diff --git a/backend/CLAUDE.md b/backend/CLAUDE.md
index e2fb625..2ce79f2 100644
--- a/backend/CLAUDE.md
+++ b/backend/CLAUDE.md
@@ -78,11 +78,11 @@ def validate_deck(cards, config: DeckConfig | None = None):
 
 ## Dependency Injection for Services
 
-**Services use constructor-based dependency injection for repositories and other services.**
+**Services use constructor-based dependency injection for repositories, services, and external resources.**
 
-Services still use DI for data access, but config comes from method parameters (request).
+Services use DI for data access and external dependencies, but config comes from method parameters (request).
 
-### Required Pattern
+### Required Pattern: Services and Repositories
 
 ```python
 class DeckService:
@@ -109,12 +109,78 @@ class DeckService:
         ...
 ```
 
+### Required Pattern: Factory Injection for External Resources
+
+For external resources (Redis, databases, engines), inject factory functions rather than instances:
+
+```python
+from collections.abc import AsyncIterator, Callable
+
+# Type aliases for factories
+RedisFactory = Callable[[], AsyncIterator["Redis"]]
+EngineFactory = Callable[[GameState], GameEngine]
+TokenVerifier = Callable[[str], UUID | None]
+
+class ConnectionManager:
+    """External resources injected as factories."""
+
+    def __init__(
+        self,
+        conn_ttl_seconds: int = 3600,
+        redis_factory: RedisFactory | None = None,  # Factory, not instance
+    ) -> None:
+        self.conn_ttl_seconds = conn_ttl_seconds
+        # Fall back to global only if not provided
+        self._get_redis = redis_factory if redis_factory is not None else get_redis
+
+    async def register_connection(self, sid: str, user_id: UUID) -> None:
+        async with self._get_redis() as redis:  # Use injected factory
+            await redis.hset(...)
+```
+
+```python
+class GameService:
+    """Engine created via injected factory for testability."""
+
+    def __init__(
+        self,
+        state_manager: GameStateManager | None = None,
+        engine_factory: EngineFactory | None = None,  # Factory for engine creation
+    ) -> None:
+        self._state_manager = state_manager or game_state_manager
+        self._engine_factory = engine_factory or self._default_engine_factory
+
+    async def execute_action(self, game_id: str, player_id: str, action: Action):
+        state = await self.get_game_state(game_id)
+        engine = self._engine_factory(state)  # Create via factory
+        result = await engine.execute_action(state, player_id, action)
+```
+
+### Global Singletons with DI Support
+
+Create global instances for production, but always support injection for testing:
+
+```python
+# Global singleton for production use
+connection_manager = ConnectionManager()
+
+# Tests inject mocks via constructor - no patching needed
+def test_register_connection(mock_redis):
+    @asynccontextmanager
+    async def mock_redis_factory():
+        yield mock_redis
+
+    manager = ConnectionManager(redis_factory=mock_redis_factory)
+    # Test with injected mock...
+```
+
 ### Why This Matters
 
 1. **Testability**: Dependencies can be mocked without patching globals
 2. **Offline Fork**: Services can be swapped for local implementations
 3. **Explicit Dependencies**: Constructor shows all requirements
 4. **Stateless Operations**: Config comes from request, not server state
+5. **No Monkey Patching**: Tests use constructor injection, not `patch()`
 
 ---
 
@@ -253,6 +319,55 @@ def test_paralyzed_pokemon_cannot_attack():
 | `tests/core/` | Core engine tests | No |
 | `tests/api/` | API endpoint tests | Yes |
 
+### No Monkey Patching - Use Dependency Injection
+
+**Never use `patch()` or `monkeypatch` for unit tests.** Instead, inject mocks via constructor.
+
+```python
+# WRONG - Monkey patching
+async def test_execute_action():
+    with patch.object(game_service, "_create_engine") as mock:
+        mock.return_value = mock_engine
+        result = await game_service.execute_action(...)
+
+# CORRECT - Dependency injection
+@pytest.fixture
+def mock_engine():
+    engine = MagicMock()
+    engine.execute_action = AsyncMock(return_value=ActionResult(success=True))
+    return engine
+
+@pytest.fixture
+def game_service(mock_state_manager, mock_engine):
+    return GameService(
+        state_manager=mock_state_manager,
+        engine_factory=lambda state: mock_engine,  # Inject via constructor
+    )
+
+async def test_execute_action(game_service, mock_engine):
+    result = await game_service.execute_action(...)
+    mock_engine.execute_action.assert_called_once()
+```
+
+### Factory Fixtures for External Resources
+
+```python
+@pytest.fixture
+def mock_redis():
+    redis = AsyncMock()
+    redis.hset = AsyncMock()
+    redis.hget = AsyncMock(return_value=None)
+    return redis
+
+@pytest.fixture
+def connection_manager(mock_redis):
+    @asynccontextmanager
+    async def mock_redis_factory():
+        yield mock_redis
+
+    return ConnectionManager(redis_factory=mock_redis_factory)
+```
+
 ### Use Seeded RNG for Determinism
 
 ```python
@@ -276,6 +391,12 @@ app/
 │   ├── config.py   # RulesConfig and sub-configs
 │   └── engine.py   # GameEngine orchestrator
 ├── services/       # Business logic (uses repositories)
+│   ├── game_service.py       # Game orchestration (uses engine_factory DI)
+│   ├── connection_manager.py # WebSocket tracking (uses redis_factory DI)
+│   └── ...
+├── socketio/       # WebSocket real-time communication
+│   ├── server.py   # Socket.IO server setup and handlers
+│   └── auth.py     # AuthHandler class (uses token_verifier DI)
 ├── repositories/   # Data access layer
 │   ├── protocols.py    # Repository protocols (interfaces)
 │   └── postgres/       # PostgreSQL implementations
@@ -322,6 +443,9 @@ app/
 | Hardcoded magic numbers | Use values from config parameter |
 | Tests without docstrings | Always explain what and why |
 | Unit tests in `tests/services/` | Use `tests/unit/` for no-DB tests |
+| Using `patch()` in unit tests | Inject mocks via constructor DI |
+| `async with get_redis()` in method body | Inject `redis_factory` via constructor |
+| Importing dependencies inside functions | Import at module level, inject via constructor |
 
 ---
 
@@ -330,3 +454,5 @@ app/
 - `app/core/AGENTS.md` - Detailed core engine guidelines
 - `app/core/README.md` - Core module documentation
 - `app/core/effects/README.md` - Effect system documentation
+- `app/services/README.md` - Services layer patterns
+- `app/socketio/README.md` - WebSocket module documentation
diff --git a/backend/app/services/README.md b/backend/app/services/README.md
index 9bd0fa2..6512a56 100644
--- a/backend/app/services/README.md
+++ b/backend/app/services/README.md
@@ -19,6 +19,8 @@ Business logic layer between API endpoints and data access.
 | `DeckValidator` | Pure deck validation functions | CardService (for lookups) |
 | `UserService` | User profile management | Direct DB access |
 | `GameStateManager` | Game state (Redis + Postgres) | Redis, AsyncSession |
+| `GameService` | Game orchestration + actions | GameStateManager, CardService, engine_factory |
+| `ConnectionManager` | WebSocket connection tracking | redis_factory |
 | `JWTService` | Token creation/verification | Settings |
 | `TokenStore` | Refresh token storage | Redis |
 
@@ -93,6 +95,50 @@ Benefits:
 - **Offline Fork**: Swap PostgresRepository for LocalRepository
 - **Decoupling**: Services don't know about SQLAlchemy
 
+### Factory Injection for External Resources
+
+For external resources (Redis, GameEngine), inject factory functions rather than instances:
+
+```python
+from collections.abc import AsyncIterator, Callable
+
+# Type aliases for factories
+RedisFactory = Callable[[], AsyncIterator["Redis"]]
+EngineFactory = Callable[[GameState], GameEngine]
+
+class ConnectionManager:
+    """External resources injected as factories."""
+
+    def __init__(
+        self,
+        conn_ttl_seconds: int = 3600,
+        redis_factory: RedisFactory | None = None,
+    ) -> None:
+        self.conn_ttl_seconds = conn_ttl_seconds
+        self._get_redis = redis_factory if redis_factory is not None else get_redis
+
+class GameService:
+    """Engine created via injected factory for testability."""
+
+    def __init__(
+        self,
+        state_manager: GameStateManager | None = None,
+        engine_factory: EngineFactory | None = None,
+    ) -> None:
+        self._state_manager = state_manager or game_state_manager
+        self._engine_factory = engine_factory or self._default_engine_factory
+```
+
+Global singletons are created for production, but constructors always support injection:
+
+```python
+# Production singleton
+connection_manager = ConnectionManager()
+
+# Tests inject mocks - no patching needed
+manager = ConnectionManager(redis_factory=mock_redis_factory)
+```
+
 ### Pure Validation Functions
 
 Validation logic is extracted into pure functions for reuse:
@@ -183,6 +229,27 @@ await manager.persist_to_db(game_id, game_state)
 | `tests/unit/services/` | Pure unit tests, mocked deps | No |
 | `tests/services/` | Integration tests | Yes (testcontainers) |
 
+**No monkey patching** - Always use constructor DI for mocks:
+
+```python
+# WRONG - monkey patching
+async def test_something():
+    with patch.object(service, "_method") as mock:
+        ...
+
+# CORRECT - constructor injection
+@pytest.fixture
+def mock_redis():
+    return AsyncMock()
+
+@pytest.fixture
+def connection_manager(mock_redis):
+    @asynccontextmanager
+    async def mock_redis_factory():
+        yield mock_redis
+    return ConnectionManager(redis_factory=mock_redis_factory)
+```
+
 ### Unit Test Example
 
 ```python
diff --git a/backend/app/services/connection_manager.py b/backend/app/services/connection_manager.py
index 185d3a8..e8823f2 100644
--- a/backend/app/services/connection_manager.py
+++ b/backend/app/services/connection_manager.py
@@ -35,14 +35,22 @@ Example:
 """
 
 import logging
+from collections.abc import AsyncIterator, Callable
 from dataclasses import dataclass
 from datetime import UTC, datetime
+from typing import TYPE_CHECKING
 from uuid import UUID
 
 from app.db.redis import get_redis
 
+if TYPE_CHECKING:
+    from redis.asyncio import Redis
+
 logger = logging.getLogger(__name__)
 
+# Type alias for redis factory - a callable that returns an async context manager
+RedisFactory = Callable[[], AsyncIterator["Redis"]]
+
 # Redis key patterns
 CONN_PREFIX = "conn:"
 USER_CONN_PREFIX = "user_conn:"
@@ -98,13 +106,20 @@ class ConnectionManager:
         conn_ttl_seconds: TTL for connection records in Redis.
     """
 
-    def __init__(self, conn_ttl_seconds: int = DEFAULT_CONN_TTL_SECONDS) -> None:
+    def __init__(
+        self,
+        conn_ttl_seconds: int = DEFAULT_CONN_TTL_SECONDS,
+        redis_factory: RedisFactory | None = None,
+    ) -> None:
         """Initialize the ConnectionManager.
 
