Exercises 13 · AI Voice Agent Backend

Start from workflow/starter.json (endpoint, normalizer, availability, formatter) or your finished lesson workflow.

Exercise 1: Parallel tool calls

Task: Vapi can send two tool calls in one request (e.g. lookup_customer and check_availability). Make the workflow reply with one response containing both results.

Done when: a Vapi payload with two calls returns results with two entries, each with the right toolCallId.

Hint

When items take different Switch branches, Format for Platform runs once per branch. Put a Merge (mode Append, number of inputs = 5) before it, so all branches are combined first.

Exercise 2: Alert the owner

Task: Replace Notify Owner (Exercise 2) with an SMS, Telegram or email to the business owner containing the caller's number, intent and follow-up reason.

Done when: WITH_CREDS=1 ./test/test.sh (the emergency transcript) triggers the alert.

Hint

Use the fields from Call Record. For SMS, Twilio's Send SMS node; for free testing, Telegram.

Exercise 3: Real calendar

Task: Make check_availability and book_appointment use Google Calendar instead of static data.

Done when: booking via the API creates a calendar event, and availability excludes busy times from the calendar.

Hint

Google Calendar → Get Many events for the day (busy times), then compute free slots in a Code node. Book with Create Event. Keep the same { ok, message } contract so nothing else changes.

Stretch: Verify the caller is really Retell

Reject requests without a valid X-Retell-Signature header (Retell signs the body with your API key). This stops anyone who finds your URL from booking appointments.