What they did
The poster cancelled their Whoop membership but still had the strap, and wanted to keep their health data private. They tried an existing open alternative, missed the original app's look, and instead had Claude Opus 5.5 write a full copy of it for iOS and Android, working only from screenshots and screen recordings of the official app. The apps send readings to the poster's own web service, which retrains the metric models every night. Beyond the core health and stress views, the build covers automatic activity tracking, a logbook, fitness plans, a homemade fitness-age score and a federated community tab meant to let anyone host a backend in a Docker container.
How it went
About 60 hours of work in total: roughly 23.5 hours on the iOS app, 18 on model tuning, 10 on the backend and 9 on the Android app. The poster puts usage at about 2.4 billion tokens, which Claude estimated at around $800 at API prices, mostly cache reads, all covered by a Max 5x plan. Readings like respiratory rate, blood oxygen, resting heart rate, HRV and skin temperature land within about 5% of the official app, and the fitness-age score within 8 to 10%. Everything except the strength trainer is done. It works with the MG, 4.0 and 5.0 straps.
Worth knowing
The poster will not publish the code, so none of this can be checked, and the demo video shows a simulator with fake data. The post names the model and plan but not the harness; it was posted to r/ClaudeCode. Past history can come from a Whoop data export, and the poster says the app can run fully offline with sync limited to the home network.