Add npx validation

src/fastmcp/client/transports.py

@@ -7,7 +7,7 @@ from pathlib import Path
 from typing import (
     TypedDict,
 )
-
+import shutil
 from mcp import ClientSession, StdioServerParameters
 from mcp.client.session import (
     ListRootsFnT,
@@ -341,6 +341,10 @@ class NpxStdioTransport(StdioTransport):
             env_vars: Additional environment variables
             use_package_lock: Whether to use package-lock.json (--prefer-offline)
         """
+        # verify npx is installed
+        if shutil.which("npx") is None:
+            raise ValueError("Command 'npx' not found")
+
         # Basic validation
         if project_directory and not Path(project_directory).exists():
             raise NotADirectoryError(