Book calendar appointments with availability checks in Google Calendar

Description

Quick overview
This workflow is triggered by another n8n workflow to verify a requested appointment slot and, if available, books a 30‑minute event in Google Calendar, returning a booked/unbooked response with formatted date/time details.
Full setup guide: https://uzzaman.dev/guides/n8n-google-calendar-availability-booking/

How it works
Receives appointment details (name, phone, service type, preferred date, preferred time) when called by another n8n workflow.
Calls a separate “Check Slot Availability” workflow to determine whether the requested slot is available.
If the slot is available, builds ISO start/end timestamps for a 30-minute appointment window.
Creates the appointment as a Google Calendar event with a summary and description populated from the input details.
Returns a success payload with booked: true and formatted preferred date/time, or returns booked: false with an error code when the slot or calendar booking fails.

Setup
Create or identify a “Check Slot Availability” workflow and update the referenced workflow ID/name so it returns an available boolean.
Add a Google Calendar OAuth2 credential and set the target calendar (email/ID) in the Google Calendar event creation step.
Ensure the calling workflow passes name, service_type, preferred_date (ISO date), preferred_time (HH:mm), and phone, and adjust the default 30-minute duration if needed.

Requirements
External AI voice platform (Vapi, Retell, or similar) & Published appointment management sub-workflows
MCP tools for Vapi and Retell voice agents
A configured "Check Slot Availability" sub-workflow (or your own equivalent)
Calling workflow must pass name, service_type, preferred_date, preferred_time, phone

Customization
Change the default 30-minute appointment duration in the event-creation step
Swap the Google Calendar node for a different calendar/CRM system
Adjust the fields included in the event summary/description