         Args:
             conn_ttl_seconds: How long to keep connection records in Redis.
+            redis_factory: Optional factory for Redis connections. If not provided,
+                uses the default get_redis from app.db.redis. Useful for testing.
         """
         self.conn_ttl_seconds = conn_ttl_seconds
+        self._get_redis = redis_factory if redis_factory is not None else get_redis
 
     def _conn_key(self, sid: str) -> str:
         """Generate Redis key for a connection."""
@@ -142,7 +157,7 @@ class ConnectionManager:
         user_id_str = str(user_id)
         now = datetime.now(UTC).isoformat()
 
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             # Check for existing connection and clean it up
             old_sid = await redis.get(self._user_conn_key(user_id_str))
             if old_sid and old_sid != sid:
@@ -204,7 +219,7 @@ class ConnectionManager:
         Args:
             sid: Socket.IO session ID.
         """
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             conn_key = self._conn_key(sid)
 
             # Get connection data for cleanup
@@ -250,7 +265,7 @@ class ConnectionManager:
         Example:
             await manager.join_game("abc123", "game-456")
         """
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             conn_key = self._conn_key(sid)
 
             # Check connection exists
@@ -291,7 +306,7 @@ class ConnectionManager:
         Example:
             game_id = await manager.leave_game("abc123")
         """
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             conn_key = self._conn_key(sid)
 
             # Get current game
@@ -330,7 +345,7 @@ class ConnectionManager:
         """
         now = datetime.now(UTC).isoformat()
 
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             conn_key = self._conn_key(sid)
 
             # Check exists and update atomically
@@ -370,7 +385,7 @@ class ConnectionManager:
             if info:
                 print(f"User {info.user_id} connected at {info.connected_at}")
         """
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             conn_key = self._conn_key(sid)
             data = await redis.hgetall(conn_key)
 
@@ -401,7 +416,7 @@ class ConnectionManager:
         """
         user_id_str = str(user_id)
 
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             user_conn_key = self._user_conn_key(user_id_str)
             sid = await redis.get(user_conn_key)
 
@@ -448,7 +463,7 @@ class ConnectionManager:
             for conn in connections:
                 # Send state update to each participant
         """
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             game_conns_key = self._game_conns_key(game_id)
             sids = await redis.smembers(game_conns_key)
 
@@ -530,7 +545,7 @@ class ConnectionManager:
             logger.info(f"Cleaned up {count} stale connections")
         """
         count = 0
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             # Scan for all connection keys
             async for key in redis.scan_iter(match=f"{CONN_PREFIX}*"):
                 sid = key[len(CONN_PREFIX) :]
@@ -555,7 +570,7 @@ class ConnectionManager:
             Number of active connections.
         """
         count = 0
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             async for _ in redis.scan_iter(match=f"{CONN_PREFIX}*"):
                 count += 1
         return count
@@ -569,7 +584,7 @@ class ConnectionManager:
         Returns:
             Number of connected participants.
         """
-        async with get_redis() as redis:
+        async with self._get_redis() as redis:
             game_conns_key = self._game_conns_key(game_id)
             return await redis.scard(game_conns_key)
 
diff --git a/backend/app/services/game_service.py b/backend/app/services/game_service.py
index 5578ce4..105b8f1 100644
--- a/backend/app/services/game_service.py
+++ b/backend/app/services/game_service.py
@@ -36,6 +36,7 @@ Example:
 """
 
 import logging
+from collections.abc import Callable
 from dataclasses import dataclass, field
 from typing import Any
 from uuid import UUID
@@ -51,6 +52,9 @@ from app.services.game_state_manager import GameStateManager, game_state_manager
 
 logger = logging.getLogger(__name__)
 
+# Type alias for engine factory - takes GameState, returns GameEngine
+EngineFactory = Callable[[GameState], GameEngine]
+
 
 # =============================================================================
 # Exceptions
@@ -180,12 +184,14 @@ class GameService:
     Attributes:
         _state_manager: GameStateManager for persistence.
         _card_service: CardService for card definitions.
+        _engine_factory: Factory function to create GameEngine instances.
     """
 
     def __init__(
         self,
         state_manager: GameStateManager | None = None,
         card_service: CardService | None = None,
+        engine_factory: EngineFactory | None = None,
     ) -> None:
         """Initialize the GameService.
 
@@ -196,12 +202,16 @@ class GameService:
         Args:
             state_manager: GameStateManager instance. Uses global if not provided.
             card_service: CardService instance. Uses global if not provided.
+            engine_factory: Optional factory for creating GameEngine instances.
+                If not provided, uses the default _default_engine_factory method.
+                Useful for testing with mock engines.
         """
         self._state_manager = state_manager or game_state_manager
         self._card_service = card_service or get_card_service()
+        self._engine_factory = engine_factory or self._default_engine_factory
 
-    def _create_engine_for_game(self, game: GameState) -> GameEngine:
-        """Create a GameEngine configured for a specific game's rules.
+    def _default_engine_factory(self, game: GameState) -> GameEngine:
+        """Default factory for creating a GameEngine from game state.
 
         The engine is created on-demand using the rules stored in the
         game state. This ensures each game uses its own configuration.
@@ -412,8 +422,8 @@ class GameService:
         if not isinstance(action, ResignAction) and state.current_player_id != player_id:
             raise NotPlayerTurnError(game_id, player_id, state.current_player_id)
 
-        # Create engine with this game's rules
-        engine = self._create_engine_for_game(state)
+        # Create engine with this game's rules via factory
+        engine = self._engine_factory(state)
 
         # Execute the action
         result: ActionResult = await engine.execute_action(state, player_id, action)
diff --git a/backend/app/socketio/README.md b/backend/app/socketio/README.md
new file mode 100644
index 0000000..1477885
--- /dev/null
+++ b/backend/app/socketio/README.md
@@ -0,0 +1,120 @@
+# Socket.IO Module
+
+Real-time WebSocket communication for active game sessions.
+
+## Architecture
+
+```
+Client <--WebSocket--> Socket.IO Server <--> GameService <--> GameEngine
+                            |
+                            v
+                      ConnectionManager <--> Redis (presence tracking)
+```
+
+## Components
+
+| Component | Responsibility | DI Pattern |
+|-----------|---------------|------------|
+| `server.py` | Socket.IO server setup, event handlers | Uses `auth_handler` |
+| `auth.py` | `AuthHandler` class for JWT authentication | `token_verifier`, `conn_manager` |
+
+## AuthHandler
+
+Handles JWT-based authentication with injectable dependencies:
+
+```python
+class AuthHandler:
+    def __init__(
+        self,
+        token_verifier: TokenVerifier | None = None,  # Callable[[str], UUID | None]
+        conn_manager: ConnectionManager | None = None,
+    ) -> None:
+        self._token_verifier = token_verifier or verify_access_token
+        self._connection_manager = conn_manager or connection_manager
+
+# Global singleton for production
+auth_handler = AuthHandler()
+
+# Tests inject mocks via constructor
+handler = AuthHandler(
+    token_verifier=lambda token: user_id,
+    conn_manager=mock_connection_manager,
+)
+```
+
+### Methods
+
+| Method | Purpose |
+|--------|---------|
+| `authenticate_connection(sid, auth)` | Validate JWT, return `AuthResult` |
+| `setup_authenticated_session(sio, sid, user_id)` | Save session, register connection |
+| `cleanup_authenticated_session(sid)` | Unregister connection, return user_id |
+
+## Event Handlers
+
+All handlers are in the `/game` namespace:
+
+| Event | Direction | Purpose |
+|-------|-----------|---------|
+| `connect` | Client→Server | Authenticate and establish session |
+| `disconnect` | Client→Server | Clean up session and notify opponent |
+| `game:join` | Client→Server | Join/rejoin a game session |
+| `game:action` | Client→Server | Execute a game action |
+| `game:resign` | Client→Server | Resign from game |
+| `game:heartbeat` | Client→Server | Keep connection alive |
+
+## Client Connection
+
+```javascript
+const socket = io("wss://api.example.com", {
+    auth: { token: "JWT_ACCESS_TOKEN" }
+});
+
+socket.on("connect", () => {
+    socket.emit("game:join", { game_id: "uuid" });
+});
+
+socket.on("game:state", (state) => {
+    // Full game state update
+});
+
+socket.on("auth_error", (error) => {
+    // { code: "invalid_token", message: "..." }
+});
+```
+
+## Testing
+
+Use dependency injection - no monkey patching:
+
+```python
+@pytest.fixture
+def mock_token_verifier():
+    return MagicMock(return_value=uuid4())
+
+@pytest.fixture
+def mock_connection_manager():
+    cm = AsyncMock()
+    cm.register_connection = AsyncMock()
+    return cm
+
+@pytest.fixture
+def auth_handler(mock_token_verifier, mock_connection_manager):
+    return AuthHandler(
+        token_verifier=mock_token_verifier,
+        conn_manager=mock_connection_manager,
+    )
+
+async def test_authenticate_success(auth_handler, mock_token_verifier):
+    """Test successful authentication with valid JWT."""
+    result = await auth_handler.authenticate_connection("sid", {"token": "valid"})
+    assert result.success
+    mock_token_verifier.assert_called_once_with("valid")
+```
+
+## See Also
+
+- `app/services/connection_manager.py` - Connection presence tracking
+- `app/services/game_service.py` - Game orchestration
+- `app/schemas/ws_messages.py` - WebSocket message schemas
+- `CLAUDE.md` - Architecture guidelines
diff --git a/backend/app/socketio/__init__.py b/backend/app/socketio/__init__.py
index 1e5469f..aae08b2 100644
--- a/backend/app/socketio/__init__.py
+++ b/backend/app/socketio/__init__.py
@@ -23,12 +23,12 @@ Usage:
 """
 
 from app.socketio.auth import (
+    AuthHandler,
     AuthResult,
-    authenticate_connection,
-    cleanup_authenticated_session,
+    auth_handler,
+    extract_token,
     get_session_user_id,
     require_auth,
-    setup_authenticated_session,
 )
 from app.socketio.server import create_socketio_app, sio
 
