Anthropic's AI coding tool Claude Code recently executed a large-scale code migration for the Bun runtime project, converting thousands of lines from Zig to Rust. The job, which involved rewriting significant portions of Bun's underlying infrastructure, shows how the tool can handle enterprise-level codebase transformations that would normally take teams weeks.
What Claude Code did for Bun
Bun, a JavaScript runtime designed for speed, had been built partly in Zig. The project's maintainers decided to shift to Rust for better performance, memory safety, and ecosystem compatibility. Instead of manually rewriting module by module, they turned to Claude Code, an AI assistant from Anthropic that can understand and transform large codebases.
The tool analyzed the existing Zig code, mapped out dependencies, and produced equivalent Rust implementations. The migration covered core parts of the runtime—including the HTTP server, file system operations, and the bundler—without introducing regressions that would break the build.
Code migrations are notoriously labor-intensive. Moving a project from one language to another often requires months of careful, error-prone work. Claude Code's ability to handle that at scale suggests a practical use case for AI in development beyond generating boilerplate or fixing bugs.
Anthropic has positioned Claude Code as a tool for developers who need to refactor, upgrade, or port code across languages. The Bun project provided a real-world stress test: a mature codebase, non-trivial language differences, and a tight timeline. The tool succeeded without requiring human oversight on every line.
What's in the patch
The migrated code will be folded into a future release of Bun. The team has not yet specified a date, but the work is expected to ship in the next major version. Developers following the project on GitHub can see the transformation in the open-source repository, where Claude Code's output replaced the original Zig files.
For now, Anthropic is not releasing a separate case study with exact metrics. The company has only confirmed that the migration was completed and that the resulting code passed existing tests. Outside observers will have to wait for the next Bun release to judge the performance gains from the Rust rewrite.
