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fastmcp/docs/patterns/cli.mdx
Jeremiah Lowin d52d6d8fff
 Support factory functions in fastmcp run (#1384)
2025-08-06 13:12:52 -04:00

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---
title: FastMCP CLI
sidebarTitle: CLI
description: Learn how to use the FastMCP command-line interface
icon: terminal
---
import { VersionBadge } from "/snippets/version-badge.mdx"
FastMCP provides a command-line interface (CLI) that makes it easy to run, develop, and install your MCP servers. The CLI is automatically installed when you install FastMCP.
```bash
fastmcp --help
```
## Commands Overview
| Command | Purpose | Dependency Management |
| ------- | ------- | --------------------- |
| `run` | Run a FastMCP server directly | **Supports:** Local files, factory functions, URLs, MCP configs. **Deps:** Uses your local environment directly. With `--python`, `--with`, `--project`, or `--with-requirements`: Runs via `uv run` subprocess |
| `dev` | Run a server with the MCP Inspector for testing | **Supports:** Local files only. **Deps:** Always runs via `uv run` subprocess (never uses your local environment); dependencies must be specified or available in a uv-managed project |
| `install` | Install a server in MCP client applications | **Supports:** Local files only. **Deps:** Creates an isolated environment; dependencies must be explicitly specified with `--with` and/or `--with-editable` |
| `inspect` | Generate a JSON report about a FastMCP server | **Supports:** Local files only. **Deps:** Uses your current environment; you are responsible for ensuring all dependencies are available |
| `version` | Display version information | N/A |
## `fastmcp run`
Run a FastMCP server directly or proxy a remote server.
```bash
fastmcp run server.py
```
<Tip>
By default, this command runs the server directly in your current Python environment. You are responsible for ensuring all dependencies are available. When using `--python`, `--with`, `--project`, or `--with-requirements` options, it runs the server via `uv run` subprocess instead.
</Tip>
### Options
| Option | Flag | Description |
| ------ | ---- | ----------- |
| Transport | `--transport`, `-t` | Transport protocol to use (`stdio`, `http`, or `sse`) |
| Host | `--host` | Host to bind to when using http transport (default: 127.0.0.1) |
| Port | `--port`, `-p` | Port to bind to when using http transport (default: 8000) |
| Path | `--path` | Path to bind to when using http transport (default: `/mcp/` or `/sse/` for SSE) |
| Log Level | `--log-level`, `-l` | Log level (DEBUG, INFO, WARNING, ERROR, CRITICAL) |
| No Banner | `--no-banner` | Disable the startup banner display |
| Python Version | `--python` | Python version to use (e.g., 3.10, 3.11) |
| Additional Packages | `--with` | Additional packages to install (can be used multiple times) |
| Project Directory | `--project` | Run the command within the given project directory |
| Requirements File | `--with-requirements` | Requirements file to install dependencies from |
### Entrypoints
<VersionBadge version="2.3.5" />
The `fastmcp run` command supports the following entrypoints:
1. **[Inferred server instance](#inferred-server-instance)**: `server.py` - imports the module and looks for a FastMCP object named `mcp`, `server`, or `app`. Errors if no such object is found.
2. **[Explicit server object](#explicit-server-object)**: `server.py:custom_name` - imports and uses the specified server object
3. **[Factory function](#factory-function)**: `server.py:create_server` - calls the specified function (sync or async) to create a server instance
4. **[Remote server proxy](#remote-server-proxy)**: `https://example.com/mcp-server` - connects to a remote server and creates a **local proxy server**
5. **MCP configuration file**: `mcp.json` - runs servers defined in a standard MCP configuration file
<Warning>
Note: When using `fastmcp run` with a local file, it **completely ignores** the `if __name__ == "__main__"` block. This means:
- Any setup code in `__main__` will NOT run
- Server configuration in `__main__` is bypassed
- `fastmcp run` finds your server object/factory and runs it with its own transport settings
If you need setup code to run, use the **factory pattern** instead.