@@ -37,10 +37,10 @@ __all__ = [
     "create_socketio_app",
     "sio",
     # Auth
+    "AuthHandler",
     "AuthResult",
-    "authenticate_connection",
-    "cleanup_authenticated_session",
+    "auth_handler",
+    "extract_token",
     "get_session_user_id",
     "require_auth",
-    "setup_authenticated_session",
 ]
diff --git a/backend/app/socketio/auth.py b/backend/app/socketio/auth.py
index 125afec..9dfc797 100644
--- a/backend/app/socketio/auth.py
+++ b/backend/app/socketio/auth.py
@@ -16,7 +16,7 @@ Session Data:
 
 Example:
     # In connect handler:
-    auth_result = await authenticate_connection(sid, auth)
+    auth_result = await auth_handler.authenticate_connection(sid, auth)
     if not auth_result.success:
         return False  # Reject connection
 
@@ -26,15 +26,19 @@ Example:
 """
 
 import logging
+from collections.abc import Callable
 from dataclasses import dataclass
 from datetime import UTC, datetime
 from uuid import UUID
 
-from app.services.connection_manager import connection_manager
+from app.services.connection_manager import ConnectionManager, connection_manager
 from app.services.jwt_service import verify_access_token
 
 logger = logging.getLogger(__name__)
 
+# Type alias for token verifier - takes token string, returns user_id or None
+TokenVerifier = Callable[[str], UUID | None]
+
 
 @dataclass
 class AuthResult:
@@ -53,6 +57,133 @@ class AuthResult:
     error_message: str | None = None
 
 
+class AuthHandler:
+    """Handler for Socket.IO authentication with injectable dependencies.
+
+    This class encapsulates authentication logic with dependencies injected
+    via constructor, making it testable without monkey patching.
+
+    Attributes:
+        _token_verifier: Function to verify JWT tokens.
+        _connection_manager: ConnectionManager for tracking connections.
+    """
+
+    def __init__(
+        self,
+        token_verifier: TokenVerifier | None = None,
+        conn_manager: ConnectionManager | None = None,
+    ) -> None:
+        """Initialize AuthHandler with dependencies.
+
+        Args:
+            token_verifier: Function to verify tokens. Uses jwt_service if not provided.
+            conn_manager: ConnectionManager instance. Uses global if not provided.
+        """
+        self._token_verifier = token_verifier or verify_access_token
+        self._connection_manager = conn_manager or connection_manager
+
+    async def authenticate_connection(
+        self,
+        sid: str,
+        auth: dict[str, object] | None,
+    ) -> AuthResult:
+        """Authenticate a Socket.IO connection using JWT.
+
+        Extracts the JWT from auth data, validates it, and returns the result.
+        Does NOT modify socket session - caller should handle that.
+
+        Args:
+            sid: Socket session ID (for logging).
+            auth: Authentication data from connect event.
+
+        Returns:
+            AuthResult with success status and user_id or error details.
+        """
+        token = extract_token(auth)
+
+        if token is None:
+            logger.debug(f"Connection {sid}: No token provided")
+            return AuthResult(
+                success=False,
+                error_code="missing_token",
+                error_message="Authentication token required",
+            )
+
+        user_id = self._token_verifier(token)
+
+        if user_id is None:
+            logger.debug(f"Connection {sid}: Invalid or expired token")
+            return AuthResult(
+                success=False,
+                error_code="invalid_token",
+                error_message="Invalid or expired token",
+            )
+
+        logger.debug(f"Connection {sid}: Authenticated as user {user_id}")
+        return AuthResult(
+            success=True,
+            user_id=user_id,
+        )
+
+    async def setup_authenticated_session(
+        self,
+        sio: object,
+        sid: str,
+        user_id: UUID,
+        namespace: str = "/game",
+    ) -> None:
+        """Set up socket session with authenticated user data.
+
+        Saves user_id and authentication timestamp to the socket session,
+        and registers the connection with ConnectionManager.
+
+        Args:
+            sio: Socket.IO AsyncServer instance (required).
+            sid: Socket session ID.
+            user_id: Authenticated user's UUID.
+            namespace: Socket.IO namespace.
+        """
+        session_data = {
+            "user_id": str(user_id),
+            "authenticated_at": datetime.now(UTC).isoformat(),
+        }
+        await sio.save_session(sid, session_data, namespace=namespace)
+
+        await self._connection_manager.register_connection(sid, user_id)
+
+        logger.info(f"Session established: sid={sid}, user_id={user_id}")
+
+    async def cleanup_authenticated_session(
+        self,
+        sid: str,
+        namespace: str = "/game",
+    ) -> str | None:
+        """Clean up session data on disconnect.
+
+        Unregisters the connection from ConnectionManager and returns
+        the user_id for any additional cleanup needed.
+
+        Args:
+            sid: Socket session ID.
+            namespace: Socket.IO namespace.
+
+        Returns:
+            user_id if session was authenticated, None otherwise.
+        """
+        conn_info = await self._connection_manager.unregister_connection(sid)
+
+        if conn_info:
+            logger.info(f"Session cleaned up: sid={sid}, user_id={conn_info.user_id}")
+            return conn_info.user_id
+
+        logger.debug(f"No session to clean up for {sid}")
+        return None
+
+
+# Global singleton instance
+auth_handler = AuthHandler()
+
+
 def extract_token(auth: dict[str, object] | None) -> str | None:
     """Extract JWT token from Socket.IO auth data.
 
@@ -96,131 +227,6 @@ def extract_token(auth: dict[str, object] | None) -> str | None:
     return None
 
 
-async def authenticate_connection(
-    sid: str,
-    auth: dict[str, object] | None,
-) -> AuthResult:
-    """Authenticate a Socket.IO connection using JWT.
-
-    Extracts the JWT from auth data, validates it, and returns the result.
-    Does NOT modify socket session - caller should handle that.
-
-    Args:
-        sid: Socket session ID (for logging).
-        auth: Authentication data from connect event.
-
-    Returns:
-        AuthResult with success status and user_id or error details.
-
-    Example:
-        result = await authenticate_connection(sid, auth)
-        if result.success:
-            await sio.save_session(sid, {"user_id": str(result.user_id)})
-        else:
-            logger.warning(f"Auth failed: {result.error_message}")
-            return False  # Reject connection
-    """
-    # Extract token from auth data
-    token = extract_token(auth)
-
-    if token is None:
-        logger.debug(f"Connection {sid}: No token provided")
-        return AuthResult(
-            success=False,
-            error_code="missing_token",
-            error_message="Authentication token required",
-        )
-
-    # Validate the token
-    user_id = verify_access_token(token)
-
-    if user_id is None:
-        logger.debug(f"Connection {sid}: Invalid or expired token")
-        return AuthResult(
-            success=False,
-            error_code="invalid_token",
-            error_message="Invalid or expired token",
-        )
-
-    logger.debug(f"Connection {sid}: Authenticated as user {user_id}")
-    return AuthResult(
-        success=True,
-        user_id=user_id,
-    )
-
-
-async def setup_authenticated_session(
-    sio: object,
-    sid: str,
-    user_id: UUID,
-    namespace: str = "/game",
-) -> None:
-    """Set up socket session with authenticated user data.
-
-    Saves user_id and authentication timestamp to the socket session,
-    and registers the connection with ConnectionManager.
-
-    Args:
-        sio: Socket.IO AsyncServer instance.
-        sid: Socket session ID.
-        user_id: Authenticated user's UUID.
-        namespace: Socket.IO namespace.
-
-    Example:
-        if auth_result.success:
-            await setup_authenticated_session(sio, sid, auth_result.user_id)
-    """
-    # Import here to avoid circular dependency
-    from app.socketio.server import sio as server_sio
-
-    # Use provided sio or fall back to server sio
-    socket_server = sio if sio is not None else server_sio
-
-    # Save to socket session
-    session_data = {
-        "user_id": str(user_id),
-        "authenticated_at": datetime.now(UTC).isoformat(),
-    }
-    await socket_server.save_session(sid, session_data, namespace=namespace)
-
-    # Register with ConnectionManager
-    await connection_manager.register_connection(sid, user_id)
-
-    logger.info(f"Session established: sid={sid}, user_id={user_id}")
-
-
-async def cleanup_authenticated_session(
-    sid: str,
-    namespace: str = "/game",
-) -> str | None:
-    """Clean up session data on disconnect.
-
-    Unregisters the connection from ConnectionManager and returns
-    the user_id for any additional cleanup needed.
-
-    Args:
-        sid: Socket session ID.
-        namespace: Socket.IO namespace.
-
-    Returns:
-        user_id if session was authenticated, None otherwise.
-
-    Example:
-        user_id = await cleanup_authenticated_session(sid)
-        if user_id:
-            # Notify opponent, etc.
-    """
-    # Unregister from ConnectionManager
-    conn_info = await connection_manager.unregister_connection(sid)
-
-    if conn_info:
-        logger.info(f"Session cleaned up: sid={sid}, user_id={conn_info.user_id}")
-        return conn_info.user_id
-
-    logger.debug(f"No session to clean up for {sid}")
-    return None
-
-
 async def get_session_user_id(
     sio: object,
     sid: str,
diff --git a/backend/app/socketio/server.py b/backend/app/socketio/server.py
index 3f4127c..b409e8e 100644
--- a/backend/app/socketio/server.py
+++ b/backend/app/socketio/server.py
@@ -33,12 +33,7 @@ from typing import TYPE_CHECKING
 import socketio
 
 from app.config import settings
-from app.socketio.auth import (
-    authenticate_connection,
-    cleanup_authenticated_session,
-    require_auth,
-    setup_authenticated_session,
-)
+from app.socketio.auth import auth_handler, require_auth
 
 if TYPE_CHECKING:
     from fastapi import FastAPI
@@ -84,7 +79,7 @@ async def connect(
         None is treated as True (accept).
     """
     # Authenticate the connection
-    auth_result = await authenticate_connection(sid, auth)
+    auth_result = await auth_handler.authenticate_connection(sid, auth)
 
     if not auth_result.success:
         logger.warning(
@@ -103,7 +98,7 @@ async def connect(
         return False
 
