MCP Integration

Fermi supports the Model Context Protocol (MCP) for connecting to external tool servers. MCP servers provide additional tools that the agent can use alongside its built-in tools.

Configuration

MCP servers can be configured in several places (all optional, all user-edited — none are created by fermi init):

• ~/.fermi/mcp.json — global MCP servers
• <project>/.mcp.json — project-local MCP servers
• mcp_servers key in settings.json — global or project-local, supports local overrides

The examples below use mcp.json. A nested { "mcpServers": { ... } } wrapper is also accepted in the JSON files.

Format

The file is a JSON object where each key is a server name and each value is a server configuration:

{
  "filesystem": {
    "transport": "stdio",
    "command": "npx",
    "args": ["-y", "@modelcontextprotocol/server-filesystem", "/path/to/allowed/dir"]
  },
  "github": {
    "transport": "stdio",
    "command": "npx",
    "args": ["-y", "@modelcontextprotocol/server-github"],
    "env": {
      "GITHUB_TOKEN": "${GITHUB_TOKEN}"
    }
  }
}

Configuration Fields

Field	Type	Required	Description
transport	"stdio" or "sse"	No	Transport protocol. Default: "stdio".
command	string	Yes (stdio)	The command to run the MCP server.
args	string[]	No	Arguments passed to the command.
url	string	Yes (sse)	URL for SSE transport servers.
env	object	No	Environment variables passed to the server process. Supports ${VAR} syntax to reference your shell environment.
env_allowlist	string[]	No	List of environment variable names to pass through from the parent process.
sensitive_tools	string[]	No	Tool names that should be treated as sensitive (may require extra confirmation).

Environment Variable Resolution

Environment variables in the env field support the ${VAR} syntax:

{
  "env": {
    "API_KEY": "${MY_API_KEY}"
  }
}

This resolves ${MY_API_KEY} from your shell environment at startup. If the variable is not set, Fermi does not fail — it warns and skips that variable (for mcp.json) or silently omits it (for settings.json), and the server still launches without it.

Transport Types

stdio (Default)

The most common transport. Fermi spawns the MCP server as a child process and communicates via stdin/stdout.

{
  "my-server": {
    "transport": "stdio",
    "command": "node",
    "args": ["path/to/server.js"]
  }
}

SSE

For servers that run as a separate HTTP service. Fermi connects to the server’s SSE endpoint.

{
  "remote-server": {
    "transport": "sse",
    "url": "http://localhost:3000/sse"
  }
}

Using MCP Tools

Once configured, MCP tools are available to the agent automatically. They appear alongside the built-in tools. You do not need to do anything special to use them — the agent discovers and calls MCP tools as needed.

You can also run /mcp inside Fermi to connect the configured servers on demand and list the discovered tools. This works before your first agent turn, which makes it a useful quick verification step.

Example: Adding a Database Tool

{
  "sqlite": {
    "transport": "stdio",
    "command": "npx",
    "args": ["-y", "@modelcontextprotocol/server-sqlite", "path/to/database.db"]
  }
}

After saving this to ~/.fermi/mcp.json and restarting Fermi, run /mcp to verify the SQLite tools were discovered. The agent will then be able to call them during normal turns.