Jarred Sumner, creator of the Bun JavaScript runtime, announced he rewrote the project from Zig to Rust using a pre-release version of Anthropic's Claude Fable 5 over just 11 days. He claimed the task would have taken three engineers approximately a year to complete manually.
The rewrite comes after Anthropic acquired Bun in December 2025, with Sumner and other team members now working at the AI company. This dramatic acceleration illustrates how advanced AI models are reshaping software development cycles.
Sumner disclosed the acquisition and his Anthropic affiliation in his announcement. The pre-release nature of the Claude model suggests this capability may not yet be available to the broader public.
Such a rapid language migration could set a precedent for other major open-source projects, potentially altering how maintainers approach large-scale codebase changes. Critics may question the reliability and maintainability of AI-generated code at this scale.
The long-term stability of the rewritten codebase remains unproven. Without third-party verification, claims about development time savings rely solely on Sumner's account.