</Warning>
#### Inferred Server Instance
If you provide a path to a file, `fastmcp run` will load the file and look for a FastMCP server instance stored as a variable named `mcp`, `server`, or `app`. If no such object is found, it will raise an error.
For example, if you have a file called `server.py` with the following content:
```python server.py
from fastmcp import FastMCP
mcp = FastMCP("MyServer")
```
You can run it with:
```bash
fastmcp run server.py
```
#### Explicit Server Object
If your server is stored as a variable with a custom name, or you want to be explicit about which server to run, you can use the following syntax to load a specific server object:
```bash
fastmcp run server.py:custom_name
```
For example, if you have a file called `server.py` with the following content:
```python
from fastmcp import FastMCP
my_server = FastMCP("CustomServer")
@my_server.tool
def hello() -> str:
return "Hello from custom server!"
```
You can run it with:
```bash
fastmcp run server.py:custom_name
```
#### Factory Function
<VersionBadge version="2.11.2" />
Since `fastmcp run` ignores the `if __name__ == "__main__"` block, you can use a factory function to run setup code before your server starts. Factory functions are called without any arguments and must return a FastMCP server instance. Both sync and async factory functions are supported.
The syntax for using a factory function is the same as for an explicit server object: `fastmcp run server.py:factory_fn`. FastMCP will automatically detect that you have identified a function rather than a server Instance
For example, if you have a file called `server.py` with the following content:
```python
from fastmcp import FastMCP
async def create_server() -> FastMCP:
mcp = FastMCP("MyServer")
@mcp.tool
def add(x: int, y: int) -> int:
return x + y
# Setup that runs with fastmcp run
tool = await mcp.get_tool("add")
tool.disable()
return mcp
```
You can run it with:
```bash
fastmcp run server.py:create_server
```
#### Remote Server Proxy
FastMCP run can also start a local proxy server that connects to a remote server. This is useful when you want to run a remote server locally for testing or development purposes, or to use with a client that doesn't support direct connections to remote servers.
To start a local proxy, you can use the following syntax:
```bash
fastmcp run https://example.com/mcp
```
#### MCP Configuration
FastMCP can also run servers defined in a standard MCP configuration file. This is useful when you want to run multiple servers from a single file, or when you want to use a client that doesn't support direct connections to remote servers.
To run a MCP configuration file, you can use the following syntax:
```bash
fastmcp run mcp.json
```
This will run all the servers defined in the file.
## `fastmcp dev`
Run a MCP server with the [MCP Inspector](https://github.com/modelcontextprotocol/inspector) for testing.
```bash
fastmcp dev server.py
```
<Tip>
This command always runs your server via `uv run` subprocess (never your local environment) to work with the MCP Inspector. All dependencies must be explicitly specified using the `--with` and/or `--with-editable` options, or be available in a uv-managed project.
</Tip>
<Warning>
The `dev` command is a shortcut for testing a server over STDIO only. When the Inspector launches, you may need to:
1. Select "STDIO" from the transport dropdown
2. Connect manually
This command does not support HTTP testing. To test a server over Streamable HTTP or SSE:
1. Start your server manually with the appropriate transport using either the command line:
```bash
fastmcp run server.py --transport http
```
or by setting the transport in your code:
```bash
python server.py # Assuming your __main__ block sets Streamable HTTP transport
```
2. Open the MCP Inspector separately and connect to your running server
</Warning>
### Options
| Option | Flag | Description |
| ------ | ---- | ----------- |
| Editable Package | `--with-editable`, `-e` | Directory containing pyproject.toml to install in editable mode |
| Additional Packages | `--with` | Additional packages to install (can be used multiple times) |
| Inspector Version | `--inspector-version` | Version of the MCP Inspector to use |
| UI Port | `--ui-port` | Port for the MCP Inspector UI |
| Server Port | `--server-port` | Port for the MCP Inspector Proxy server |
| Python Version | `--python` | Python version to use (e.g., 3.10, 3.11) |
| Project Directory | `--project` | Run the command within the given project directory |
| Requirements File | `--with-requirements` | Requirements file to install dependencies from |
### Entrypoints
The `dev` command supports local FastMCP server files only:
1. **Inferred server instance**: `server.py` - imports the module and looks for a FastMCP object named `mcp`, `server`, or `app`. Errors if no such object is found.
