AI Rewrote 1 Million Lines of Code in 11 Days? The Amazing Transformation of 'Bun'

Imagine this: you have a massive library with over a million books, and you need to translate them into another language. What if you could finish this colossal task, which would take years for a human to do, in just 11 days? Recently, something just as amazing actually happened in the field of software development.

The AI model ‘Claude’ has completely rewritten the core foundation of ‘Bun’ (a JavaScript runtime, which is a programming language that runs in web browsers) into an entirely new language called ‘Rust’ (a system programming language that emphasizes memory safety and performance), totaling over 1 million lines of code [Source 9, Source 13]. In this article, we’ll easily explain why this large-scale code migration is important and what it means for our daily lives.

Why is this important?

“Bun” is a tool that helps developers execute JavaScript or TypeScript code faster and more efficiently [Source 3, Source 4]. But why swap this vital tool from its original language to Rust?

The biggest reasons are ‘safety’ and ‘speed.’ The Rust language allows for safer computer memory management, reducing the frequency of programs unexpectedly crashing [Source 3, Source 10]. It is also advantageous for performance optimization. Indeed, after this rewrite, ‘Claude Code’ (an AI-assisted programming tool) saw its startup speed in Linux environments improve by 10% compared to before [Source 1, Source 7]. While this might feel subtle to general users like us, it is a technically significant advancement.

Putting it simply: It’s like changing a recipe

Let’s use an analogy. Imagine you run a large restaurant serving meals to thousands of people. At first, you crafted your recipes precisely using a tool called ‘Zig.’ However, because you wanted to deliver meals more safely and efficiently, you decided to change your recipes entirely to ‘Rust,’ a new tool most trusted by chefs around the world.

In the past, humans would have had to rewrite this vast collection of recipes one by one. But this time, a ‘superhuman AI assistant’ named Claude wrote the recipes for you. Jarred Sumner, the founder of Bun, set up about 50 AI workflows and directed Claude Code to work non-stop for 11 days to translate over 1 million lines of code into Rust [Source 12, Source 13]. If humans had done it, it would have taken three people a year to finish; this time, it was completed in a short period with AI [Source 16].

Current Situation: The era where AI directly manages code

Starting from Claude Code version 2.1.181, this new Rust-based Bun runtime is now included and provided [Source 1, Source 7]. Developers continue to write code as before, but the engine running in the background has been replaced with a much safer and faster Rust-based engine.

Of course, not everyone is applauding this massive AI-driven code modification. Some voices express concern that the validation process for AI-generated code might be insufficient [Source 13]. However, Anthropic (the creator of Claude) has proven through this project just how successfully AI can handle complex and massive software projects [Source 9, Source 16].

What will happen in the future?

This case shows that AI has gone beyond simply answering questions or writing text, and can now become the ‘subject of engineering’ that directly changes the foundations of complex technology [Source 9, Source 10]. In the future, when the apps or services we use are updated to be safer and faster, it is highly likely that an AI colleague will be working day and night behind the scenes, modifying code alongside human developers.

Moving forward, we will be greeted by a software environment that is faster and more powerful, brought about by complex technical transitions created by AI. Change has already begun, and its speed is far exceeding our expectations.

MindTickleBytes AI Reporter’s Perspective

This event isn’t just about changing one programming language. The fact that AI accomplished a grueling task that would have taken humans a year to do in just 11 days signifies that the very definition of ‘software maintenance’ has changed. Now, rather than fearing technological change, it is time to contemplate how to cleverly utilize AI, our tool, to accelerate the future we desire.

References

1. Claude Code uses Bun written in Rust now - simonwillison.net
2. Claude Code uses Bun written in Rust now - daily.dev

3. [Claude Code uses Bun written in Rust now

   DeepHorus](https://www.deephorus.com/blog/2026-07-19-claude-code-uses-bun-written-in-rust-now/)

4. [Claude Code uses Bun written in Rust now

   AINews](https://www.ainews.tech/article/2058)

5. [Rewriting Bun in Rust

   Bun Blog](https://bun.com/blog/bun-in-rust)

6. Claude Code adopts Rust-based Bun runtime for faster startup …
7. Claude Code adopts Bun runtime rewritten in Rust, speed …

8. [Bun Rewrites in Rust: Technical Review of the Zig-to-Rust Migration

   Fawad Hussain Syed](https://fawadhs.dev/blog/bun-rust-rewrite-technical-review)

9. Claude Rewrites Bun’s Million Lines of Code in 11 Days for $165,000, Setting a New Benchmark for AI-Assisted Programming — BigGo Finance
10. Burned $160,000, Wrote 1M Lines of Code Nonstop: How Bun’s Founder Rewrote the Entire JavaScript Runtime Foundation Using Claude AI

11. [AI Porting: Claude Rewrites Bun Codebase in Rust

    heise online](https://www.heise.de/en/news/AI-Porting-Claude-Rewrites-Bun-Codebase-in-Rust-11294318.html)

12. How Bun’s founder rewrote the codebase in Rust with Claude
13. Zig creator calls Bun’s Claude Rust rewrite ‘unreviewed slop’

14. [Why not rewrite claude-code in Rust? So, Anthropic acquires Bun team because cla…

    Hacker News](https://news.ycombinator.com/item?id=48019019)

15. One Anthropic Engineer Rewrites Bun In Rust In 11 Days With AI, Says Would’ve Taken 3 Engineers A Year Earlier