A voice agent that books appointments is a system where someone talks, a model understands what they are asking for, calls real tools, and hangs up with an appointment on a calendar. I built one in a lab repo: ElevenLabs Agents handles the voice—transcription, turn-taking, synthesis—Cal.com handles the calendar, and in between sits a Fastify backend whose only job is to make sure the model never has to guess. Not what day it is, not which time the chosen option corresponds to, not how that time is written in ISO with an offset. This post is the whole walkthrough: the architecture, the three tools, the prompt, and how I tested it without burning the 15 voice minutes the free plan gives you each month.…