Former Twitter CEO Jack Dorsey 'vibe codes' second app in a week; this one tracks your Vitamin D

Jack Dorsey

Jack Dorsey, Twitter co-founder and blockchain evangelist, is on a weekend app-building spree. After releasing Bitchat, a peer-to-peer encrypted messaging platform just days ago, he’s back with another surprise drop: Sun Day, a health-focused app that tracks your UV exposure and estimates your vitamin D intake.

Available now for iOS via TestFlight, Sun Day taps into your location to show UV index and local sunlight hours. You can input details like skin type and clothing coverage, then toggle when you’re in or out of the sun. The app will estimate how much vitamin D your body’s producing — and warn you when you’re at risk of burning. Think quantified sunbathing, with some biohacker vibes.

The app is open source, with its code published on GitHub, and it’s the latest in Dorsey’s experimental “weekend builds” — side projects he’s coding using Goose, an open-source AI coding tool that’s part of a growing wave of “vibe coding” platforms. These tools, including Windsurf and Cursor, are fast becoming the new frontier in AI development, and big tech has taken notice. Just last week, OpenAI’s $3 billion bid for Windsurf collapsed, paving the way for Google DeepMind to hire the startup’s CEO and senior devs in a $2.4 billion talent deal.

Dorsey says his aim is to learn through building — and given the pace he’s moving at, it’s hard to argue. He seems to be leaning into sovereign tech, user privacy, and wellness — a mix of Web3, personal health tracking, and AI-assisted solo development.