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The fastmcp CLI is installed automatically with FastMCP. It’s the primary way to run, test, install, and interact with MCP servers from your terminal.

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Commands at a Glance

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Server Targets

Most commands need to know which server to talk to. You pass a “server spec” as the first argument, and FastMCP resolves the right transport automatically. URLs connect to a running HTTP server:
Python files are loaded directly — no mcp.run() boilerplate needed. FastMCP finds a server instance named mcp, server, or app in the file, or you can specify one explicitly:
Config files work too — both FastMCP’s own fastmcp.json format and standard MCP config files with an mcpServers key:
Stdio commands connect to any MCP server that speaks over standard I/O. Use --command instead of a positional argument:

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Name-Based Resolution

If your servers are already configured in an editor or tool, you can refer to them by name. FastMCP scans configs from Claude Desktop, Claude Code, Cursor, Gemini CLI, and Goose:
When the same name appears in multiple configs, use the source:name form to be specific:
Run fastmcp discover to see what’s available on your machine.

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Authentication

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Prefect Horizon Account

Use the top-level account commands to manage the credential for Prefect Horizon.
fastmcp login first uses HORIZON_API_KEY or a valid stored key when one is available. When login needs a new key, it shows a verification URL and code. It opens a browser when the terminal supports it. If the browser does not open, use the shown URL and code on another device. To switch accounts, run fastmcp logout before you run fastmcp login again. Login stores only the personal Horizon API key. It does not select or store a deployment organization. fastmcp whoami gets the current user from Horizon. fastmcp logout attempts to revoke the stored key and always removes its local credential. Set HORIZON_API_KEY to use an environment credential instead. The CLI gives that value first precedence and never stores it. When this variable controls the session, logout does not revoke or remove any credential. Remove the variable from your environment to sign out. Use --json for stable command results. During JSON login, the verification challenge goes to stderr and the final result goes to stdout. JSON mode does not open a browser or ask a question.

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MCP Server Authentication

When targeting an HTTP URL, the CLI enables OAuth authentication by default. If the server requires it, you’ll be guided through the flow (typically opening a browser). If it doesn’t, the setup is a silent no-op. To skip authentication entirely — useful for local development servers — pass --auth none:
You can also pass a bearer token directly. Give the token value on its own; FastMCP adds the Bearer prefix when it builds the Authorization header.

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Transport Override

FastMCP defaults to Streamable HTTP for URL targets. If the server only supports Server-Sent Events (SSE), force the older transport: