fastmcp/docs/cli/running.mdx

---
title: Running Servers
sidebarTitle: Running
description: Start, develop, and configure servers from the command line
icon: play
---

import { VersionBadge } from '/snippets/version-badge.mdx'

## Starting a Server

`fastmcp run` starts a server. Point it at a Python file, a factory function, a remote URL, or a config file:

```bash
fastmcp run server.py
fastmcp run server.py:create_server
fastmcp run https://example.com/mcp
fastmcp run fastmcp.json
```

By default, the server runs over **stdio** — the transport that MCP clients like Claude Desktop expect. To serve over HTTP instead, specify the transport:

```bash
fastmcp run server.py --transport http
fastmcp run server.py --transport http --host 0.0.0.0 --port 9000
```

### Entrypoints

FastMCP supports several ways to locate and start your server:

**Inferred instance** — FastMCP imports the file and looks for a variable named `mcp`, `server`, or `app`:

```bash
fastmcp run server.py
```

**Explicit instance** — point at a specific variable:

```bash
fastmcp run server.py:my_server
```

**Factory function** — FastMCP calls the function and uses the returned server. Useful when your server needs async setup or configuration that runs before startup:

```bash
fastmcp run server.py:create_server
```

**Remote URL** — starts a local proxy that bridges to a remote server. Handy for local development against a deployed server, or for bridging a remote HTTP server to stdio:

```bash
fastmcp run https://example.com/mcp
```

**FastMCP config** — uses a `fastmcp.json` file that declaratively specifies the server, its dependencies, and deployment settings. When you run `fastmcp run` with no arguments, it auto-detects `fastmcp.json` in the current directory:

```bash
fastmcp run
fastmcp run my-config.fastmcp.json
```

See [Server Configuration](/deployment/server-configuration) for the full `fastmcp.json` format.

**MCP config** — runs servers defined in a standard MCP configuration file (any `.json` with an `mcpServers` key):

```bash
fastmcp run mcp.json
```

<Warning>
`fastmcp run` completely ignores the `if __name__ == "__main__"` block. Any setup code in that block won't execute. If you need initialization logic to run, use a [factory function](/cli/overview#factory-functions).
</Warning>

### Options

| Option | Flag | Description |
| ------ | ---- | ----------- |
| Transport | `--transport`, `-t` | `stdio` (default), `http`, or `sse` |
| Host | `--host` | Bind address for HTTP (default: `127.0.0.1`) |
| Port | `--port`, `-p` | Bind port for HTTP (default: `8000`) |
| Path | `--path` | URL path for HTTP (default: `/mcp/`) |
| Log Level | `--log-level`, `-l` | `DEBUG`, `INFO`, `WARNING`, `ERROR`, `CRITICAL` |
| No Banner | `--no-banner` | Suppress the startup banner |
| Auto-Reload | `--reload` / `--no-reload` | Watch for file changes and restart automatically |
| Reload Dirs | `--reload-dir` | Directories to watch (repeatable) |
| Skip Env | `--skip-env` | Don't set up a uv environment (use when already in one) |
| Python | `--python` | Python version to use (e.g., `3.11`) |
| Extra Packages | `--with` | Additional packages to install (repeatable) |
| Project | `--project` | Run within a specific uv project directory |
| Requirements | `--with-requirements` | Install from a requirements file |

### Dependency Management

By default, `fastmcp run` uses your current Python environment directly. When you pass `--python`, `--with`, `--project`, or `--with-requirements`, it switches to running via `uv run` in a subprocess, which handles dependency isolation automatically.

The `--skip-env` flag is useful when you're already inside an activated venv, a Docker container with pre-installed dependencies, or a uv-managed project — it prevents uv from trying to set up another environment layer.

## Previewing Apps

<VersionBadge version="3.2.0" />

`fastmcp dev apps` launches a browser-based preview UI for servers with [Prefab App tools](/apps/prefab). It starts your MCP server on one port and a local dev UI on another — giving you a live, interactive picker where you can call app tools and see their rendered output without needing a full MCP host client.

```bash
fastmcp dev apps server.py
fastmcp dev apps server.py:mcp --mcp-port 9000 --dev-port 9090
```

The picker auto-generates a form from each tool's input schema. Submit the form and the result opens in a new tab as a rendered Prefab UI.

Auto-reload is on by default — save a file and the MCP server restarts automatically.

<Tip>
`fastmcp dev apps` requires `fastmcp[apps]` — install with `pip install "fastmcp[apps]"`.
</Tip>

| Option | Flag | Description |
| ------ | ---- | ----------- |
| MCP Port | `--mcp-port` | Port for the MCP server (default: `8000`) |
| Dev Port | `--dev-port` | Port for the dev UI (default: `8080`) |
| Auto-Reload | `--reload` / `--no-reload` | Watch for file changes (default: on) |

## Development with the Inspector

`fastmcp dev inspector` launches your server inside the [MCP Inspector](https://github.com/modelcontextprotocol/inspector), a browser-based tool for interactively testing MCP servers. Auto-reload is on by default, so your server restarts when you save changes.

```bash
fastmcp dev inspector server.py
fastmcp dev inspector server.py -e . --with pandas
```

<Tip>
The Inspector always runs your server via `uv run` in a subprocess — it never uses your local environment directly. Specify dependencies with `--with`, `--with-editable`, `--with-requirements`, or through a `fastmcp.json` file.
</Tip>

<Warning>
The Inspector connects over **stdio only**. When it launches, you may need to select "STDIO" from the transport dropdown and click connect. To test a server over HTTP, start it separately with `fastmcp run server.py --transport http` and point the Inspector at the URL.
</Warning>

| Option | Flag | Description |
| ------ | ---- | ----------- |
| Editable Package | `--with-editable`, `-e` | Install a directory in editable mode |
| Extra Packages | `--with` | Additional packages (repeatable) |
| Inspector Version | `--inspector-version` | MCP Inspector version to use |
| UI Port | `--ui-port` | Port for the Inspector UI |
| Server Port | `--server-port` | Port for the Inspector proxy |
| Auto-Reload | `--reload` / `--no-reload` | File watching (default: on) |
| Reload Dirs | `--reload-dir` | Directories to watch (repeatable) |
| Python | `--python` | Python version |
| Project | `--project` | Run within a uv project directory |
| Requirements | `--with-requirements` | Install from a requirements file |

## Pre-Building Environments

`fastmcp project prepare` creates a persistent uv project from a `fastmcp.json` file, pre-installing all dependencies. This separates environment setup from server execution — install once, run many times.

```bash
# Step 1: Build the environment (slow, does dependency resolution)
fastmcp project prepare fastmcp.json --output-dir ./env

# Step 2: Run using the prepared environment (fast, no install step)
fastmcp run fastmcp.json --project ./env
```

The prepared directory contains a `pyproject.toml`, a `.venv` with all packages installed, and a `uv.lock` for reproducibility. This is particularly useful in deployment scenarios where you want deterministic, pre-built environments.