How I Built an AI Running Coach Pipeline: From Garmin Webhook to Custom GPT

The Problem I'm a runner who uses Intervals.icu to track training metrics — CTL (Chronic Training Load), ATL (Acute Training Load), TSB (Training Stress Balance), VDOT, HR zones. Great data platform, but it doesn't tell you what to do next . ChatGPT can reason about training — but without data, it hallucinates. It'll suggest pace zones for a runner it knows nothing about. I needed a bridge. The Architecture Garmin Watch → Intervals.icu (direct sync, not Strava) → Webhook (ACTIVITY_UPLOADED / CALENDAR_UPDATED) → Next.js API Route → Processing Pipeline → PostgreSQL (Prisma) → Custom GPT (via Actions API) → Plans written back to Intervals.icu calendar Stack: Next.js 16 (App Router), TypeScript strict, Prisma 7, PostgreSQL 16, grammy (Telegram bots), Mistral API + OpenAI fallback, Docker Compose + Caddy. The Webhook Handler When Intervals.icu fires a webhook, the handler needs to: Validate HMAC-SHA256 signature Route by event type (activity uploaded, calendar updated, activity deleted) Ret