Moving Beyond the Black Box: How I Built a Real-Time Voice Fitness Coach using Next.js 15, Convex, & Vapi.ai

Moving Beyond the Black Box: How I Built a Real-Time Voice Fitness Coach using Next.js 15, Convex, & Vapi.ai

I've used a lot of fitness apps. I've followed their calorie numbers, done their workout splits, and trusted their recommendations - all without ever understanding where any of it came from. The number just appeared, authoritative and unexplained, and I was supposed to trust it.

Eventually, I stopped. Not because the numbers were wrong. Because I had no reason to believe they were right.

That frustration became FitExplain - a full-stack web application that generates personalized fitness and nutrition plans through a voice conversation, and then shows you exactly how every number was calculated. No black box. No mystery algorithm. Just metabolic science, made visible.

This post is a complete technical breakdown of how I built it.