Bun ditches Zig for Rust with help from Claude Fable 5, writes over a million lines of code in 11 days
The JavaScript runtime Bun has completed a full rewrite from Zig to Rust, with Anthropic’s Fable 5 model handling most of the 1.1 million lines of code in just 11 days.
- Bun’s core runtime has been fully rewritten from Zig to Rust, a major architectural shift.
- Anthropic’s Fable 5 AI model generated over 1.1 million lines of Rust code in just 11 days.
- The rewrite aims to improve memory safety, maintainability, and ecosystem compatibility.
- This marks one of the largest AI-assisted code migrations in a production JavaScript tool.
The JavaScript toolchain Bun has undergone a complete rewrite, transitioning its core runtime from the Zig programming language to Rust. This ambitious migration was largely automated using Anthropic’s Fable 5 AI model, which generated over 1.1 million lines of Rust code in a span of just 11 days. The shift reflects a broader industry trend toward Rust for performance-critical systems, particularly in tooling where memory safety and execution speed are paramount.
The rewrite was not a gradual refactor but a full replacement of Bun’s existing codebase, demonstrating the potential of AI-assisted development in large-scale software projects. While the original implementation in Zig was already highly performant, the Rust version aims to improve maintainability, security, and compatibility with the broader Rust ecosystem. Early benchmarks suggest comparable or improved performance, though the team has not yet released detailed metrics.
Anthropic’s involvement highlights the growing role of AI in core software development workflows, where models like Fable 5 can handle repetitive, large-scale code generation tasks. This project also serves as a case study for how AI can accelerate migration efforts between programming languages, a process typically fraught with complexity and risk.
Rust’s memory safety and performance benefits are now accessible to Bun’s user base, enabling safer and faster JavaScript tooling.
Companies relying on Bun may see improved stability and reduced maintenance overhead due to the Rust rewrite.
Demonstrates the growing role of AI in core software development, potentially increasing productivity in large-scale migrations.
Shows how AI can accelerate complex software engineering tasks, setting a precedent for future tooling migrations.
- Zig
- A systems programming language focused on performance, safety, and clarity, often used for low-level tooling.
- Rust
- A systems programming language emphasizing memory safety, concurrency, and performance, widely adopted in performance-critical applications.
- Fable 5
- Anthropic’s AI model designed for code generation and software development assistance.
ProgrammingReturn on Attention: Why AI Code Reviews Are Wearing Us Out
ProgrammingMCP tool design: Practical approaches and tradeoffs
Rewriting Bun in Rust
Quoting Kenton Varda
ProgrammingAI Wrote a Thread-Safe Counter. The CPU Made It 5x Slower.
AI ToolsLocal Video Summarization Pipeline: Processing Frames with SmolVLM2-2.2B
A new open-source pipeline leverages the SmolVLM2-2.2B model to generate useful video summaries locally, running efficiently on consumer GPUs.
SAG-AFTRA Slams Meta AI for Using Your Instagram Photos, Says Opt Out - Variety
SAG-AFTRA has criticized Meta for allegedly using Instagram photos to train AI models without user consent, urging an opt-out mechanism.
Artificial intelligence raises new questions for Minnesota campaigns - InForum
Minnesota campaigns face new ethical and regulatory questions as AI tools become more prevalent in political messaging and outreach.
Microsoft's AI Drive Saw Its Carbon Emissions Grow By 25 Percent In 2025 - Engadget
Microsoft reports a 25% increase in carbon emissions in 2025, directly linked to its rapid AI infrastructure expansion.
Neuro-symbolic artificial intelligence in medicine - Nature
Neuro-symbolic artificial intelligence is being applied in medicine, according to a recent report in Nature. This technology combines symbolic AI and neural networks to improve medical diagnosis and treatment.
Accenture Federal Services Wins $821M CDAO Task Order for War Data Platform - GovCon Wire
Accenture Federal Services won an $821 million task order to build a data platform for the US Department of Defense's Chief Digital and Artificial Intelligence Office.