Bun's creator says he rewrote Bun from Zig to Rust using a Claude Fable 5 prerelease version in 11 days, noting it would've taken three engineers “about a year”
bun.comJarred Sumner
Context & Ripple Effects
Related coverage has already put Claude’s coding tools in progressively more demanding settings: Anthropic described parallel agents producing a Rust-based C compiler, while a hands-on account reported complex project work with a Fable 5 prerelease.
This report extends that arc from greenfield or task-focused generation to a claimed migration of an existing codebase. It also arrives as coverage characterizes Claude Code as a leading product in the emerging AI coding-tools market.
First-order effects
Bun’s maintainer now has a Rust rewrite produced with substantial AI assistance to review, test, integrate, and maintain; the reported schedule changes the immediate engineering trade-off around the migration.
The claim gives Anthropic a high-visibility developer-tools case study for its prerelease Fable 5 model, beyond the company’s own account of agent-built compiler work.
Second-order effects
AI coding vendors will face a higher bar for demonstrating that agents can handle repository-scale changes, not merely generate isolated functions or prototypes; verification and maintainability become central comparison points.
Teams considering language migrations or other broad refactors may increasingly test agent-assisted workflows, while preserving human review because a fast rewrite does not by itself establish production correctness.
Third-order effects
If comparable results can be independently repeated, software modernization could shift from being primarily constrained by implementation labor toward being constrained by specification, testing, code review, and operational accountability.
The durable competitive advantage in coding agents may move beyond raw code generation toward reliable multi-step execution across large codebases, including tools that make generated changes auditable and safe to merge.
The trend: This is one data point in the shift of AI coding tools from developer assistants toward agents used for repository-scale engineering work.
Seems to suggest that if it stops being the case that there's 3 labs which are all roughly equally good, competing each others margins away, the provider of the best model could probably get away with charging *a lot* more than they currently are. [image]
I have never actively wanted a programming language to fail but thanks to Andrew Kelley's scurrilous, outrageous, classless post on @jarredsumner 's bun rewrite, I got there. Port every project from Zig to Rust.
The Bun rewrite was dirt cheap even at Fable pricing. For comparison, it took 3-5 engineers (about half the core team) about ~14 months to rewrite @Turborepo in Rust between 2023 and 2024. When you consider the opportunity cost on top of eng salaries, it actually cost several
I'm glad the Bun rewrite blog post focused mostly on the methodology of the port! It's a good post! Good work @jarredsumner. My favorite part is the little detail about the importance of clean context agents for doing different tasks to avoid biases. I've found this is a good
this should be a huge update in your model of software engineering: rewrites can be good, cheap and fast of course most apps are not as testable and verifiable as Bun, but the models will continue to get better at filling those gaps
Frankly, I've never fully understood the hype around Zig, beyond the fact that it is more modern than C/C++. For a casual project, Zig might be fine. But for a large-scale production project with complex ownership and lifetime issues, Rust is probably a better option. For Bun
Recommended reading. $165,000 for the port from Zig => Rust. Full of interesting details and lots to learn on how to leverage AI (in general, not just for such monumental tasks). I love using Bun => even more bullish now
Well worth reading. Made me think about token cost per ported line. Bun -> Rust: $160,000/1,009,257 =$0.16 ~$0.16 Kubernetes -> TypeScript: $3,044/126,642 = ~$0.02 But then Jarred took 11 days, I took 2 months. Also very different ports.
Jarred used Claude Code Dynamic Workflows to rewrite Bun's massive codebase from Zig to Rust. Completed in 11 days. Tests passing for all 6 platforms. Many memory leaks solved by switching to Rust. 5.9 billion uncached input tokens, 690 million output tokens, and 72 billion
I appreciate the thoughtful blog post. As someone that bet on bun early in multiple ways, including dropping everything to build electrobun pre-LLM-agents and investing 2+ years on and off on it, the last few months where it sounded like humans were no longer looking at the bun […
ok upon reading this it clears everything up! amazing. specifically, when Jarred announced he rewrote Bun in Rust it was before any announcement of Fable so hearing that, assuming it was Opus, had me thinking he was smoking crack Fable doing this makes more sense
I love the part of the bun rewrite that shows what's possible and pushes people to ask their models for more I am concerned about the part where people try to do this for things that 1) aren't already designed/architected 2) don't have such a comprehensive test suite and
I remember when at my previous company Doctrine we rewrote our frontend from AngularJS 1.7 to React. It was a huge project and a big decision. It took roughly six months and a team of talented software engineers. That's why the Bun rewrite is so interesting to me. Bun was ~535k
1) Amazing read. Make the time for it. 2) I particularly like this visual for test/CI progress. 3) Curious how an Opus-level model would fare on the same work. [video]