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A Horizon server is an MCP endpoint over Streamable HTTP. Connecting any client comes down to two things: the server URL and how the client authenticates. Once a client has both, it discovers the server’s tools, resources, and prompts and can call them. Every hosted server has a stable URL ending in /mcp, shown on its Deployments page, of the form https://<server-name>.fastmcp.app/mcp. The examples below use https://weather-mcp.fastmcp.app/mcp; substitute your own. Horizon’s dashboard generates ready-to-paste configuration for each supported client. Open a server, choose Connect, and pick your client to get the exact snippet with your URL and any required credentials already filled in. This page explains what those snippets contain so you can read, adapt, and troubleshoot them.

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Authentication

How a client proves its identity depends on the server’s authentication mode and on what the client supports. For the full model, see Authentication. A server with Horizon authentication enabled requires every caller to authenticate, and there are two ways a client does so:
  • Interactive sign-in. Clients that support authenticated MCP connections open a Horizon sign-in flow the first time they connect. The user signs in, and the client holds the resulting credential. This is the smoothest path for desktop and CLI clients that prompt for authorization.
  • API key. Clients that cannot sign in interactively send a Horizon API key as a bearer token. Keys start with fmcp_ and go in the Authorization header: 
    Authorization: Bearer fmcp_...
A server with Horizon authentication disabled applies no Horizon access check, so a public server needs no credential and a server that implements its own authentication expects whatever its code requires. The rest of this page shows configuration for a server with Horizon authentication enabled; when it is disabled, omit the Horizon credential or supply the one your server expects.

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Desktop clients

Claude Desktop and Cursor read MCP servers from a JSON config. Add your server under mcpServers with its URL. Include an Authorization header only when the server needs an API key; clients that sign in interactively leave it out. For Claude Desktop, open Settings → Developer → Edit Config, add the server, and restart Claude: 
{
  "mcpServers": {
    "weather-mcp": {
      "url": "https://weather-mcp.fastmcp.app/mcp",
      "transport": "http",
      "headers": {
        "Authorization": "Bearer fmcp_..."
      }
    }
  }
}
For Cursor, the config is the same shape without the transport field: 
{
  "mcpServers": {
    "weather-mcp": {
      "url": "https://weather-mcp.fastmcp.app/mcp"
    }
  }
}
Cursor also accepts a one-click install link. The Connect dialog generates a cursor:// deeplink that adds the server without editing config by hand.

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CLI clients

Command-line clients add a server with a single command. Each takes a name, the URL, and the HTTP transport, and accepts --env flags for values your client passes through. Claude Code authenticates interactively: run the command, then type /mcp in Claude Code to complete sign-in. 
claude mcp add --scope local --transport http weather-mcp https://weather-mcp.fastmcp.app/mcp
Gemini CLI: 
gemini mcp add weather-mcp https://weather-mcp.fastmcp.app/mcp --transport http
Codex: 
codex mcp add --url https://weather-mcp.fastmcp.app/mcp weather-mcp

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ChatGPT

ChatGPT connects to MCP servers from its web interface rather than a config file. Enable Developer mode in settings, create a new app, choose OAuth for authentication, and paste your server URL into the MCP Server URL field. ChatGPT then runs the Horizon sign-in flow and authorizes the requested permissions.

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When a connection fails

A failed connection usually stops at the gateway before reaching your server code. The status tells you where:
The client sent no credential, an unsupported Authorization scheme, or an expired, revoked, or malformed token. Confirm the client is signed in, or that the API key is current and sent as Authorization: Bearer fmcp_.... See Authentication.
The caller authenticated, but access settings do not allow them to use the server or the requested capability. Check organization membership, server access, and capability policy in Authorization.
The URL maps to no live server, or Horizon is concealing a server from a caller without discovery access. Confirm the URL on the Deployments page and the caller’s access.
Some clients cache the server’s tool list. Reconnect or refresh the client’s capabilities. If a tool is still absent, confirm the live deployment selects the build whose manifest contains it.