MCP Inspector Connection Error with FastMCP on Windows 11

[abhishekbuild]

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?

[cliffhall]

@1abhi6 can you change those arguments to run ./main.py --no-banner or even run main.py --nobanner

I'm afraid that backslash is being interpreted as an escape sequence, not a path separator.

[abhishekbuild]

I changed the arguments to run main.py --no-banner and it worked!. Thank you very much.

[AbidYousuf]

I am testing FastMCP with MCP Inspector on Windows 11 and facing a connection timeout issue. The MCP server starts successfully, but Inspector fails to connect.

Environment

OS: Windows 11

Python: 3.10+

fastmcp: >= 2.12.4

Node.js: v22.20.0

npm: 10.9.3

uv: 0.8.22

Transport: stdio

Steps to Reproduce

From project root directory:

uv run fastmcp dev main.py

Inspector opens at:

http://localhost:6274

Click Connect

After ~40–50 seconds, Inspector shows:

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

Sample Code
from fastmcp import FastMCP
import random

mcp = FastMCP("Testing MCP Inspector")

@mcp.tool
def roll(n_dice: int = 1) -> list[int]:
return [random.randint(1, 6) for _ in range(n_dice)]

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

if name == "main":
mcp.run()

Expected Behavior

Inspector should connect successfully and list the available tools.

Actual Behavior

Inspector times out and displays:

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

[cliffhall]

I am testing FastMCP with MCP Inspector on Windows 11 and facing a connection timeout issue. The MCP server starts successfully, but Inspector fails to connect.

@AbidYousuf Can you please open a new issue and include a screenshot showing what you've entered in the sidebar and the URL bar.