Claude throwing an error "Server disconnected"

[ANewProfile]

So I was interested in learning to make MCPs, and I started with a simple python file called "FastMCP-Intro.py":

from fastmcp import FastMCP

mcp = FastMCP("LearningDemo")


@mcp.tool()
def add(a: int, b: int):
    """Add two numbers"""
    return a + b

After running fastmcp install FastMCP-Intro.py in the correct working directory, my terminal says: Added server 'LearningDemo' to Claude config and Successfully installed LearningDemo in Claude app, indicating that the installation was successful. Looking at Claude's config file, it does contain:

"LearningDemo": {
      "command": "uv",
      "args": [
        "run",
        "--with",
        "fastmcp",
        "fastmcp",
        "run",
        "/Users/path/to/correct/directory/FastMCP-Intro.py"
      ]
}

However, after launching the Claude desktop app, the Settings say that there was an Error, with the message "Server disconnected". I also tried with the config:

"RealDemo": {
      "command": "npx",
      "args": [
        "-y",
        "@modelcontextprotocol/server-filesystem",
        "/Users/path/to/correct/directory/FastMCP-Intro.py"
      ]
}

but this returned the same error.

However, public MCPs that I was previously using still worked, connecting like normal.

When I ran fastmcp dev FastMCP-Intro.py and accessed the MCP Inspector from my browser, the MCP worked fine. I was able to use the "add" tool to add two numbers, with no troubles. On the sidebar, there were three errors:
[03/29/25 13:47:39] INFO Processing request of type server.py:534 ListResourcesRequest,
[03/29/25 13:47:48] INFO Processing request of type server.py:534 ListToolsRequest, and
[03/29/25 13:48:02] INFO Processing request of type server.py:534 CallToolRequest, but the MCP tool still worked perfectly fine in the Inspector.

I searched the documentation, but couldn't find anything indicating why this might be happening. Does anyone know how to fix this? Am I just making some stupid oversight mistake?

[jeremysub]

Hello:

I spent a great deal of time trying to figure this out when I started playing with this. The process is non-intuitive currently. Hopefully this improves over time. I ended up with a config like this for a simple Hello World MCP Server:

    "hello-server": {
      "command": "uv",
      "args": [
        "run",
        "--with",
        "mcp[cli]",
        "mcp",
        "run",
        "/Users/jeremysublett/Repos/MCPTests/HelloWorldServer/server.py"
      ]
    },

I've also needed multiple "--with" commands for additional libraries that may be needed. Here's my corresponding script.

from mcp.server.fastmcp import FastMCP
from namer import get_randomized_name

# Create an MCP server
mcp = FastMCP("hello-server")

# Add a simple tool
@mcp.tool()
def say_hello(name: str) -> str:
    """Says hello to a user by name."""
    return f"Hello, {name}!"

@mcp.tool()
def get_random_name() -> str:
    """Returns a random name."""
    return get_randomized_name()

if __name__ == "__main__":
    mcp.run()

Hope this helps you out.

[ANewProfile]

The code

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("LearningDemo")


@mcp.tool()
def add(a: int, b: int):
    """Add two numbers"""
    return a + b

with the implementation

"LearningDemo": {
      "command": "uv",
      "args": [
        "run",
        "--with",
        "mcp[cli]",
        "mcp",
        "run",
        "/Users/path/to/correct/directory/FastMCP-Intro.py"
      ]
}

doesn't work either. I tried running uv run --with mcp[cli] mcp run /Users/path/to/correct/directory/FastMCP-Intro.py in my terminal, but it returned with an error and said: zsh: no matches found: mcp[cli].

Running uv run --with mcp mcp run /Users/path/to/correct/directory/FastMCP-Intro.py or uv run --with fastmcp fastmcp run /Users/path/to/correct/directory/FastMCP-Intro.py both don't return with an error, but nothing showed in the terminal and I ended up CTRL+C'ing out of it.

[ANewProfile]

I fixed it by installing uv with brew (brew install uv), you can see the answer here

[zzstoatzz]

@ANewProfile @jeremysub we should make this more clear by archiving this repo but all this functionality now lives in mcp's python SDK directly

and

zsh: no matches found: mcp[cli]

fwiw you needed quotes "mcp[cli]"

and FastMCP accepts a dependencies: list[str] kwarg that are used when starting the process (this kwarg is also in mcp's SDK now)