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Posted by u/kylemacomber 4 months ago
Launch HN: Bitrig (YC S25) – Build Swift apps on your iPhone
Hi HN, we’re Kyle, Jacob, and Tim. We’re building Bitrig (https://www.bitrig.app).

Bitrig lets you create native Swift apps for your phone, on your phone, just by chatting with AI. It’s like Lovable for iPhone apps.

Here's a video of Bitrig in action: https://youtu.be/CUlWhF3ERME

We created SwiftUI at Apple to help developers make better apps with less code. Bitrig lets anyone build at this level of polish. If you've thought about making an iPhone app, Bitrig is the easiest possible way to get started with Swift.

Bitrig uses Claude Sonnet 4.0 with a simple system prompt and tool definitions to generate native Swift code. Normally running this on an iPhone would require compiling and signing it with Xcode, and you can’t run Xcode on an iPhone. So we did something… creative. We wrote a custom Swift interpreter! Among other things this lets you instantly preview your app in Bitrig and share it with just a URL.

If you have a paid Apple developer account, you can connect it with Bitrig. We’ll compile your app on our server and upload it to App Store Connect, so you can distribute it on TestFlight or the App Store. This last step also gives you a fully optimized build of your app that you can install right on your Home Screen.

We think there’s something electric about building apps directly on your phone. We hope you give Bitrig a try!

We’re ingesting Apple’s SDK frameworks into Bitrig piece by piece. If you try to build something and hit a missing framework, let us know and we’ll prioritize adding it.

Download Bitrig on the App Store: https://apps.apple.com/us/app/bitrig/id6747835910

chrislattner · 4 months ago
Super impressive app and experience, it is incredible that you can get Swift to do this with such interactivity! Rebuilding Swift to be interpreted is a bold move,

-Chris

zahirbmirza · 4 months ago
This app is really quite fascinating as well as amazing. I have been using it for a week now. I do like how it is so good with Swift and iOS; you have done a great job of removing so much of the confusion that LLMs must face when trying to develop for a demanding platform. The API insight the app has is a testament to your deep background and foresight into iOS development. I really love this.

Also, I have been taking the code and advice from Bitrig into Cursor, because it solves problems Cursor struggles with no matter what model it calls.

Your work is empowering, but more than that, it is inspiring, because I can now start to envisage what computer human interaction will look like in the near future.

You have taken away a layer of complexity to creation and that is to be applauded. I think your work is a landmark. I think Bitrig will unleash a new era of creative expression. I wish that I was part of your team.

kylemacomber · 4 months ago
Wow that's really high praise. Thank you.
debdut · 4 months ago
This is crazy, you guys basically rebuilt Swift (interpreter instead of compiler) and also the frameworks. This is the level of engineering that does the magic of "it just works". I, personally, am very interested in swift, and would love to work for free for you guys.
kylemacomber · 4 months ago
One clarification: we did not rebuild any of the frameworks. As quickly as possible, the interpreter calls out to the compiled frameworks in the OS. So Bitrig is calling into the real SwiftUI, the real Foundation, the real MapKit, etc.
tomaskafka · 4 months ago
A subset of swift, it seems - the instructions say to avoid async/await for example, but impressive nonetheless!
iddan · 4 months ago
When I was in middle school I remember a friend lieing that there is “a mobile app to create mobile apps”. I thought it’s an amazing idea, but it was way too early. I love how this hallucination became a reality. Good luck!
Jotalea · 4 months ago
Not a lie anymore! There's CodeAssist for Android

https://play.google.com/store/apps/details?id=com.tyron.code

And well, the app the post is about, for iOS.

FlamingMoe · 4 months ago
Neat. I just tried it out and one thing I was pleasantly surprised about was the styling it came up with. It was sleek, and it did not have the purply gradienty tailwindy styling that I always get from Claude. Did you do much to adjust the prompt for custom styles?
kylemacomber · 4 months ago
Styling apps is an area we're excited to spend time exploring. Today in our system prompt we say "ALWAYS make the design Apple-like. Use clean typography and consistent padding/spacing." However, tbh it's not something we carefully tested.

For the most part, I think the look and feel of the apps benefits from SwiftUI baking Apple's design system into the defaults so heavily.

VenturingVole · 4 months ago
This is really cool of you to openly share the prompt - props for that.

Really cool product, as someone currently attempting to build a somewhat similar internal tool I have an understanding of some of the pain points involved.

Please don't allow yourselves to be bought out by Apple in the way Buddy Build were back in 2018 though! (and then shut down)

kylemacomber · 4 months ago
On a personal note, I wanted to thank the HN community. I’ve been reading HN since college (for over 15 years now!) and it’s been formative in my development as a software engineer and leader.

I don’t pipe up very often, but I visit HN almost every day. Many of the books I read, blogs I frequent, and podcasts I listen to, I found via HN. I think it’s fair to say, if it weren’t for HN, that Bitrig wouldn’t be Bitrig and SwiftUI wouldn’t be SwiftUI.

codruterdei · 4 months ago
That’s great and I’m glad you feel good here. I think it’s time for you to share what you found over the years on HN with us. Your curated list, if you will.
kylemacomber · 4 months ago
This isn't exhaustive, but quickly top of my head:

- How to Win Friends and Influence People — This was really eye opening to me and helped me more productively engage in a large organization like Apple.

- Sam Harris's podcast — I'm always interested to hear his takes on the controversial topics of the day as well as more philosophical and philanthropic ones.

- The Knowledge Project podcast — I don't listen to this as much anymore but a few years ago I was a regular listener.

- Simon Willison’s blog — The highest signal to noise way I've found to stay up to date with developments in AI.

rufugee · 4 months ago
Hello fellow lurker. I for one am glad you are here ;-)

And I echo the thanks. HN has made me a better technical professional in so many ways.

spike021 · 4 months ago
Would you consider having a way to export work done in this app to be included as part of regular Xcode projects?

I've been working on a project and sometimes when I'm away from my laptop I have ideas I want to try in SwiftUI but only have my phone on me. What I end up doing is just writing my idea in text and then waiting till I'm back at my desk to give it a shot. Being able to spin up a quick prototype and preview it on the go could be cool.

I don't think my dog would enjoy me lugging my laptop on our walks but my phone is no big deal ;).

kylemacomber · 4 months ago
Totally! I have the same use case when I’m, say, at the park supervising my kids.

You can export your entire project as a single swift file from the share sheet. This makes it pretty straightforward to import the code into an Xcode project when you’re back at your Mac

jaccola · 4 months ago
It doesn't seem like this is the direction you are going, but this feels like what Siri could become (though I have little faith given the history).

With your sunscreen example, I should be able to just ask Siri to do exactly this, and it could tap me in my pocket, show me a custom UI to log periodically and then disappear.

Not sure on limitations of APIs on iOS but definitely feels like there is space for a better voice assistant, just like how Raycast have created a better Spotlight on Mac.

Looks very cool so far!

jacobx · 4 months ago
Yeah, I think that whole area of dynamically generating UI is really interesting. And having a Swift interpreter like we're using would unlock doing that with native UI components.

Our background is more in making developer tools, so bitrig was the first product we were excited about, but I think there's a lot of overlap with dynamic UI generation, and we'd love to explore that too.

ignoramous · 4 months ago
Nice. Are you folks thinking about Kotlin/Flutter on Android too, or is it going to be exclusively focused on the ecosystem around Swift?
spike021 · 4 months ago
Maybe this paired with something like App Clips? I don't see them used much these days but could make them more useful.