Cannot start client with npx

[nmoeller]

Describe the bug
I wanted to try running the client sample with the server-everything mcp server.
Unfortunatly i always have the issue that i get the following error message :

[[WinError 2] The system cannot find the file specified](https://discuss.python.org/t/filenotfounderror-winerror-2-the-system-cannot-find-the-file-specified/9757)

The issues seems to be related that it cannot find npx, but i have installed node,npm and npx.
When i am running npx -v in a standalone console it responds with 10.8.0. Also when debugging the get_default_environment() contains my Path variable which contains the path to node / npx.

Do you have any idea of what could be wrong ? Could somebody verify that this works on other Machines as expected and it's due to my machine ?

from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client
from mcp import types

# Create server parameters for stdio connection
server_params = StdioServerParameters(
    command="npx", # Executable
    args=["-y @modelcontextprotocol/server-everything"], # Optional command line arguments
    env=None # Optional environment variables
)

# Optional: create a sampling callback
async def handle_sampling_message(message: types.CreateMessageRequestParams) -> types.CreateMessageResult:
    return types.CreateMessageResult(
        role="assistant",
        content=types.TextContent(
            type="text",
            text="Hello, world! from model",
        ),
        model="gpt-3.5-turbo",
        stopReason="endTurn",
    )

async def run():
    async with stdio_client(server_params) as (read, write):
        async with ClientSession(read, write, sampling_callback=handle_sampling_message) as session:
            # Initialize the connection
            await session.initialize()

            # List available prompts
            prompts = await session.list_prompts()

            # Get a prompt
            prompt = await session.get_prompt("example-prompt", arguments={"arg1": "value"})

            # List available resources
            resources = await session.list_resources()

            # List available tools
            tools = await session.list_tools()

            # Read a resource
            content, mime_type = await session.read_resource("file://some/path")

            # Call a tool
            result = await session.call_tool("tool-name", arguments={"arg1": "value"})

if __name__ == "__main__":
    import asyncio
    asyncio.run(run())

[dsp-ant]

I would try providing the absolute path to NPX. If that works then the culprit would be path handling

[s-konnex-engine]

Note the following from the python-sdk examples:

import shutil
import ... # Rest of imports

class Server:
    """Manages MCP server connections and tool execution."""
    def __init__(self, name: str, config: dict[str, Any]) -> None: # Notice the Server class takes a name and a config (json)
        self.name: str = name
        self.config: dict[str, Any] = config  # config is applied to self.config in the Server class instance
        # REST OF THE CONSTRUCTOR

    async def initialize(self) -> None:
        """Initialize the server connection."""
        command = (
            shutil.which("npx")    # <------------------------------- THIS IS THE FIX
            if self.config["command"] == "npx"
            else self.config["command"]
        )
        if command is None:
            raise ValueError("The command must be a valid string and cannot be None.")

        server_params = StdioServerParameters(
            command=command,
            args=self.config["args"],
            env={**os.environ, **self.config["env"]}
            if self.config.get("env")
            else None,
        )
        try:
            stdio_transport = await self.exit_stack.enter_async_context(
                stdio_client(server_params)
            )
            read, write = stdio_transport
            session = await self.exit_stack.enter_async_context(
                ClientSession(read, write)
            )
            await session.initialize()
            self.session = session
        except Exception as e:
            logging.error(f"Error initializing server {self.name}: {e}")
            await self.cleanup()
            raise

    # REST OF SERVER CLASS...
)

I'm on windows and this is the only thing that worked for me. Even the absolute path or extracting the absolute directory path from environment variables and joining the command to create the path didn't work. I'm not sure this is windows specific but my instincts are telling me that it is.

the full solution on one line looks as follows:

command = (shutil.which("npx") if self.config["command"] == "npx" else self.config["command"])

So for your simple functional example that isn't using a class with a config you could just use command = shutil.which("npx") just before the server_params line and command = command in the server_params initialization. however, using the Server class and self.config results in a more robust solution that's capable of handling both typescript and python based servers. If you want to stay in the realm of functions just create a function that takes a dict[str, Any] (json) and returns the correct command.

e.g.

async def get_correct_command(config: dict[str, Any]) -> str:
    command = (
        shutil.which("npx") 
        if config["command"] == "npx" 
        else config["command"]
    )
    return command

then

config = "everything": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-everything"]
}

server_params = StdioServerParameters(
    command=get_correct_command(config), # Correct command returned
    args=["-y @modelcontextprotocol/server-everything"], # Optional command line arguments
    env=None # Optional environment variables
)```

[Kludex]

This is fixed on #327, and it should be available on version 1.5.0.