MCP Inspector Connection Error with FastMCP on Windows 11

[1abhi6]

Hi team,

I am testing FastMCP with MCP Inspector and facing a connection issue.
Here are my setup details:

• OS: Windows 11

• Python: >=3.13

• Dependencies:

  requires-python = ">=3.13"
  dependencies = [
      "fastmcp>=2.12.4",
  ]

• Node.js: v22.20.0

• NPM: 10.9.3

• uv: 0.8.22

• MCP Transport: stdio

Steps I tried

1. Ran:

   uv run fastmcp dev main.py

2. MCP Inspector opened at: http://localhost:6274/

3. When I click the Connect button in Inspector, I get:

   Connection Error - Check if your MCP server is running and proxy token is correct
   

I also checked a similar issue here: [modelcontextprotocol/inspector#724](#724), but the suggestions didn’t solve the problem.

My code:

from fastmcp import FastMCP
import random

# Create MCP server instance
mcp = FastMCP("Testing MCP Inspector")


# Roll a dice tool
@mcp.tool
def roll(n_dice: int = 1) -> list[int]:
    """Roll n_dice 6 sided dice and return the result"""
    return [random.randint(1, 6) for _ in range(n_dice)]


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


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

MCP Inspector Snapshot:

Expected Behavior

Inspector should connect successfully to the FastMCP server.

Actual Behavior

Inspector shows a Connection Error even though the server starts.

Question

Is this a bug with FastMCP on Windows 11, or am I missing any extra configuration for Inspector to connect properly?