AG-UI Frontend Tools: The Server

What this lesson demonstrates

Frontend tools invert where the work happens. The Foundry agent still runs on the server, but the tools it wants to call are implemented on the client. When the model requests one, the AG-UI handler streams that request to the connected client over SSE; the client executes it and streams the result back.

For that to work, the server must not auto-execute function tools. That is the whole lesson, and it comes down to a single flag.

The real code

newHostedAgent builds the Foundry agent with DisableFuncAutoCall: true:

func newHostedAgent(endpoint, model string, cred azcore.TokenCredential) *agent.Agent {
    return foundryprovider.NewAgent(
        endpoint,
        cred,
        foundryprovider.ModelDeployment(model),
        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 a provider attaches middleware that auto-executes function tools locally. DisableFuncAutoCall: true turns that middleware off. The model’s tool request then flows out through the AG-UI handler to the client instead of being run server-side. buildHandler mounts the usual aguiprovider.NewJSONHTTPHandler at /.

What to notice

• The server registers no tools. Notice agent.Config here has no Tools field set — the server doesn’t own the implementations. It only needs to not swallow the calls, which is exactly what DisableFuncAutoCall guarantees.
• The handler is identical to the earlier servers. As with backend tools, the AG-UI wiring doesn’t change; the behavior difference is entirely a property of the agent config. That symmetry is the point: one protocol, and where a tool runs is a config decision.
• The offline test asserts the flag is set. main_test.go builds the same agent with a fake credential and checks that DisableFuncAutoCall is on and the handler is non-nil — no server started, no network call. The live server binds :8888 only under AF_LIVE.

How it maps to the Microsoft Agent Framework Go SDK

agent.Config.DisableFuncAutoCall is the toggle that decides whether the framework runs function tools in-process or surfaces them as unresolved calls. Combined with aguiprovider.NewJSONHTTPHandler, an unresolved call becomes an AG-UI event the client fulfills — letting a server-hosted Foundry agent drive tools that only the front end can run (browser APIs, local state, user devices).

Run it

go run ./tutorial/02-agents/agui/step03_frontend_tools/server, then in another terminal go run ./tutorial/02-agents/agui/step03_frontend_tools/client. Needs az login + FOUNDRY_PROJECT_ENDPOINT (+ FOUNDRY_MODEL). Offline tests run with go test ./...; the live bind skips without AF_LIVE=1.

Next: AG-UI Human-in-the-Loop — The Server