Someone reverse engineered the Bluetooth protocol of a $7 smart ring from Temu, built their own iOS app from scratch, and open sourced the whole thing. The app keeps your health data local, has an optional AI coach, and costs nothing beyond whatever you spend on API keys. I’ve been thinking about this for a couple of days now and I can’t quite let it go.

The backstory is worth understanding. The person who built it started by looking at the Google Fitbit Air, which wraps an LLM around your health data and gives you daily briefs and a conversational coach. The concept is genuinely good. The execution involves paying $100 upfront, then $10 a month, and handing Google a continuous stream of your heart rate, sleep cycles, and whatever else the band picks up. Whoop is worse: up to $360 a year, and your biometric data sitting on their servers indefinitely. There’s no world in which that ends well. Health insurers are already creative enough without being handed a granular record of your cardiovascular fitness.