     # Set up authenticated session and register connection
-    await setup_authenticated_session(sio, sid, auth_result.user_id, namespace="/game")
+    await auth_handler.setup_authenticated_session(sio, sid, auth_result.user_id, namespace="/game")
 
     logger.info(f"Client authenticated to /game: sid={sid}, user_id={auth_result.user_id}")
     return True
@@ -119,7 +114,7 @@ async def disconnect(sid: str) -> None:
         sid: Socket session ID of disconnecting client.
     """
     # Clean up session and get user info
-    user_id = await cleanup_authenticated_session(sid, namespace="/game")
+    user_id = await auth_handler.cleanup_authenticated_session(sid, namespace="/game")
 
     if user_id:
         logger.info(f"Client disconnected from /game: sid={sid}, user_id={user_id}")
diff --git a/backend/tests/socketio/test_auth.py b/backend/tests/socketio/test_auth.py
index 073c1c0..422a4fa 100644
--- a/backend/tests/socketio/test_auth.py
+++ b/backend/tests/socketio/test_auth.py
@@ -4,19 +4,17 @@ This module tests JWT-based authentication for WebSocket connections,
 including token extraction, validation, and session management.
 """
 
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock
 from uuid import uuid4
 
 import pytest
 
 from app.socketio.auth import (
+    AuthHandler,
     AuthResult,
-    authenticate_connection,
-    cleanup_authenticated_session,
     extract_token,
     get_session_user_id,
     require_auth,
-    setup_authenticated_session,
 )
 
 
@@ -106,95 +104,143 @@ class TestExtractToken:
 
 
 class TestAuthenticateConnection:
-    """Tests for connection authentication."""
+    """Tests for connection authentication via AuthHandler."""
 
-    @pytest.mark.asyncio
-    async def test_authenticate_success_with_valid_token(self) -> None:
+    @pytest.fixture
+    def mock_token_verifier(self) -> MagicMock:
+        """Create a mock token verifier function."""
+        return MagicMock()
+
+    @pytest.fixture
+    def mock_connection_manager(self) -> AsyncMock:
+        """Create a mock ConnectionManager."""
+        cm = AsyncMock()
+        cm.register_connection = AsyncMock()
+        cm.unregister_connection = AsyncMock()
+        return cm
+
+    @pytest.fixture
+    def auth_handler(
+        self,
+        mock_token_verifier: MagicMock,
+        mock_connection_manager: AsyncMock,
+    ) -> AuthHandler:
+        """Create AuthHandler with injected mocks."""
+        return AuthHandler(
+            token_verifier=mock_token_verifier,
+            conn_manager=mock_connection_manager,
+        )
+
+    async def test_authenticate_success_with_valid_token(
+        self,
+        auth_handler: AuthHandler,
+        mock_token_verifier: MagicMock,
+    ) -> None:
         """Test successful authentication with a valid JWT.
 
         A valid access token should result in AuthResult with
         success=True and the user_id from the token.
         """
         user_id = uuid4()
+        mock_token_verifier.return_value = user_id
 
-        with patch("app.socketio.auth.verify_access_token") as mock_verify:
-            mock_verify.return_value = user_id
+        result = await auth_handler.authenticate_connection("test-sid", {"token": "valid-token"})
 
-            result = await authenticate_connection("test-sid", {"token": "valid-token"})
+        assert result.success is True
+        assert result.user_id == user_id
+        assert result.error_code is None
+        mock_token_verifier.assert_called_once_with("valid-token")
 
-            assert result.success is True
-            assert result.user_id == user_id
-            assert result.error_code is None
-            mock_verify.assert_called_once_with("valid-token")
-
-    @pytest.mark.asyncio
-    async def test_authenticate_fails_with_missing_token(self) -> None:
+    async def test_authenticate_fails_with_missing_token(
+        self,
+        auth_handler: AuthHandler,
+    ) -> None:
         """Test authentication failure when no token is provided.
 
         Connections without any auth data should fail with
         a 'missing_token' error code.
         """
-        result = await authenticate_connection("test-sid", None)
+        result = await auth_handler.authenticate_connection("test-sid", None)
 
         assert result.success is False
         assert result.user_id is None
         assert result.error_code == "missing_token"
         assert "required" in result.error_message.lower()
 
-    @pytest.mark.asyncio
-    async def test_authenticate_fails_with_empty_auth(self) -> None:
+    async def test_authenticate_fails_with_empty_auth(
+        self,
+        auth_handler: AuthHandler,
+    ) -> None:
         """Test authentication failure with empty auth object.
 
         An auth object without any token fields should fail.
         """
-        result = await authenticate_connection("test-sid", {})
+        result = await auth_handler.authenticate_connection("test-sid", {})
 
         assert result.success is False
         assert result.error_code == "missing_token"
 
-    @pytest.mark.asyncio
-    async def test_authenticate_fails_with_invalid_token(self) -> None:
+    async def test_authenticate_fails_with_invalid_token(
+        self,
+        auth_handler: AuthHandler,
+        mock_token_verifier: MagicMock,
+    ) -> None:
         """Test authentication failure with invalid/expired JWT.
 
         When verify_access_token returns None (invalid token),
         we should fail with 'invalid_token' error.
         """
-        with patch("app.socketio.auth.verify_access_token") as mock_verify:
-            mock_verify.return_value = None  # Token validation failed
+        mock_token_verifier.return_value = None
 
-            result = await authenticate_connection("test-sid", {"token": "invalid-token"})
+        result = await auth_handler.authenticate_connection("test-sid", {"token": "invalid-token"})
 
-            assert result.success is False
-            assert result.user_id is None
-            assert result.error_code == "invalid_token"
-            assert (
-                "invalid" in result.error_message.lower()
-                or "expired" in result.error_message.lower()
-            )
+        assert result.success is False
+        assert result.user_id is None
+        assert result.error_code == "invalid_token"
+        assert (
+            "invalid" in result.error_message.lower() or "expired" in result.error_message.lower()
+        )
 
-    @pytest.mark.asyncio
-    async def test_authenticate_extracts_token_from_bearer(self) -> None:
+    async def test_authenticate_extracts_token_from_bearer(
+        self,
+        auth_handler: AuthHandler,
+        mock_token_verifier: MagicMock,
+    ) -> None:
         """Test that authentication works with Bearer format.
 
         The authenticate function should handle Bearer token format
         through the extract_token function.
         """
         user_id = uuid4()
+        mock_token_verifier.return_value = user_id
 
-        with patch("app.socketio.auth.verify_access_token") as mock_verify:
-            mock_verify.return_value = user_id
+        result = await auth_handler.authenticate_connection(
+            "test-sid", {"authorization": "Bearer my-token"}
+        )
 
-            result = await authenticate_connection("test-sid", {"authorization": "Bearer my-token"})
-
-            assert result.success is True
-            mock_verify.assert_called_once_with("my-token")
+        assert result.success is True
+        mock_token_verifier.assert_called_once_with("my-token")
 
 
 class TestSetupAuthenticatedSession:
     """Tests for session setup after authentication."""
 
-    @pytest.mark.asyncio
-    async def test_setup_saves_session_data(self) -> None:
+    @pytest.fixture
+    def mock_connection_manager(self) -> AsyncMock:
+        """Create a mock ConnectionManager."""
+        cm = AsyncMock()
+        cm.register_connection = AsyncMock()
+        return cm
+
+    @pytest.fixture
+    def auth_handler(self, mock_connection_manager: AsyncMock) -> AuthHandler:
+        """Create AuthHandler with injected mock."""
+        return AuthHandler(conn_manager=mock_connection_manager)
+
+    async def test_setup_saves_session_data(
+        self,
+        auth_handler: AuthHandler,
+    ) -> None:
         """Test that session setup saves user_id and timestamp.
 
         After authentication, the socket session should contain
@@ -203,22 +249,21 @@ class TestSetupAuthenticatedSession:
         user_id = uuid4()
         mock_sio = AsyncMock()
 
-        with patch("app.socketio.auth.connection_manager") as mock_cm:
-            mock_cm.register_connection = AsyncMock()
+        await auth_handler.setup_authenticated_session(mock_sio, "test-sid", user_id)
 
-            await setup_authenticated_session(mock_sio, "test-sid", user_id)
+        mock_sio.save_session.assert_called_once()
+        call_args = mock_sio.save_session.call_args
+        assert call_args.args[0] == "test-sid"
 
-            # Verify session was saved
-            mock_sio.save_session.assert_called_once()
-            call_args = mock_sio.save_session.call_args
-            assert call_args.args[0] == "test-sid"
+        session_data = call_args.args[1]
+        assert session_data["user_id"] == str(user_id)
+        assert "authenticated_at" in session_data
 
-            session_data = call_args.args[1]
-            assert session_data["user_id"] == str(user_id)
-            assert "authenticated_at" in session_data
-
-    @pytest.mark.asyncio
-    async def test_setup_registers_with_connection_manager(self) -> None:
+    async def test_setup_registers_with_connection_manager(
+        self,
+        auth_handler: AuthHandler,
+        mock_connection_manager: AsyncMock,
+    ) -> None:
         """Test that session setup registers with ConnectionManager.
 
