Jack Dorsey is shipping. On Sunday, the Twitter co-founder shared a link on X (formerly Twitter) to a new app called Sun Day, which he claims helps users track their vitamin D intake. It’s the second new app he’s debuted in a week, and it’s all thanks to the help of AI. 

Sun Day calculates how long users can safely soak up the sun before burning their skin using the UV index of the user’s location, cloud cover, sunrise and sunset times, skin tone, and even what kind of clothes they’re wearing. This feature is no doubt helpful to people like Dorsey, who casually mentions later in the thread that he does not use sunscreen. 

The app also tracks a user’s time outdoors to supposedly estimate how much vitamin D they’re absorbing. This is based on “UV exposure using a multi-factor model based on scientific research,” according to the app’s GitHub page. Just how accurate that model is remains unclear.

Future updates will improve calculations with factors like height, weight, and the user’s actual vitamin D blood levels. For now, anyone who is curious can try Sun Day themselves on iOS via TestFlight or poke around its code on GitHub.

Like Dorsey’s other recent “weekend project,” BitChat, Sun Day was built using Goose, the AI coding assistant developed by Block, Dorsey’s payments company. And both projects are part of a broader trend Dorsey seems into right now, “vibe coding.”

Vibe coding is a newish approach where developers rely heavily on AI assistants to generate and debug code using natural language prompts. This allows developers to focus more on the overall “vibe” of what they’re trying to accomplish with an app rather than the technical specifics of its code. 

The previous Sunday, Dorsey announced the beta launch of BitChat, a messaging app built for peer-to-peer conversations over Bluetooth mesh networks instead of the internet that does not require any phone numbers, emails, or any permanent IDs to function. 

“[B]itchat addresses the need for resilient, private communication that doesn’t depend on centralized infrastructure,” Dorsey explained in a white paper published to his GitHub page. “By leveraging Bluetooth Low Energy mesh networking, bitchat enables direct peer-to-peer messaging within physical proximity, with automatic message relay extending the effective range beyond direct Bluetooth connections.”

But BitChat has already run into skepticism. 

In a recent blog post, Supernetworks CEO Alex Radocea pointed out a major flaw: the app does not currently have any real way to verify who a user is talking to. 

“In cryptography, details matter,” Radocea wrote. “A protocol that has the right vibes can have fundamental substance flaws that compromise everything it claims to protect.”

Since the blog post, the app’s GitHub page has added a warning stating that the app “has not received external security review and may contain vulnerabilities and does not necessarily meet its stated security goals.” 

It’s just a reminder that vibecoding might be a way for developers to brute-force their way to a functional product, but users might want to think twice before installing these apps and taking on the potential security risks.