Jarred Sumner, the creator of the JavaScript runtime Bun, completed a full rewrite of the project's codebase from Zig to Rust in 11 days using AI agents. The migration addressed significant stability issues, as Bun had previously experienced memory leaks and crashes due to Zig not being a memory-safe language. Jarred utilized Anthropic's Fable, an AI agent system, to handle the conversion of 535,496 lines of code across 1,448 files. The process involved 64 parallel AI agents working in four shards to maintain efficiency while avoiding conflicting git commands.

The technical workflow began with a three-hour preparation phase, resulting in a 600-line porting guide to map Zig patterns to Rust. After the initial rewrite, the team ran two separate adversarial reviews by Claude to verify the changes. Despite the initial code being non-compilable, the agents resolved 16,000 compiler errors autonomously over a 12-hour period. Final validation included running the project's existing test suite, which ensured the new Rust codebase maintained feature parity with the original.

The project incurred an API cost of $165,000, consuming 5.9 billion uncached input tokens, 690 million output tokens, and 72 billion cached input tokens. While this cost is significant, Jarred estimates that a manual rewrite by a small team would have taken at least a year and halted all other development. The successful migration demonstrates that complex code rewrites can be executed rapidly provided the codebase has an extremely robust test suite and an experienced engineer providing direction. This blueprint for AI-assisted migration suggests a potential shift in how software teams manage large-scale technical debt, though experts note it requires high-stakes investment and strong domain expertise to ensure project success.

Jarred Sumner, the creator of the JavaScript runtime Bun, completed a full rewrite of the project's codebase from Zig to Rust in 11 days using AI agents. The migration addressed significant stability issues, as Bun had previously experienced memory leaks and crashes due to Zig not being a memory-safe language. Jarred utilized Anthropic's Fable, an AI agent system, to handle the conversion of 535,496 lines of code across 1,448 files. The process involved 64 parallel AI agents working in four shards to maintain efficiency while avoiding conflicting git commands.

The technical workflow began with a three-hour preparation phase, resulting in a 600-line porting guide to map Zig patterns to Rust. After the initial rewrite, the team ran two separate adversarial reviews by Claude to verify the changes. Despite the initial code being non-compilable, the agents resolved 16,000 compiler errors autonomously over a 12-hour period. Final validation included running the project's existing test suite, which ensured the new Rust codebase maintained feature parity with the original.

The project incurred an API cost of $165,000, consuming 5.9 billion uncached input tokens, 690 million output tokens, and 72 billion cached input tokens. While this cost is significant, Jarred estimates that a manual rewrite by a small team would have taken at least a year and halted all other development. The successful migration demonstrates that complex code rewrites can be executed rapidly provided the codebase has an extremely robust test suite and an experienced engineer providing direction. This blueprint for AI-assisted migration suggests a potential shift in how software teams manage large-scale technical debt, though experts note it requires high-stakes investment and strong domain expertise to ensure project success.