         The connection should be tracked in ConnectionManager for
@@ -227,53 +272,63 @@ class TestSetupAuthenticatedSession:
         user_id = uuid4()
         mock_sio = AsyncMock()
 
-        with patch("app.socketio.auth.connection_manager") as mock_cm:
-            mock_cm.register_connection = AsyncMock()
+        await auth_handler.setup_authenticated_session(mock_sio, "test-sid", user_id)
 
-            await setup_authenticated_session(mock_sio, "test-sid", user_id)
-
-            mock_cm.register_connection.assert_called_once_with("test-sid", user_id)
+        mock_connection_manager.register_connection.assert_called_once_with("test-sid", user_id)
 
 
 class TestCleanupAuthenticatedSession:
     """Tests for session cleanup on disconnect."""
 
-    @pytest.mark.asyncio
-    async def test_cleanup_unregisters_connection(self) -> None:
+    @pytest.fixture
+    def mock_connection_manager(self) -> AsyncMock:
+        """Create a mock ConnectionManager."""
+        return AsyncMock()
+
+    @pytest.fixture
+    def auth_handler(self, mock_connection_manager: AsyncMock) -> AuthHandler:
+        """Create AuthHandler with injected mock."""
+        return AuthHandler(conn_manager=mock_connection_manager)
+
+    async def test_cleanup_unregisters_connection(
+        self,
+        auth_handler: AuthHandler,
+        mock_connection_manager: AsyncMock,
+    ) -> None:
         """Test that cleanup unregisters from ConnectionManager.
 
         On disconnect, the connection should be removed from
         ConnectionManager to update presence tracking.
         """
-        with patch("app.socketio.auth.connection_manager") as mock_cm:
-            mock_conn_info = MagicMock()
-            mock_conn_info.user_id = "user-123"
-            mock_cm.unregister_connection = AsyncMock(return_value=mock_conn_info)
+        mock_conn_info = MagicMock()
+        mock_conn_info.user_id = "user-123"
+        mock_connection_manager.unregister_connection = AsyncMock(return_value=mock_conn_info)
 
-            result = await cleanup_authenticated_session("test-sid")
+        result = await auth_handler.cleanup_authenticated_session("test-sid")
 
-            assert result == "user-123"
-            mock_cm.unregister_connection.assert_called_once_with("test-sid")
+        assert result == "user-123"
+        mock_connection_manager.unregister_connection.assert_called_once_with("test-sid")
 
-    @pytest.mark.asyncio
-    async def test_cleanup_returns_none_for_unknown_session(self) -> None:
+    async def test_cleanup_returns_none_for_unknown_session(
+        self,
+        auth_handler: AuthHandler,
+        mock_connection_manager: AsyncMock,
+    ) -> None:
         """Test cleanup returns None for non-existent sessions.
 
         If the connection wasn't registered (e.g., auth failed),
         cleanup should return None gracefully.
         """
-        with patch("app.socketio.auth.connection_manager") as mock_cm:
-            mock_cm.unregister_connection = AsyncMock(return_value=None)
+        mock_connection_manager.unregister_connection = AsyncMock(return_value=None)
 
-            result = await cleanup_authenticated_session("unknown-sid")
+        result = await auth_handler.cleanup_authenticated_session("unknown-sid")
 
-            assert result is None
+        assert result is None
 
 
 class TestGetSessionUserId:
     """Tests for session user_id retrieval."""
 
-    @pytest.mark.asyncio
     async def test_get_session_user_id_returns_id(self) -> None:
         """Test retrieving user_id from authenticated session.
 
@@ -290,7 +345,6 @@ class TestGetSessionUserId:
         assert result == "user-123"
         mock_sio.get_session.assert_called_once_with("test-sid", namespace="/game")
 
-    @pytest.mark.asyncio
     async def test_get_session_user_id_returns_none_for_missing(self) -> None:
         """Test that missing session returns None.
 
@@ -304,7 +358,6 @@ class TestGetSessionUserId:
 
         assert result is None
 
-    @pytest.mark.asyncio
     async def test_get_session_user_id_handles_exception(self) -> None:
         """Test that exceptions are caught and return None.
 
@@ -322,7 +375,6 @@ class TestGetSessionUserId:
 class TestRequireAuth:
     """Tests for the require_auth helper."""
 
-    @pytest.mark.asyncio
     async def test_require_auth_returns_user_id_for_authenticated(self) -> None:
         """Test that require_auth returns user_id for valid sessions.
 
@@ -336,7 +388,6 @@ class TestRequireAuth:
 
         assert result == "user-123"
 
-    @pytest.mark.asyncio
     async def test_require_auth_returns_none_for_unauthenticated(self) -> None:
         """Test that require_auth returns None for unauthenticated sessions.
 
diff --git a/backend/tests/unit/services/test_connection_manager.py b/backend/tests/unit/services/test_connection_manager.py
index 7fbbf84..f7fc08e 100644
--- a/backend/tests/unit/services/test_connection_manager.py
+++ b/backend/tests/unit/services/test_connection_manager.py
@@ -1,12 +1,16 @@
 """Tests for ConnectionManager service.
 
 This module tests WebSocket connection tracking with Redis. Since these are
-unit tests, we mock the Redis operations to test the ConnectionManager logic
+unit tests, we inject a mock Redis factory to test the ConnectionManager logic
 without requiring a real Redis instance.
+
+Uses dependency injection pattern - no monkey patching required.
 """
 
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
 from datetime import UTC, datetime, timedelta
-from unittest.mock import AsyncMock, patch
+from unittest.mock import AsyncMock
 from uuid import uuid4
 
 import pytest
@@ -21,15 +25,13 @@ from app.services.connection_manager import (
 )
 
 
-@pytest.fixture
-def manager() -> ConnectionManager:
-    """Create a ConnectionManager instance for testing."""
-    return ConnectionManager(conn_ttl_seconds=3600)
-
-
 @pytest.fixture
 def mock_redis() -> AsyncMock:
-    """Create a mock Redis client."""
+    """Create a mock Redis client.
+
+    This mock provides all the Redis operations used by ConnectionManager
+    with sensible defaults that can be overridden per-test.
+    """
     redis = AsyncMock()
     redis.hset = AsyncMock()
     redis.hget = AsyncMock()
@@ -46,6 +48,21 @@ def mock_redis() -> AsyncMock:
     return redis
 
 
+@pytest.fixture
+def manager(mock_redis: AsyncMock) -> ConnectionManager:
+    """Create a ConnectionManager with injected mock Redis.
+
+    The mock Redis is injected via the redis_factory parameter,
+    eliminating the need for monkey patching in tests.
+    """
+
+    @asynccontextmanager
+    async def mock_redis_factory() -> AsyncIterator[AsyncMock]:
+        yield mock_redis
+
+    return ConnectionManager(conn_ttl_seconds=3600, redis_factory=mock_redis_factory)
+
+
 class TestConnectionInfoDataclass:
     """Tests for the ConnectionInfo dataclass."""
 
@@ -87,8 +104,8 @@ class TestConnectionInfoDataclass:
     def test_is_stale_with_custom_threshold(self) -> None:
         """Test is_stale with a custom threshold.
 
-        The threshold can be adjusted for different use cases like
-        more aggressive cleanup or more lenient timeout.
+        The threshold parameter allows callers to customize what counts
+        as stale for different use cases.
         """
         now = datetime.now(UTC)
         last_seen = now - timedelta(seconds=60)
@@ -96,7 +113,7 @@ class TestConnectionInfoDataclass:
             sid="test-sid",
             user_id="user-123",
             game_id=None,
-            connected_at=now,
+            connected_at=last_seen,
             last_seen=last_seen,
         )
 
@@ -125,23 +142,20 @@ class TestRegisterConnection:
         sid = "test-sid-123"
         user_id = str(uuid4())
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        await manager.register_connection(sid, user_id)
 
-            await manager.register_connection(sid, user_id)
+        # Verify connection hash was created
+        conn_key = f"{CONN_PREFIX}{sid}"
+        mock_redis.hset.assert_called()
+        hset_call = mock_redis.hset.call_args
+        assert hset_call.args[0] == conn_key
+        mapping = hset_call.kwargs["mapping"]
+        assert mapping["user_id"] == user_id
+        assert mapping["game_id"] == ""
 
-            # Verify connection hash was created
-            conn_key = f"{CONN_PREFIX}{sid}"
-            mock_redis.hset.assert_called()
-            hset_call = mock_redis.hset.call_args
-            assert hset_call.args[0] == conn_key
-            mapping = hset_call.kwargs["mapping"]
-            assert mapping["user_id"] == user_id
-            assert mapping["game_id"] == ""
-
-            # Verify user-to-connection mapping was created
-            user_conn_key = f"{USER_CONN_PREFIX}{user_id}"
-            mock_redis.set.assert_called_with(user_conn_key, sid)
+        # Verify user-to-connection mapping was created
+        user_conn_key = f"{USER_CONN_PREFIX}{user_id}"
+        mock_redis.set.assert_called_with(user_conn_key, sid)
 
     @pytest.mark.asyncio
     async def test_register_connection_replaces_old_connection(
@@ -167,15 +181,12 @@ class TestRegisterConnection:
             "last_seen": datetime.now(UTC).isoformat(),
         }
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        await manager.register_connection(new_sid, user_id)
 
-            await manager.register_connection(new_sid, user_id)
-
-            # Verify old connection was cleaned up (delete called for old conn key)
-            old_conn_key = f"{CONN_PREFIX}{old_sid}"
-            delete_calls = [call.args[0] for call in mock_redis.delete.call_args_list]
-            assert old_conn_key in delete_calls
+        # Verify old connection was cleaned up (delete called for old conn key)
+        old_conn_key = f"{CONN_PREFIX}{old_sid}"
+        delete_calls = [call.args[0] for call in mock_redis.delete.call_args_list]
+        assert old_conn_key in delete_calls
 
     @pytest.mark.asyncio
     async def test_register_connection_accepts_uuid(
@@ -191,15 +202,12 @@ class TestRegisterConnection:
         sid = "test-sid"
         user_uuid = uuid4()
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        await manager.register_connection(sid, user_uuid)
 
-            await manager.register_connection(sid, user_uuid)
-
-            # Verify user_id was converted to string
-            hset_call = mock_redis.hset.call_args
-            mapping = hset_call.kwargs["mapping"]
-            assert mapping["user_id"] == str(user_uuid)
+        # Verify user_id was converted to string
+        hset_call = mock_redis.hset.call_args
+        mapping = hset_call.kwargs["mapping"]
+        assert mapping["user_id"] == str(user_uuid)
 
 
 class TestUnregisterConnection:
@@ -213,17 +221,14 @@ class TestUnregisterConnection:
     ) -> None:
         """Test that unregistering unknown connection returns None.
 