2. **Explicit server object**: `server.py:custom_name` - imports and uses the specified server object
3. **Factory function**: `server.py:create_server` - calls the specified function (sync or async) to create a server instance
<Warning>
The `dev` command **only supports local files** - no URLs, remote servers, or MCP configuration files.
</Warning>
**Examples**
```bash
# Run dev server with editable mode and additional packages
fastmcp dev server.py -e . --with pandas --with matplotlib
# Run dev server with specific Python version
fastmcp dev server.py --python 3.11
# Run dev server with requirements file
fastmcp dev server.py --with-requirements requirements.txt
# Run dev server within a specific project directory
fastmcp dev server.py --project /path/to/project
```
## `fastmcp install`
<VersionBadge version="2.10.3" />
Install a MCP server in MCP client applications. FastMCP currently supports the following clients:
- **Claude Code** - Installs via Claude Code's built-in MCP management system
- **Claude Desktop** - Installs via direct configuration file modification
- **Cursor** - Installs via deeplink that opens Cursor for user confirmation
- **MCP JSON** - Generates standard MCP JSON configuration for manual use
```bash
fastmcp install claude-code server.py
fastmcp install claude-desktop server.py
fastmcp install cursor server.py
fastmcp install mcp-json server.py
```
Note that for security reasons, MCP clients usually run every server in a completely isolated environment. Therefore, all dependencies must be explicitly specified using the `--with` and/or `--with-editable` options (following `uv` conventions) or by attaching them to your server in code via the `dependencies` parameter. You should not assume that the MCP server will have access to your local environment.
<Warning>
**`uv` must be installed and available in your system PATH**. Both Claude Desktop and Cursor run in isolated environments and need `uv` to manage dependencies. On macOS, install `uv` globally with Homebrew for Claude Desktop compatibility: `brew install uv`.
</Warning>
<Note>
**Python Version Considerations**: The install commands now support the `--python` option to specify a Python version directly. You can also use `--project` to run within a specific project directory or `--with-requirements` to install dependencies from a requirements file.
</Note>
<Tip>
**FastMCP `install` commands focus on local server files with STDIO transport.** For remote servers running with HTTP or SSE transport, use your client's native configuration - FastMCP's value is simplifying the complex local setup with dependencies and `uv` commands.
</Tip>
### Options
| Option | Flag | Description |
| ------ | ---- | ----------- |
| Server Name | `--server-name`, `-n` | Custom name for the server (defaults to server's name attribute or file name) |
| Editable Package | `--with-editable`, `-e` | Directory containing pyproject.toml to install in editable mode |
| Additional Packages | `--with` | Additional packages to install (can be used multiple times) |
| Environment Variables | `--env` | Environment variables in KEY=VALUE format (can be used multiple times) |
| Environment File | `--env-file`, `-f` | Load environment variables from a .env file |
| Python Version | `--python` | Python version to use (e.g., 3.10, 3.11) |
| Project Directory | `--project` | Run the command within the given project directory |
| Requirements File | `--with-requirements` | Requirements file to install dependencies from |
### Entrypoints
The `install` command supports local FastMCP server files only:
1. **Inferred server instance**: `server.py` - imports the module and looks for a FastMCP object named `mcp`, `server`, or `app`. Errors if no such object is found.
