MCP client with npx not working when env parameter of StdioServerParameters is set

[maxguru]

MCP client code using StdioServerParameters with npx command is not working (with any MCP server) when its env parameter is set to something other than None.

Demo code:

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

server_params = StdioServerParameters(
        command="npx",
        args=["-y", "@modelcontextprotocol/server-filesystem", "/home"],
        env={} # fail
        # env=None # no fail
)

async def run():
        async with stdio_client(server_params) as (read, write):
                async with ClientSession(read, write) as session:
                        await session.initialize()
                        tools = await session.list_tools()

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

Fail output:

sh: 1: mcp-server-filesystem: not found
***hangs***

No fail output:

Secure MCP Filesystem Server running on stdio
Allowed directories: [ '/home' ]

Interestingly, running npm i @modelcontextprotocol/server-filesystem and using node directly works fine:

server_params = StdioServerParameters(
        command="node",
        args=["/home/user/node_modules/@modelcontextprotocol/server-filesystem/dist/index.js", "/home"],
        env={} # no fail
)

Desktop:

• OS: Tested on Gentoo and Ubuntu 24.04.2 LTS
• Version: mcp-1.4.1

[maxguru]

Perhaps this line:
https://github.com/modelcontextprotocol/python-sdk/blob/5a54d824591ae73a5785c4fbce627d103c5c7df8/src/mcp/client/stdio.py#L106C55-L106C78
Needs to be changed from this:

env=server.env if server.env is not None else get_default_environment(),

to this:

env={**get_default_environment(), **server.env} if server.env is not None else get_default_environment(),

I've created pull request #327

[Kludex]

That's I guess because npx needs some environment variable to work properly?

[Kludex]

The current behavior makes sense to me. It doesn't let secrets that may be available via env vars visible to the MCP server.

[maxguru]

Your assertion that current (buggy) implementation doesn't expose secrets is false. If get_default_environment() exposes secrets then it is already doing that in the current implementation in the case when server.env is None.

[Kludex]

Oh sorry, that makes sense. 👍

[Kludex]

• Closed by Use default environment even when env is provided #327