MCP Client connecting to an already running MCP Server?

[sidharthrajaram]

In the example, the MCP Client is responsible for starting the MCP server process like:

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

# Create server parameters for stdio connection
server_params = StdioServerParameters(
    command="python", # Executable
    args=["example_server.py"], # Optional command line arguments
    env=None # Optional environment variables
)

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

Let's say I have the MCP server running already (i.e. uv run example_server.py).

How can MCP clients connect to that server?

Thank you!

[phpmac]

It may not be possible for the time being, because there is no port, and we are waiting for an official response.

[dsp-ant]

The STDIO transport uses stdin/stdout. This means it connects by creating a subprocess which is a 1:1 relationship to the client starting it, since the subprocess is owned by the host application and talk over the channels (stdin/stdout) that are defined at process creation time. Other clients cannot connect to it, since it's not a network or unix socket process. To put it differently: you cannot connect to stdin/stdout of a running process.

If you want multiple clients to connect to one server, you must use HTTP+SSE transport and use sse_client().

[sidharthrajaram]

Thanks @dsp-ant. I was able to do this by using SSE as the transport protocol rather than stdio.

[sidharthrajaram]

@phpmac @witt3rd @logan-markewich @Spycsh , in case you folks are interested, I put up a working pattern of this using SSE here: https://github.com/sidharthrajaram/mcp-sse