares=frozenset([(0, 0), (1, 0), (1, 1), (2, 1)]))
+        assert piece.name == "F"
+        assert piece.size == 4
+        assert len(piece.squares) == 4
+
+    def test_piece_size(self):
+        piece = Piece(name="I4", squares=frozenset([(0, 0), (1, 0), (2, 0), (3, 0)]))
+        assert piece.size == 4
+
+    def test_standard_pieces_count(self):
+        assert len(STANDARD_PIECES) == 21
+
+    def test_standard_pieces_total_squares(self):
+        total = sum(p.size for p in STANDARD_PIECES)
+        assert total == 89
+
+    def test_piece_orientations_via_pieceset(self):
+        piece_set = PieceSet(STANDARD_PIECES)
+        orientations = piece_set.get_orientations(0)
+        assert len(orientations) > 0
+        for orient in orientations:
+            assert isinstance(orient, PieceOrientation)
+
+
+class TestPieceSet:
+    def test_piece_set_creation(self):
+        piece_set = PieceSet(STANDARD_PIECES)
+        assert piece_set.num_pieces == 21
+        assert len(piece_set.pieces) == 21
+
+    def test_piece_set_custom(self):
+        custom = [p for p in STANDARD_PIECES if p.size <= 3]
+        piece_set = PieceSet(custom)
+        assert piece_set.num_pieces == len(custom)
+
+    def test_piece_set_get_piece_id(self):
+        piece_set = PieceSet(STANDARD_PIECES)
+        piece_id = piece_set.get_piece_id("F5")
+        assert piece_set.get_piece(piece_id).name == "F5"
+
+    def test_piece_set_get_orientations(self):
+        piece_set = PieceSet(STANDARD_PIECES)
+        for i in range(piece_set.num_pieces):
+            orientations = piece_set.get_orientations(i)
+            assert len(orientations) > 0
+
+
+class TestGenerateOrientations:
+    def test_square_piece_one_orientation(self):
+        piece = Piece("O4", frozenset([(0, 0), (0, 1), (1, 0), (1, 1)]))
+        orientations = generate_orientations(piece)
+        assert len(orientations) == 1
+
+    def test_line_piece_two_orientations(self):
+        piece = Piece("I3", frozenset([(0, 0), (1, 0), (2, 0)]))
+        orientations = generate_orientations(piece)
+        assert len(orientations) == 2
+
+    def test_L_piece_four_orientations(self):
+        piece = Piece("L3", frozenset([(0, 0), (0, 1), (1, 0)]))
+        orientations = generate_orientations(piece)
+        assert len(orientations) == 4
+
+    def test_all_orientations_unique(self):
+        piece_set = PieceSet(STANDARD_PIECES)
+        for piece_id in range(piece_set.num_pieces):
+            orientations = piece_set.get_orientations(piece_id)
+            unique_cells = set()
+            for orient in orientations:
+                key = tuple(sorted(orient.squares))
+                assert key not in unique_cells, f"Duplicate orientation in piece {piece_id}"
+                unique_cells.add(key)