Frontend Tools

What this lesson demonstrates

This is the mirror image of the backend-tools lesson. Here the tool lives on the client, not the server. The client advertises a tool (get_user_location) to the AG-UI server on every run; when the model decides to call it, the server does not execute it — it streams the tool call back down to the client, which runs the Go function locally and returns the result. This is how a UI lends the agent a capability only the client has: GPS, the DOM, the logged-in user’s session, a local file picker.

The lesson is a client+server pair, and the load-bearing detail is a single server flag.

The server: don’t auto-run the tool

foundryprovider.AgentConfig{
    Instructions: "You are a helpful assistant.",
    Config: agent.Config{
        Name:                "AGUIAssistant",
        DisableFuncAutoCall: true, // client owns the tools, so don't auto-run them here
    },
}

Normally the provider attaches middleware that auto-executes function tools server-side. DisableFuncAutoCall: true turns that off. Now when the model asks for a tool, the AG-UI handler forwards the request to the client instead of trying to run it. The server itself registers no tool — it only hosts the model.

The client: the tool runs here

func newFrontendTool() tool.Tool {
    return functool.MustNew(functool.Config{
        Name:        "get_user_location",
        Description: "Get the user's current location from GPS.",
    }, func(ctx context.Context, in struct{}) (string, error) {
        return "Amsterdam, Netherlands (52.37°N, 4.90°E)", nil
    })
}

The handler is a closure that runs in this process — that is the whole point. The tool is registered via agent.Config.Tools on the AG-UI-backed agent, so its name, description, and schema are advertised to the server on every run. Ask “Where am I?” and the model calls get_user_location, which executes right here on the client.

What to notice / the gotcha

Both halves must agree. If the server forgets DisableFuncAutoCall: true, it will try to run a tool it doesn’t own and the call fails; if the client forgets to register the tool, the model never sees it. The tool round-trip itself is handled transparently by the provider middleware — the client’s streaming loop is identical to earlier lessons; the local execution happens under the hood.

How it maps to the Microsoft Agent Framework

Frontend tools are how the framework keeps client-only context out of the server. The model, hosted against Azure AI Foundry, can reason about “where the user is” without the server ever knowing the user’s coordinates — the client answers that call and returns only what it chooses to. It is the same functool + agent.Config.Tools authoring you already know; only which side holds the handler, plus the server’s opt-out flag, differs.

Run it

go run ./tutorial/02-agents/agui/step03_frontend_tools/server   # terminal 1
go run ./tutorial/02-agents/agui/step03_frontend_tools/client   # terminal 2

The tool and agent wiring build/test offline against a dummy endpoint; the live run is gated behind AF_LIVE=1.

Next: Human In Loop