2. **Explicit server object**: `server.py:custom_name` - imports and uses the specified server object
3. **Factory function**: `server.py:create_server` - calls the specified function (sync or async) to create a server instance
<Note>
Factory functions are particularly useful for install commands since they allow setup code to run that would otherwise be ignored when the MCP client runs your server.
</Note>
<Warning>
The `install` command **only supports local files** - no URLs, remote servers, or MCP configuration files. For remote servers, use your MCP client's native configuration.
</Warning>
**Examples**
```bash
# Auto-detects server object (looks for 'mcp', 'server', or 'app')
fastmcp install claude-desktop server.py
# Uses specific server object
fastmcp install claude-desktop server.py:my_server
# With custom name and dependencies
fastmcp install claude-desktop server.py:my_server --server-name "My Analysis Server" --with pandas
# Install in Claude Code with environment variables
fastmcp install claude-code server.py --env API_KEY=secret --env DEBUG=true
# Install in Cursor with environment variables
fastmcp install cursor server.py --env API_KEY=secret --env DEBUG=true
# Install with environment file
fastmcp install cursor server.py --env-file .env
# Install with specific Python version
fastmcp install claude-desktop server.py --python 3.11
# Install with requirements file
fastmcp install claude-code server.py --with-requirements requirements.txt
# Install within a project directory
fastmcp install cursor server.py --project /path/to/project
# Generate MCP JSON configuration
fastmcp install mcp-json server.py --name "My Server" --with pandas
# Copy JSON configuration to clipboard
fastmcp install mcp-json server.py --copy
```
### MCP JSON Generation
The `mcp-json` subcommand generates standard MCP JSON configuration that can be used with any MCP-compatible client. This is useful when:
- Working with MCP clients not directly supported by FastMCP
- Creating configuration for CI/CD environments
- Sharing server configurations with others
- Integration with custom tooling
The generated JSON follows the standard MCP server configuration format used by Claude Desktop, VS Code, Cursor, and other MCP clients, with the server name as the root key:
```json
{
"server-name": {
"command": "uv",
"args": [
"run",
"--with",
"fastmcp",
"fastmcp",
"run",
"/path/to/server.py"
],
"env": {
"API_KEY": "value"
}
}
}
```
<Note>
To use this configuration with your MCP client, you'll typically need to add it to the client's `mcpServers` object. Consult your client's documentation for any specific configuration requirements or formatting needs.
</Note>
**Options specific to mcp-json:**
| Option | Flag | Description |
| ------ | ---- | ----------- |
| Copy to Clipboard | `--copy` | Copy configuration to clipboard instead of printing to stdout |
## `fastmcp inspect`
<VersionBadge version="2.9.0" />
Generate a detailed JSON report about a FastMCP server, including information about its tools, prompts, resources, and capabilities.
```bash
fastmcp inspect server.py
```
### Options
| Option | Flag | Description |
| ------ | ---- | ----------- |
| Output File | `--output`, `-o` | Output file path for the JSON report (default: server-info.json) |
### Entrypoints
The `inspect` command supports local FastMCP server files only:
1. **Inferred server instance**: `server.py` - imports the module and looks for a FastMCP object named `mcp`, `server`, or `app`. Errors if no such object is found.
2. **Explicit server object**: `server.py:custom_name` - imports and uses the specified server object
3. **Factory function**: `server.py:create_server` - calls the specified function (sync or async) to create a server instance
<Warning>
The `inspect` command **only supports local files** - no URLs, remote servers, or MCP configuration files.
</Warning>
**Examples**
```bash
# Auto-detect server object
fastmcp inspect server.py
# Specify server object
fastmcp inspect server.py:my_server
# Custom output location
fastmcp inspect server.py --output analysis.json
```
## `fastmcp version`
Display version information about FastMCP and related components.
```bash
fastmcp version
```
### Options
| Option | Flag | Description |
| ------ | ---- | ----------- |
| Copy to Clipboard | `--copy` | Copy version information to clipboard |
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