-        If the connection doesn't exist, we should return None rather than
-        raising an error.
+        Unregistering a non-existent connection should be a no-op
+        and return None to indicate nothing was cleaned up.
         """
         mock_redis.hgetall.return_value = {}
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.unregister_connection("unknown-sid")
 
-            result = await manager.unregister_connection("unknown-sid")
-
-            assert result is None
+        assert result is None
 
     @pytest.mark.asyncio
     async def test_unregister_cleans_up_all_data(
@@ -231,44 +236,39 @@ class TestUnregisterConnection:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that unregistering cleans up all related Redis data.
+        """Test that unregistering cleans up all Redis data.
 
-        Cleanup should remove:
+        Unregistration should remove:
         1. Connection hash
-        2. User-to-connection mapping (if it still points to this sid)
-        3. Game connection set membership
+        2. User-to-connection mapping
+        3. Remove from game connection set (if in a game)
         """
         sid = "test-sid"
         user_id = "user-123"
         game_id = "game-456"
+        now = datetime.now(UTC)
 
-        # Mock: connection exists with game
         mock_redis.hgetall.return_value = {
             "user_id": user_id,
             "game_id": game_id,
-            "connected_at": datetime.now(UTC).isoformat(),
-            "last_seen": datetime.now(UTC).isoformat(),
+            "connected_at": now.isoformat(),
+            "last_seen": now.isoformat(),
         }
-        mock_redis.get.return_value = sid  # user mapping points to this sid
+        mock_redis.get.return_value = sid  # User's current connection
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.unregister_connection(sid)
 
-            result = await manager.unregister_connection(sid)
+        assert result is not None
+        assert result.user_id == user_id
+        assert result.game_id == game_id
 
-            # Verify result
-            assert result is not None
-            assert result.sid == sid
-            assert result.user_id == user_id
-            assert result.game_id == game_id
-
-            # Verify cleanup
-            mock_redis.srem.assert_called()  # Removed from game set
-            mock_redis.delete.assert_called()  # Connection deleted
+        # Verify cleanup calls
+        mock_redis.delete.assert_called()
+        mock_redis.srem.assert_called_with(f"{GAME_CONNS_PREFIX}{game_id}", sid)
 
 
 class TestGameAssociation:
-    """Tests for game join/leave operations."""
+    """Tests for game association methods."""
 
     @pytest.mark.asyncio
     async def test_join_game_adds_to_game_set(
@@ -276,25 +276,22 @@ class TestGameAssociation:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that joining a game adds the connection to the game's set.
+        """Test that joining a game adds connection to game's set.
 
-        When a connection joins a game, it should:
-        1. Update the connection's game_id
-        2. Add the sid to the game's connection set
+        The connection should be tracked in the game's connection set
+        for broadcasting updates to all participants.
         """
         sid = "test-sid"
         game_id = "game-123"
 
         mock_redis.exists.return_value = True
+        mock_redis.hget.return_value = ""  # Not in a game yet
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.join_game(sid, game_id)
 
-            result = await manager.join_game(sid, game_id)
-
-            assert result is True
-            mock_redis.hset.assert_called_with(f"{CONN_PREFIX}{sid}", "game_id", game_id)
-            mock_redis.sadd.assert_called_with(f"{GAME_CONNS_PREFIX}{game_id}", sid)
+        assert result is True
+        mock_redis.sadd.assert_called_with(f"{GAME_CONNS_PREFIX}{game_id}", sid)
+        mock_redis.hset.assert_called_with(f"{CONN_PREFIX}{sid}", "game_id", game_id)
 
     @pytest.mark.asyncio
     async def test_join_game_returns_false_for_unknown_connection(
@@ -304,18 +301,13 @@ class TestGameAssociation:
     ) -> None:
         """Test that joining a game fails for unknown connections.
 
-        If the connection doesn't exist, we should return False rather
-        than creating orphan game association data.
+        Non-existent connections should not be able to join games.
         """
         mock_redis.exists.return_value = False
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.join_game("unknown-sid", "game-123")
 
-            result = await manager.join_game("unknown-sid", "game-123")
-
-            assert result is False
-            mock_redis.sadd.assert_not_called()
+        assert result is False
 
     @pytest.mark.asyncio
     async def test_join_game_leaves_previous_game(
@@ -325,25 +317,22 @@ class TestGameAssociation:
     ) -> None:
         """Test that joining a new game leaves the previous game.
 
-        When switching games, the connection should be removed from the
-        old game's connection set before joining the new one.
+        A connection can only be in one game at a time, so joining
+        a new game should automatically leave any previous game.
         """
         sid = "test-sid"
-        old_game = "game-old"
-        new_game = "game-new"
+        old_game = "old-game"
+        new_game = "new-game"
 
         mock_redis.exists.return_value = True
-        mock_redis.hget.return_value = old_game  # Currently in old game
+        mock_redis.hget.return_value = old_game
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        await manager.join_game(sid, new_game)
 
-            await manager.join_game(sid, new_game)
-
-            # Verify left old game
-            mock_redis.srem.assert_called_with(f"{GAME_CONNS_PREFIX}{old_game}", sid)
-            # Verify joined new game
-            mock_redis.sadd.assert_called_with(f"{GAME_CONNS_PREFIX}{new_game}", sid)
+        # Should remove from old game
+        mock_redis.srem.assert_called_with(f"{GAME_CONNS_PREFIX}{old_game}", sid)
+        # Should add to new game
+        mock_redis.sadd.assert_called_with(f"{GAME_CONNS_PREFIX}{new_game}", sid)
 
     @pytest.mark.asyncio
     async def test_leave_game_removes_from_set(
@@ -351,25 +340,21 @@ class TestGameAssociation:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that leaving a game removes the connection from the set.
+        """Test that leaving a game removes connection from set.
 
-        Leave should:
-        1. Remove sid from game's connection set
-        2. Clear game_id on connection record
+        The connection should be removed from the game's set and
+        its game_id should be cleared.
         """
         sid = "test-sid"
         game_id = "game-123"
 
         mock_redis.hget.return_value = game_id
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.leave_game(sid)
 
-            result = await manager.leave_game(sid)
-
-            assert result == game_id
-            mock_redis.srem.assert_called_with(f"{GAME_CONNS_PREFIX}{game_id}", sid)
-            mock_redis.hset.assert_called_with(f"{CONN_PREFIX}{sid}", "game_id", "")
+        assert result == game_id
+        mock_redis.srem.assert_called_with(f"{GAME_CONNS_PREFIX}{game_id}", sid)
+        mock_redis.hset.assert_called_with(f"{CONN_PREFIX}{sid}", "game_id", "")
 
     @pytest.mark.asyncio
     async def test_leave_game_returns_none_when_not_in_game(
@@ -377,20 +362,16 @@ class TestGameAssociation:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that leave_game returns None when not in a game.
+        """Test that leaving when not in a game returns None.
 
-        If the connection isn't associated with any game, we should
-        return None without making unnecessary Redis calls.
+        If the connection isn't in a game, leave_game should return
+        None to indicate no game was left.
         """
-        mock_redis.hget.return_value = ""  # No game_id
+        mock_redis.hget.return_value = ""
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.leave_game("test-sid")
 
-            result = await manager.leave_game("test-sid")
-
-            assert result is None
-            mock_redis.srem.assert_not_called()
+        assert result is None
 
 
 class TestHeartbeat:
@@ -404,25 +385,18 @@ class TestHeartbeat:
     ) -> None:
         """Test that heartbeat updates the last_seen timestamp.
 
-        Heartbeats keep the connection alive by updating the timestamp
-        and refreshing TTLs on Redis records.
+        Heartbeats keep connections alive by updating timestamps
+        and refreshing TTLs on Redis keys.
         """
         sid = "test-sid"
         mock_redis.exists.return_value = True
         mock_redis.hget.return_value = "user-123"
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.update_heartbeat(sid)
 
-            result = await manager.update_heartbeat(sid)
-
-            assert result is True
-            # Verify last_seen was updated
-            hset_call = mock_redis.hset.call_args
-            assert hset_call.args[0] == f"{CONN_PREFIX}{sid}"
-            assert hset_call.args[1] == "last_seen"
-            # Verify TTL was refreshed
-            mock_redis.expire.assert_called()
+        assert result is True
+        mock_redis.hset.assert_called()
+        mock_redis.expire.assert_called()
 
     @pytest.mark.asyncio
     async def test_update_heartbeat_returns_false_for_unknown(
@@ -430,18 +404,16 @@ class TestHeartbeat:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that heartbeat returns False for unknown connections.
+        """Test that heartbeat fails for unknown connections.
 
-        If the connection doesn't exist, we shouldn't try to update it.
+        Unknown connections should not be able to send heartbeats,
+        returning False to indicate failure.
         """
         mock_redis.exists.return_value = False
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.update_heartbeat("unknown-sid")
 
-            result = await manager.update_heartbeat("unknown-sid")
-
-            assert result is False
+        assert result is False
 
 
 class TestQueryMethods:
@@ -453,13 +425,13 @@ class TestQueryMethods:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that get_connection returns ConnectionInfo for valid sid.
+        """Test that get_connection returns ConnectionInfo.
 
