FastMCP demo server (Windows, no admin)

Use this project from cmd.exe with a local venv.

Setup

python -m venv venv
venv\Scripts\activate
python -m pip install --upgrade pip
python -m pip install uv
python -m uv pip install "fastmcp[apps]"

Check install

python -m uv run --active fastmcp version

Run your server file (main.py)

python -m uv run --active python main.py

FastMCP developer mode commands

fastmcp dev expects a subcommand (inspector or apps) plus a server spec.

python -m uv run --active fastmcp dev inspector main.py
python -m uv run --active fastmcp dev apps main.py

If needed, you can be explicit about the server object:

python -m uv run --active fastmcp dev inspector main.py:mcp
python -m uv run --active fastmcp dev apps main.py:mcp

Why uv run fastmcp dev main.py failed

main.py was interpreted as a command argument in the wrong position. Use dev inspector or dev apps before the file path.

Troubleshooting: spawn UNKNOWN when opening Inspector

If you see this on Windows:

• MCP Inspector is up and running at http://localhost:6274/...
• then Node crashes with Error: spawn UNKNOWN

your server is usually fine; the failure is from auto-opening the browser.

Fix 0 (recommended): Disable Inspector auto-open

set MCP_AUTO_OPEN_ENABLED=false
python -m uv run --active fastmcp dev inspector main.py:mcp

Then copy the printed http://localhost:... URL and open it manually.

Fix 1: Set browser executable explicitly

set "BROWSER=C:\Program Files\Google\Chrome\Application\chrome.exe"
python -m uv run --active fastmcp dev inspector main.py:mcp

If you use Edge:

set "BROWSER=C:\Program Files (x86)\Microsoft\Edge\Application\msedge.exe"
python -m uv run --active fastmcp dev inspector main.py:mcp

Fix 2: Run server directly (no Inspector auto-open)

python -m uv run --active python main.py

Fix 3: If Inspector prints a localhost URL, open it manually

Copy the printed http://localhost:... URL and open it in your browser.

venv\Scripts\python.exe -m uv run --active fastmcp install claude-desktop main.py:mcp Successfully installed 'Demo Server' in Claude Desktop