Developer builds voice appointment-booking agent using ElevenLabs, Cal.com, and Fastify

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A software developer has created a voice-based appointment booking system that integrates ElevenLabs for speech handling, Cal.com for calendar management, and a Fastify backend to control the interaction flow. The agent conducts conversations in Spanish, guiding users through a fixed ten-step process from greeting to confirmed booking without the AI model ever handling date formatting or payload assembly directly. A key design decision places a TypeScript backend between the AI agent and the Cal.com API, ensuring all timezone arithmetic, slot selection, and booking payloads are handled deterministically in code rather than left to the language model. The system returns tool errors as HTTP 200 responses with readable sentences to prevent the model from improvising mid-call, and booking actions require explicit user confirmation to avoid irreversible mistakes. The full project code is publicly available on GitHub, and the post details the architecture, prompting strategy, and how the developer tested the system while staying within ElevenLabs' free-tier voice minute limits.

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