-        The returned ConnectionInfo should have all fields populated
-        from the Redis hash.
+        The returned info should contain all the stored connection data.
         """
         sid = "test-sid"
         now = datetime.now(UTC)
+
         mock_redis.hgetall.return_value = {
             "user_id": "user-123",
             "game_id": "game-456",
@@ -467,15 +439,12 @@ class TestQueryMethods:
             "last_seen": now.isoformat(),
         }
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.get_connection(sid)
 
-            result = await manager.get_connection(sid)
-
-            assert result is not None
-            assert result.sid == sid
-            assert result.user_id == "user-123"
-            assert result.game_id == "game-456"
+        assert result is not None
+        assert result.sid == sid
+        assert result.user_id == "user-123"
+        assert result.game_id == "game-456"
 
     @pytest.mark.asyncio
     async def test_get_connection_returns_none_for_unknown(
@@ -483,15 +452,15 @@ class TestQueryMethods:
         manager: ConnectionManager,
         mock_redis: AsyncMock,
     ) -> None:
-        """Test that get_connection returns None for unknown sid."""
+        """Test that get_connection returns None for unknown sid.
+
+        Non-existent connections should return None rather than raising.
+        """
         mock_redis.hgetall.return_value = {}
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.get_connection("unknown-sid")
 
-            result = await manager.get_connection("unknown-sid")
-
-            assert result is None
+        assert result is None
 
     @pytest.mark.asyncio
     async def test_is_user_online_returns_true_for_connected_user(
@@ -501,7 +470,7 @@ class TestQueryMethods:
     ) -> None:
         """Test that is_user_online returns True for connected users.
 
-        A user is online if they have an active connection that isn't stale.
+        Users with active, non-stale connections should be considered online.
         """
         user_id = "user-123"
         now = datetime.now(UTC)
@@ -514,12 +483,9 @@ class TestQueryMethods:
             "last_seen": now.isoformat(),
         }
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.is_user_online(user_id)
 
-            result = await manager.is_user_online(user_id)
-
-            assert result is True
+        assert result is True
 
     @pytest.mark.asyncio
     async def test_is_user_online_returns_false_for_stale_connection(
@@ -529,11 +495,11 @@ class TestQueryMethods:
     ) -> None:
         """Test that is_user_online returns False for stale connections.
 
-        Even if a connection record exists, if it's stale (no recent heartbeat),
-        the user should be considered offline.
+        Users with stale connections (no recent heartbeat) should be
+        considered offline even if their connection record exists.
         """
         user_id = "user-123"
-        old_time = datetime.now(UTC) - timedelta(minutes=5)
+        old_time = datetime.now(UTC) - timedelta(seconds=HEARTBEAT_INTERVAL_SECONDS * 4)
 
         mock_redis.get.return_value = "test-sid"
         mock_redis.hgetall.return_value = {
@@ -543,12 +509,9 @@ class TestQueryMethods:
             "last_seen": old_time.isoformat(),
         }
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.is_user_online(user_id)
 
-            result = await manager.is_user_online(user_id)
-
-            assert result is False
+        assert result is False
 
     @pytest.mark.asyncio
     async def test_get_game_connections_returns_all_participants(
@@ -564,29 +527,32 @@ class TestQueryMethods:
         now = datetime.now(UTC)
 
         mock_redis.smembers.return_value = {"sid-1", "sid-2"}
-        mock_redis.hgetall.side_effect = [
-            {
-                "user_id": "user-1",
-                "game_id": game_id,
-                "connected_at": now.isoformat(),
-                "last_seen": now.isoformat(),
-            },
-            {
-                "user_id": "user-2",
-                "game_id": game_id,
-                "connected_at": now.isoformat(),
-                "last_seen": now.isoformat(),
-            },
-        ]
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        # Use a function to return the right data based on the key queried
+        def hgetall_by_key(key: str) -> dict[str, str]:
+            if key == f"{CONN_PREFIX}sid-1":
+                return {
+                    "user_id": "user-1",
+                    "game_id": game_id,
+                    "connected_at": now.isoformat(),
+                    "last_seen": now.isoformat(),
+                }
+            elif key == f"{CONN_PREFIX}sid-2":
+                return {
+                    "user_id": "user-2",
+                    "game_id": game_id,
+                    "connected_at": now.isoformat(),
+                    "last_seen": now.isoformat(),
+                }
+            return {}
 
-            result = await manager.get_game_connections(game_id)
+        mock_redis.hgetall.side_effect = hgetall_by_key
 
-            assert len(result) == 2
-            user_ids = {conn.user_id for conn in result}
-            assert user_ids == {"user-1", "user-2"}
+        result = await manager.get_game_connections(game_id)
+
+        assert len(result) == 2
+        user_ids = {conn.user_id for conn in result}
+        assert user_ids == {"user-1", "user-2"}
 
     @pytest.mark.asyncio
     async def test_get_opponent_sid_returns_other_player(
@@ -628,38 +594,38 @@ class TestQueryMethods:
 
         mock_redis.hgetall.side_effect = hgetall_by_key
 
-        with patch("app.services.connection_manager.get_redis") as mock_get_redis:
-            mock_get_redis.return_value.__aenter__.return_value = mock_redis
+        result = await manager.get_opponent_sid(game_id, current_user)
 
-            result = await manager.get_opponent_sid(game_id, current_user)
-
-            assert result == opponent_sid
+        assert result == opponent_sid
 
 
 class TestKeyGeneration:
     """Tests for Redis key generation methods."""
 
-    def test_conn_key_format(self, manager: ConnectionManager) -> None:
+    def test_conn_key_format(self) -> None:
         """Test that connection keys have correct format.
 
         Keys should follow the pattern conn:{sid} for easy identification
         and pattern matching.
         """
+        manager = ConnectionManager()
         key = manager._conn_key("test-sid-123")
         assert key == "conn:test-sid-123"
 
-    def test_user_conn_key_format(self, manager: ConnectionManager) -> None:
+    def test_user_conn_key_format(self) -> None:
         """Test that user connection keys have correct format.
 
         Keys should follow the pattern user_conn:{user_id}.
         """
+        manager = ConnectionManager()
         key = manager._user_conn_key("user-456")
         assert key == "user_conn:user-456"
 
-    def test_game_conns_key_format(self, manager: ConnectionManager) -> None:
+    def test_game_conns_key_format(self) -> None:
         """Test that game connections keys have correct format.
 
         Keys should follow the pattern game_conns:{game_id}.
         """
+        manager = ConnectionManager()
         key = manager._game_conns_key("game-789")
         assert key == "game_conns:game-789"
diff --git a/backend/tests/unit/services/test_game_service.py b/backend/tests/unit/services/test_game_service.py
index 53916e0..f7aa19d 100644
--- a/backend/tests/unit/services/test_game_service.py
+++ b/backend/tests/unit/services/test_game_service.py
@@ -7,9 +7,11 @@ The GameService is STATELESS regarding game rules:
 - No GameEngine is stored in the service
 - Engine is created per-operation using rules from GameState
 - Rules come from frontend at game creation, stored in GameState
+
+Uses dependency injection pattern - no monkey patching required.
 """
 
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock
 from uuid import uuid4
 
 import pytest
@@ -53,19 +55,35 @@ def mock_card_service() -> MagicMock:
     return MagicMock()
 
 
+@pytest.fixture
+def mock_engine() -> MagicMock:
+    """Create a mock GameEngine.
+
+    The engine handles game logic - executing actions and checking
+    win conditions.
+    """
+    engine = MagicMock()
+    engine.execute_action = AsyncMock(
+        return_value=ActionResult(success=True, message="Action executed")
+    )
+    return engine
+
+
 @pytest.fixture
 def game_service(
     mock_state_manager: AsyncMock,
     mock_card_service: MagicMock,
+    mock_engine: MagicMock,
 ) -> GameService:
-    """Create a GameService with mocked dependencies.
+    """Create a GameService with injected mock dependencies.
 
-    Note: No engine is passed - GameService creates engines per-operation
-    using rules stored in each game's GameState.
+    The mock engine factory is injected via the engine_factory parameter,
+    eliminating the need for monkey patching in tests.
     """
     return GameService(
         state_manager=mock_state_manager,
         card_service=mock_card_service,
+        engine_factory=lambda game: mock_engine,
     )
 
 
@@ -324,9 +342,8 @@ class TestJoinGame:
 class TestExecuteAction:
     """Tests for the execute_action method.
 
-    These tests verify action execution through GameService. Since GameService
-    creates engines per-operation, we patch _create_engine_for_game to return
-    a mock engine with controlled behavior.
+    These tests verify action execution through GameService. The mock engine
+    is injected via the engine_factory parameter in the game_service fixture.
     """
 
     @pytest.mark.asyncio
@@ -334,6 +351,7 @@ class TestExecuteAction:
         self,
         game_service: GameService,
         mock_state_manager: AsyncMock,
+        mock_engine: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test successful action execution.
@@ -342,19 +360,14 @@ class TestExecuteAction:
         the state should be saved to cache.
         """
         mock_state_manager.load_state.return_value = sample_game_state
-
-        mock_engine = MagicMock()
-        mock_engine.execute_action = AsyncMock(
-            return_value=ActionResult(
-                success=True,
-                message="Attack executed",
-                state_changes=[{"type": "damage", "amount": 30}],
-            )
+        mock_engine.execute_action.return_value = ActionResult(
+            success=True,
+            message="Attack executed",
+            state_changes=[{"type": "damage", "amount": 30}],
         )
 
