Drive-thru coffee

A drive-thru order is a small state machine: add items, change them mid-flow, total the cart, and close with a name on the cup. This agent runs that whole loop by voice and mirrors the cart to the screen as it goes. It is the cookbook's first demo, so it also sets the UI scaffold every later demo copies.

The stack is Deepgram Nova-3 for STT, gpt-4o-mini for the tools, and Cartesia Sonic-2 for the voice. Turn-taking is LiveKit's inference VAD and turn detector.

Money is integer cents everywhere, formatted only at the edge, so a total never picks up a floating-point rounding bug:

def _fmt(cents: int) -> str:
    return f"${cents // 100}.{cents % 100:02d}"

The cart is one list in userdata. add_item looks the item up in the menu, appends it, and re-publishes the Order and Total; remove_item is the inverse operation, so the cart and the screen never drift:

cart = context.userdata["cart"]
cart.append(item)
_publish_cart(self.room, cart)

submit_order closes the loop: it refuses an empty cart, takes a pickup name, and publishes a Checkout card with the total, so the customer sees the order land before they pull forward.

The scaffold worth copying is UI idempotency. The playground wants exactly one mount per component id, then update for every later change, so a helper tracks which ids are already up on the room object:

def _ui_action(room: rtc.Room, component_id: str) -> Literal["mount", "update"]:
    mounted = getattr(room, "_awesome_voice_ui_mounted", None)
    if mounted is None:
        mounted = set()
        setattr(room, "_awesome_voice_ui_mounted", mounted)
    if component_id in mounted:
        return "update"
    mounted.add(component_id)
    return "mount"

Publish every change as mount and the playground stacks a new card each time instead of updating in place. Adding a shot draws a second order card on top of the first. Mount once per id, then update.

Build it from an empty folder in the full walkthrough, or talk to the finished agent at https://playground.mahimai.ca/demos/drive-thru-coffee.