12 · Agent as a Function Tool

What this lesson demonstrates

You already know function tools: a typed Go function the model may call mid-run. This lesson swaps the function for a whole agent. agenttool.New adapts an *agent.Agent into an ordinary tool.Tool, so a top-level orchestrator agent can delegate to a specialist agent exactly as it would call any other tool — no hand-written routing.

Here the orchestrator (an “Assistant” instructed to answer in French) owns a WeatherAgent as a tool, and that weather agent in turn owns a leaf weather function. Ask about Amsterdam’s weather and the call cascades two levels deep, then returns in French.

The core: an agent becomes a tool

foundryprovider.AgentConfig{
    Instructions: "You are a helpful assistant who responds in French.",
    Config: agent.Config{
        Name: "Assistant",
        // agenttool.New adapts an *agent.Agent to a tool.Tool: its Call runs the
        // inner agent with the model's {"query": ...} argument and returns the text.
        Tools: []tool.Tool{agenttool.New(weatherAgent, agenttool.Config{})},
    },
}

When the outer model decides it needs weather, it calls the WeatherAgent tool; the wrapper runs the inner agent with {"query": ...}, which may itself call the leaf weather function, and the answer bubbles back up.

What to notice

• The inner agent’s name and description become the tool’s contract. agenttool.New reuses WeatherAgent’s Name() and Description() — that’s the schema the outer model sees when deciding to delegate, so give the specialist a clear identity.
• Two levels of tool-calling, zero routing logic. The orchestrator calls the WeatherAgent tool; the weather agent calls its weather function. You wrote no dispatch code.
• Different input shape. A plain function tool wraps a single argument as Arg0; the agent-as-tool takes a {"query": "..."} object instead — its Call feeds query to the inner agent. The offline test asserts each shape.
• Construction is factored out. newWeatherAgent and newAssistant exist so the test builds both agents offline with a fake credential and asserts that wrapping the weather agent yields a tool named WeatherAgent exposing a query property.

How it maps to the SDK

functool.MustNew builds the leaf weather tool by inferring its JSON schema from a plain typed Go function; agenttool.New builds the higher-order tool from an agent. Both produce a tool.Tool, so the orchestrator treats specialist agents and plain functions uniformly. This is how you assemble specialists into a hierarchy of Foundry-backed agents without a bespoke orchestration layer.

Run it

go run ./tutorial/02-agents/agents/step12_as_function_tool

The offline structural test runs anywhere; the live end-to-end run needs az login + FOUNDRY_PROJECT_ENDPOINT and is gated behind AF_LIVE=1.

Next: 17 · Additional AI Context