-        with patch.object(game_service, "_create_engine_for_game", return_value=mock_engine):
-            action = AttackAction(attack_index=0)
-            result = await game_service.execute_action("game-123", "player-1", action)
+        action = AttackAction(attack_index=0)
+        result = await game_service.execute_action("game-123", "player-1", action)
 
         assert result.success is True
         assert result.action_type == "attack"
@@ -434,6 +447,7 @@ class TestExecuteAction:
         self,
         game_service: GameService,
         mock_state_manager: AsyncMock,
+        mock_engine: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test that resignation is allowed even when not your turn.
@@ -442,24 +456,19 @@ class TestExecuteAction:
         by either player.
         """
         mock_state_manager.load_state.return_value = sample_game_state
-
-        mock_engine = MagicMock()
-        mock_engine.execute_action = AsyncMock(
-            return_value=ActionResult(
-                success=True,
-                message="Player resigned",
-                win_result=WinResult(
-                    winner_id="player-1",
-                    loser_id="player-2",
-                    end_reason=GameEndReason.RESIGNATION,
-                    reason="Player resigned",
-                ),
-            )
+        mock_engine.execute_action.return_value = ActionResult(
+            success=True,
+            message="Player resigned",
+            win_result=WinResult(
+                winner_id="player-1",
+                loser_id="player-2",
+                end_reason=GameEndReason.RESIGNATION,
+                reason="Player resigned",
+            ),
         )
 
-        with patch.object(game_service, "_create_engine_for_game", return_value=mock_engine):
-            # player-2 resigns even though it's player-1's turn
-            result = await game_service.execute_action("game-123", "player-2", ResignAction())
+        # player-2 resigns even though it's player-1's turn
+        result = await game_service.execute_action("game-123", "player-2", ResignAction())
 
         assert result.success is True
         assert result.game_over is True
@@ -470,6 +479,7 @@ class TestExecuteAction:
         self,
         game_service: GameService,
         mock_state_manager: AsyncMock,
+        mock_engine: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test execute_action raises error for invalid actions.
@@ -478,19 +488,12 @@ class TestExecuteAction:
         InvalidActionError with the reason.
         """
         mock_state_manager.load_state.return_value = sample_game_state
-
-        mock_engine = MagicMock()
-        mock_engine.execute_action = AsyncMock(
-            return_value=ActionResult(
-                success=False,
-                message="Not enough energy to attack",
-            )
+        mock_engine.execute_action.return_value = ActionResult(
+            success=False,
+            message="Not enough energy to attack",
         )
 
-        with (
-            patch.object(game_service, "_create_engine_for_game", return_value=mock_engine),
-            pytest.raises(InvalidActionError) as exc_info,
-        ):
+        with pytest.raises(InvalidActionError) as exc_info:
             await game_service.execute_action("game-123", "player-1", AttackAction(attack_index=0))
 
         assert "Not enough energy" in exc_info.value.reason
@@ -500,6 +503,7 @@ class TestExecuteAction:
         self,
         game_service: GameService,
         mock_state_manager: AsyncMock,
+        mock_engine: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test execute_action detects game over and persists to DB.
@@ -508,58 +512,26 @@ class TestExecuteAction:
         to the database for durability.
         """
         mock_state_manager.load_state.return_value = sample_game_state
-
-        mock_engine = MagicMock()
-        mock_engine.execute_action = AsyncMock(
-            return_value=ActionResult(
-                success=True,
-                message="Final prize taken!",
-                win_result=WinResult(
-                    winner_id="player-1",
-                    loser_id="player-2",
-                    end_reason=GameEndReason.PRIZES_TAKEN,
-                    reason="All prizes taken",
-                ),
-            )
+        mock_engine.execute_action.return_value = ActionResult(
+            success=True,
+            message="Final prize taken!",
+            win_result=WinResult(
+                winner_id="player-1",
+                loser_id="player-2",
+                end_reason=GameEndReason.PRIZES_TAKEN,
+                reason="All prizes taken",
+            ),
         )
 
-        with patch.object(game_service, "_create_engine_for_game", return_value=mock_engine):
-            result = await game_service.execute_action(
-                "game-123", "player-1", AttackAction(attack_index=0)
-            )
+        result = await game_service.execute_action(
+            "game-123", "player-1", AttackAction(attack_index=0)
+        )
 
         assert result.game_over is True
         assert result.winner_id == "player-1"
         assert result.end_reason == GameEndReason.PRIZES_TAKEN
         mock_state_manager.persist_to_db.assert_called_once()
 
-    @pytest.mark.asyncio
-    async def test_execute_action_uses_game_rules(
-        self,
-        game_service: GameService,
-        mock_state_manager: AsyncMock,
-        sample_game_state: GameState,
-    ) -> None:
-        """Test that execute_action creates engine with game's rules.
-
-        The engine should be created using the rules stored in the game
-        state, not any service-level defaults.
-        """
-        mock_state_manager.load_state.return_value = sample_game_state
-
-        mock_engine = MagicMock()
-        mock_engine.execute_action = AsyncMock(
-            return_value=ActionResult(success=True, message="OK")
-        )
-
-        with patch.object(
-            game_service, "_create_engine_for_game", return_value=mock_engine
-        ) as mock_create:
-            await game_service.execute_action("game-123", "player-1", PassAction())
-
-            # Verify engine was created with the game state
-            mock_create.assert_called_once_with(sample_game_state)
-
 
 class TestResignGame:
     """Tests for the resign_game convenience method."""
@@ -569,6 +541,7 @@ class TestResignGame:
         self,
         game_service: GameService,
         mock_state_manager: AsyncMock,
+        mock_engine: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test that resign_game is a convenience wrapper for execute_action.
@@ -577,23 +550,18 @@ class TestResignGame:
         and call execute_action.
         """
         mock_state_manager.load_state.return_value = sample_game_state
-
-        mock_engine = MagicMock()
-        mock_engine.execute_action = AsyncMock(
-            return_value=ActionResult(
-                success=True,
-                message="Player resigned",
-                win_result=WinResult(
-                    winner_id="player-2",
-                    loser_id="player-1",
-                    end_reason=GameEndReason.RESIGNATION,
-                    reason="Player resigned",
-                ),
-            )
+        mock_engine.execute_action.return_value = ActionResult(
+            success=True,
+            message="Player resigned",
+            win_result=WinResult(
+                winner_id="player-2",
+                loser_id="player-1",
+                end_reason=GameEndReason.RESIGNATION,
+                reason="Player resigned",
+            ),
         )
 
-        with patch.object(game_service, "_create_engine_for_game", return_value=mock_engine):
-            result = await game_service.resign_game("game-123", "player-1")
+        result = await game_service.resign_game("game-123", "player-1")
 
         assert result.success is True
         assert result.action_type == "resign"
@@ -692,31 +660,39 @@ class TestCreateGame:
         assert "GS-002" in str(exc_info.value)
 
 
-class TestCreateEngineForGame:
-    """Tests for the _create_engine_for_game method.
+class TestDefaultEngineFactory:
+    """Tests for the _default_engine_factory method.
 
     This method is responsible for creating a GameEngine configured
     with the rules from a specific game's state.
     """
 
-    def test_create_engine_uses_game_rules(
+    def test_default_engine_uses_game_rules(
         self,
-        game_service: GameService,
+        mock_state_manager: AsyncMock,
+        mock_card_service: MagicMock,
         sample_game_state: GameState,
     ) -> None:
-        """Test that engine is created with the game's rules.
+        """Test that default engine is created with the game's rules.
 
         The engine should use the RulesConfig stored in the game state,
         not any default configuration.
         """
-        engine = game_service._create_engine_for_game(sample_game_state)
+        # Create service without mock engine to test default factory
+        service = GameService(
+            state_manager=mock_state_manager,
+            card_service=mock_card_service,
+        )
+
+        engine = service._default_engine_factory(sample_game_state)
 
         # Engine should have the game's rules
         assert engine.rules == sample_game_state.rules
 
-    def test_create_engine_with_rng_seed(
+    def test_default_engine_with_rng_seed(
         self,
-        game_service: GameService,
+        mock_state_manager: AsyncMock,
+        mock_card_service: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test that engine uses seeded RNG when game has rng_seed.
@@ -724,17 +700,21 @@ class TestCreateEngineForGame:
         When a game has an rng_seed set, the engine should use a
         deterministic RNG for replay support.
         """
+        service = GameService(
+            state_manager=mock_state_manager,
+            card_service=mock_card_service,
+        )
         sample_game_state.rng_seed = 12345
 
-        engine = game_service._create_engine_for_game(sample_game_state)
+        engine = service._default_engine_factory(sample_game_state)
 
-        # Engine should have been created (we can't easily verify seed,
-        # but we can verify it doesn't error)
+        # Engine should have been created successfully
         assert engine is not None
 
-    def test_create_engine_without_rng_seed(
+    def test_default_engine_without_rng_seed(
         self,
-        game_service: GameService,
+        mock_state_manager: AsyncMock,
+        mock_card_service: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test that engine uses secure RNG when no seed is set.
@@ -742,15 +722,20 @@ class TestCreateEngineForGame:
         Without an rng_seed, the engine should use cryptographically
         secure random number generation.
         """
+        service = GameService(
+            state_manager=mock_state_manager,
+            card_service=mock_card_service,
+        )
         sample_game_state.rng_seed = None
 
-        engine = game_service._create_engine_for_game(sample_game_state)
+        engine = service._default_engine_factory(sample_game_state)
 
         assert engine is not None
 
-    def test_create_engine_derives_unique_seed_per_action(
+    def test_default_engine_derives_unique_seed_per_action(
         self,
-        game_service: GameService,
+        mock_state_manager: AsyncMock,
+        mock_card_service: MagicMock,
         sample_game_state: GameState,
     ) -> None:
         """Test that different action counts produce different RNG sequences.
@@ -758,15 +743,19 @@ class TestCreateEngineForGame:
         For deterministic replay, each action needs a unique but
         reproducible RNG seed based on game seed + action count.
         """
+        service = GameService(
+            state_manager=mock_state_manager,
+            card_service=mock_card_service,
+        )
         sample_game_state.rng_seed = 12345
 
         # Simulate first action (action_log is empty)
         sample_game_state.action_log = []
-        engine1 = game_service._create_engine_for_game(sample_game_state)
+        engine1 = service._default_engine_factory(sample_game_state)
 
         # Simulate second action (one action in log)
         sample_game_state.action_log = [{"type": "pass"}]
-        engine2 = game_service._create_engine_for_game(sample_game_state)
+        engine2 = service._default_engine_factory(sample_game_state)
 
         # Both engines should be created successfully
         # (They will have different seeds due to action count)