AI Rewrote an App. Its Creator Declared War.

Bun executed an unprecedented digital heist, porting over half a million lines of Zig to Rust in just 11 days. This astonishing feat, unfolding between May 3-14, 2026, leveraged 64 parallel Claude agents running a pre-release Fable 5 model across four Git worktrees. The AI-driven migration saw ~535,496 lines of Zig transformed into ~682,000 lines of Rust, peaking at an astounding 58 commits per minute and culminating in 6,502 total commits.

Bun’s official rationale for this radical overhaul was clear: eradicate years of persistent 'use-after-free' memory bugs. The team aimed to exploit Rust's famously strict borrow checker for enhanced stability, directly addressing 128 known issues that had plagued the codebase. This strategic pivot promised a more robust and reliable runtime, turning potential runtime errors into compile-time guarantees.

Crucially, this AI-powered rewrite only became financially viable thanks to Bun’s acquisition by Anthropic in December 2025. This strategic integration provided Bun with unparalleled access to Anthropic’s advanced AI models, sidestepping the prohibitive estimated $165,000 token cost if purchased via public API. The project thus served as a monumental internal showcase for Anthropic’s generative AI capabilities, demonstrating its potential for large-scale code transformation.

Zig's creator, Andrew Kelly, launched a blistering counter-attack, dismantling both Bun's original codebase and its AI-generated successor. His public blog post, later edited for even greater severity, left no doubt about his profound contempt for the entire undertaking.

Kelly didn't pull punches on Bun's original Zig implementation, branding it "hacks on top of hacks" and a "prime example of how not to write Zig." He asserted the half-million lines of code were a "net liability" for his language, implying a fundamental failure in engineering long before any AI touched it.

His scorn extended to the Rust port, generated by 64 parallel Claude agents, which he dismissed as "a million lines of unreviewed slop." Kelly castigated its proponents as "tasteless AI enthusiasts," implying a profound lack of discernment for quality code and a blind embrace of automated mediocrity over human craftsmanship.

The criticism intensified with a deeply personal assault on Bun's founder, Jarred Sumner. Kelly publicly labeled Sumner a "stinky manager" and accused him of making Bun "an embarrassment for Zig," directly challenging his leadership. He blamed Sumner "partially for some of the slop we've been dealing with" and concluded by firmly stating, "I stand by my criticism of his leadership."

The battle over Bun’s rewrite isn't merely about code; it’s a clash of fundamental engineering philosophies. Andrew Kelly, Zig's creator, champions meticulous, manual engineering, demanding explicit control and minimal abstraction. Bun, under Jarred Sumner, however, embodies a fast-paced, feature-driven development model, now supercharged by 64 parallel Claude agents porting half a million lines.

Kelly didn't mince words, dismissing Bun's original Zig codebase as "hacks on top of hacks" and "unreviewed slop," arguing its pervasive bugs stemmed from poor coding practices, not Zig's inherent limitations. He famously declared the AI-driven port a "marketing ploy" by a "trillion dollar company," hinting at Anthropic's backing. For a deeper dive into his scathing assessment, read My Thoughts on the Bun Rust Rewrite - Andrew Kelley.

Amidst the vitriol, the data presents a complex, if not entirely clean, picture. The port did fix 128 known bugs, a significant functional improvement. However, it initially introduced 19 new regressions, which have since been resolved. Furthermore, approximately 4% of Bun's new Rust code, around 13,000 keywords, resides in unsafe blocks, a detail sure to raise eyebrows among Rust purists expecting absolute memory safety guarantees.

This unprecedented rewrite, performed by 64 parallel Claude agents, porting 535,496 lines of Zig to ~682,000 lines of Rust in 11 days, establishes a chilling precedent. Is this the future of rapid, large-scale code migration, or a stark warning about the accumulation of "unreviewed slop" and technical debt? The estimated $165,000 token cost, even if waived due to Anthropic's acquisition of Bun, signals a seismic shift in development economics.

Andrew Kelly's unfiltered condemnation, describing Bun's original code as "hacks on top of hacks" and the AI-generated port as "a million lines of unreviewed slop," ignited a fierce debate. Many lauded his refreshing honesty and necessary truth-telling about code quality, while others deemed it unprofessional and damaging to the reputations of both Bun and Zig. This division underscores a profound anxiety about the evolving landscape of open-source collaboration.

The central question now looms large: as AI generates more foundational code, who bears ultimate responsibility for its quality, security, and long-term maintainability? This redefines the very essence of open-source, challenging traditional notions of human contribution, peer review, and a language's reputation. The industry must confront these implications head-on, or risk a future built on opaque, AI-driven foundations.

Why did Bun switch from Zig to Rust?

Bun's creator, Jarred Sumner, cited persistent memory safety bugs like 'use-after-free' crashes as the primary reason. The goal was to leverage Rust's borrow checker to eliminate these issues at compile time.

Who is Andrew Kelly and why is he upset?

Andrew Kelly is the creator of the Zig programming language. He is upset because he believes Bun's Zig code was poorly written ('hacks on top of hacks') and that the AI-driven rewrite to Rust was a marketing stunt that unfairly tarnished Zig's reputation.

How did Bun use AI to rewrite its entire codebase?

Bun utilized 64 parallel AI agents powered by a pre-release Anthropic Claude model. These agents worked across multiple git worktrees for 11 days, porting approximately 535,000 lines of Zig to 682,000 lines of Rust.

What were Andrew Kelly's strongest criticisms of Bun?

He called Bun's codebase 'unreviewed slop' and a 'net liability' for Zig. He also heavily criticized Bun's creator, Jarred Sumner, calling him a 'stinky manager' and blaming him for making Bun an 'embarrassment for Zig'.