diff --git a/src/server/main.py b/src/server/main.py
index 6816b0d..7c2b63d 100644
--- a/src/server/main.py
+++ b/src/server/main.py
@@ -559,6 +559,31 @@ async def game_loop():
             print(f"Game loop error: {e}")
             get_logger().logger.error(f"Game loop error: {e}", exc_info=True)
 
+
+def ensure_npm_dependencies(web_dir: str) -> bool:
+    """
+    确保 npm 依赖是最新的。
+    
+    Args:
+        web_dir: web 目录路径。
+        
+    Returns:
+        True 如果安装成功，False 如果失败。
+    """
+    import platform
+    print("📦 正在检查前端依赖...")
+    try:
+        if platform.system() == "Windows":
+            subprocess.run("npm install", cwd=web_dir, shell=True, check=True)
+        else:
+            subprocess.run(["npm", "install"], cwd=web_dir, shell=False, check=True)
+        print("✅ 前端依赖已就绪")
+        return True
+    except subprocess.CalledProcessError as e:
+        print(f"⚠️ npm install 失败: {e}，继续启动...")
+        return False
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     # 初始化语言设置
@@ -603,6 +628,9 @@ async def lifespan(app: FastAPI):
         project_root = os.path.abspath(os.path.join(current_dir, '..', '..'))
         web_dir = os.path.join(project_root, 'web')
         
+        # 确保 npm 依赖是最新的（npm install 会自动跳过已安装的包，通常 <1s）。
+        ensure_npm_dependencies(web_dir)
+        
         print(f"正在启动前端开发服务 (npm run dev) 于: {web_dir}")
         # 跨平台兼容：Windows 用 shell=True + 字符串，macOS/Linux 用 shell=False + 列表。
         try:
diff --git a/tests/test_dev_mode.py b/tests/test_dev_mode.py
new file mode 100644
index 0000000..e23ab84
--- /dev/null
+++ b/tests/test_dev_mode.py
@@ -0,0 +1,76 @@
+"""Tests for dev mode functionality in the server."""
+
+import subprocess
+from unittest.mock import patch, MagicMock
+import pytest
+
+
+class TestEnsureNpmDependencies:
+    """Tests for ensure_npm_dependencies function."""
+
+    def test_npm_install_success_unix(self):
+        """Test npm install succeeds on Unix-like systems."""
+        from src.server.main import ensure_npm_dependencies
+        
+        with patch("platform.system", return_value="Darwin"), \
+             patch("subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(returncode=0)
+            
+            result = ensure_npm_dependencies("/fake/web/dir")
+            
+            assert result is True
+            mock_run.assert_called_once_with(
+                ["npm", "install"],
+                cwd="/fake/web/dir",
+                shell=False,
+                check=True
+            )
+
+    def test_npm_install_success_windows(self):
+        """Test npm install succeeds on Windows."""
+        from src.server.main import ensure_npm_dependencies
+        
+        with patch("platform.system", return_value="Windows"), \
+             patch("subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(returncode=0)
+            
+            result = ensure_npm_dependencies("/fake/web/dir")
+            
+            assert result is True
+            mock_run.assert_called_once_with(
+                "npm install",
+                cwd="/fake/web/dir",
+                shell=True,
+                check=True
+            )
+
+    def test_npm_install_failure_returns_false(self):
+        """Test npm install failure returns False but doesn't raise."""
+        from src.server.main import ensure_npm_dependencies
+        
+        with patch("platform.system", return_value="Darwin"), \
+             patch("subprocess.run") as mock_run:
+            mock_run.side_effect = subprocess.CalledProcessError(1, "npm install")
+            
+            result = ensure_npm_dependencies("/fake/web/dir")
+            
+            assert result is False
+
+    def test_npm_install_linux(self):
+        """Test npm install on Linux uses same path as macOS."""
+        from src.server.main import ensure_npm_dependencies
+        
+        with patch("platform.system", return_value="Linux"), \
+             patch("subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(returncode=0)
+            
+            result = ensure_npm_dependencies("/fake/web/dir")
+            
+            assert result is True
+            # Linux should use the same non-shell approach as macOS.
+            mock_run.assert_called_once_with(
+                ["npm", "install"],
+                cwd="/fake/web/dir",
+                shell=False,
+                check=True
+            )