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”
Disclosure: Bun was acquired by Anthropic in December 2025. I and others on the Bun team work at Anthropic.
bun.comJarred Sumner
Context & Ripple Effects
Anthropic’s related coverage has moved from Claude Code’s emergence as a commercial coding-tool leader to reports of parallel agents producing a large Rust-based C compiler, and then to hands-on accounts of Fable 5 handling complex projects. The Bun claim is a more consequential test case because it concerns a core developer tool rather than a bounded demonstration.
Bun is now inside Anthropic, so the account is both evidence of how Anthropic’s own teams are deploying a prerelease model and a vendor-affiliated claim that outside developers will want independently reproduced.
First-order effects
Bun’s team says it completed a Zig-to-Rust rewrite far faster than its prior engineering estimate, making Claude Fable 5 central to a high-visibility internal migration workflow.
Anthropic gains a prominent product-development case study for its coding agents, while also inheriting a higher bar to substantiate quality, maintainability, and review of the generated rewrite.
Second-order effects
Teams considering language migrations or large refactors will increasingly test agentic coding against conventional staffing plans, especially where work can be decomposed and continuously validated.
Competing coding-model providers will face pressure to demonstrate performance on repository-scale transformations rather than isolated code-generation benchmarks.
Third-order effects
If such results prove repeatable under independent review, software delivery could shift toward smaller teams directing, testing, and integrating agent-produced changes, with verification and architecture becoming more important bottlenecks than code drafting.
The pattern also makes evaluation standards more important: vendor-controlled examples and prerelease access alone will be insufficient for buyers making staffing or platform decisions.
The trend: This is one data point in the shift from AI coding assistants toward agentic systems positioned to execute substantial, multi-file software maintenance and migration 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]
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
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 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.
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
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
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
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]