Bun Successfully Ports Codebase from Zig to Rust in 11 Days, Powered by Anthropic's 'Fable 5' AI
Popular JavaScript runtime Bun announced the successful porting of its entire codebase from Zig to Rust, with most of the code written by AI and completed in just 11 days.
Jarred Sumner, creator of Bun, a highly popular runtime for JavaScript and TypeScript, has announced the successful porting of the entire project's codebase from Zig to Rust. Bun version 1.3.14 will be the last written in Zig, and the upcoming version 1.4 will be the first to run on an entirely Rust codebase. Behind this remarkable achievement is the use of an unreleased AI model named 'Claude Fable 5' from Anthropic, which is Bun's parent company. Sumner revealed that with the help of Fable 5, a single engineer was able to complete the entire code migration process, achieving 100% test pass rates on all platforms, in just 11 days. This code migration project was massive, involving the conversion of 1,448 Zig language code files, totaling over 535,000 lines of code (excluding comments), into Rust code, resulting in a total code diff of more than 1 million new lines. The process involved extensive AI API calls, consuming 5.9 billion input tokens and generating 690 million output tokens, with an estimated API cost of approximately $165,000 USD (around 6 million Thai Baht). Sumner stated that the main reason for switching to Rust was for the 'reliability' of the software, and this historic change demonstrates AI's potential to efficiently assist in complex software engineering tasks that historically required immense human effort.
This is a significant case study demonstrating how modern AI can successfully assist in large-scale and complex software engineering tasks, such as porting an entire project's programming language, in a short period. This could change how software is developed and maintained in the future.