use Legion.Tool to mark a module as a tool available to agents.

By default, description/0 returns the module's source code so the LLM knows what functions are available.

Overridable

• description/0 — override to return a hand-written summary instead of the source code. Defaults to the module's source code.
• description/1 — like description/0, but receives the active sandbox module, for tools whose usage differs by generated language. Preferred over description/0 when defined.
• extra_allowed_modules/0 — override to return additional modules that the sandbox should alias and permit when this tool is available. Defaults to []. Useful for tools like Legion.Tools.AgentTool that dispatch to other modules the agent needs to reference by name.

Example

defmodule MyApp.WeatherTool do
  use Legion.Tool

  def description do
    """
    WeatherTool — fetches current weather data.

    ## Functions
    - `current(city)` — returns weather JSON for the given city name.
    """
  end

  @doc "Returns current weather for a city."
  def current(city) do
    Req.get!("https://wttr.in/#{city}?format=j1").body
  end
end

External modules as tools

A module that does not use Legion.Tool (e.g. Req) can still be listed as a tool if its source code is registered at compile time, so the LLM can read what it offers:

config :legion, extra_source_modules: [Req]