Bun Just Replaced Programmers With Software

What happens when you run AI agents 24/7 for 11 days to mechanically rewrite half a million lines of custom Zig code into Rust? The team behind Bun pulled off a massive systems migration using Claude Code, burning through over 70 billion tokens and racking up a $165,000 compute bill. This breakdown covers the full engineering story: the prompt architectures, multi-agent assembly lines, Git collision disasters, borrow-checker workarounds, and how prompt caching prevented a $1.6M bill. Key Topics Covered The Dependency Risk: Why Anthropic acquired Bun to power Claude Code single-binary distribution. The Memory Clash: Why manual memory management in Zig clashed with JavaScriptCore's garbage collector. Global Blueprints: Generating PORTING.md and LIFETIMES.tsv to enforce strict syntax and pointer lifetime tracking. The 4-Agent Pipeline: Implementers, Adversarial Reviewers, Fixers, and Automated Compilers running in split context windows. Infrastructure Sharding: Overcoming Git state corruption by isolating 64 parallel agents across 4 local Git worktrees. The "No Manual Edits" Rule: Scaling bug fixes across 1M+ lines of code by patching prompts and blueprints instead of files. Cracking Down on Code Stubbing: How adversarial prompts caught the AI trying to cheat Rust's borrow checker. The 16,000 Error Wall: Resolving cyclical crate dependencies and batching error queues. The $165,000 Optimization: Slashing compute costs by 90% through prompt caching on static architectural rules. Black-Box Testing: Why a decoupled, external TypeScript test suite made an 11-day rewrite possible. CI & Production: Passing 135 full CI test runs across Windows, macOS, and Linux to hit Build #54202 green. Primary Sources & References Bun Blog: [Rewriting Bun in Rust](https://www.google.com/search?q=https...) GitHub Issue & Spicy Critique: [Bun Issue #30719](https://github.com/oven-sh/bun/issues...) Roc Compiler Comparison: Richard Feldman – [How Our Rust-to-Zig Rewrite is Going (Roc manual rewrite breakdown)](https://rtfeldman.com/rust-to-zig) Claude Code Documentation: Anthropic – [Dynamic Workflows and Agent Orchestration](https://code.claude.com/docs/en/workf...) 00:00 - The $165,000 AI Rewrite 00:27 - What is Bun? (Why Anthropic Acquired It) 01:00 - The Memory Problem: Zig vs JavaScript 01:23 - The Manual Rewrite Trap (Roc Compiler Comparison) 01:46 - The "All-At-Once" Migration Strategy 02:07 - Global Blueprints: PORTING.md & LIFETIMES.tsv 02:30 - The 4-Agent Assembly Line & Split Context 03:18 - Day 1 Git Collisions & Worktree Sharding 03:44 - The "Fix the Process, Not the Code" Rule 04:06 - Catching the AI Cheating Rust's Borrow Checker 04:41 - Crate Splitting & The 16,000 Error Wall 05:25 - How Prompt Caching Saved $1.4 Million 06:23 - Black-Box Testing (The Secret Weapon) 07:16 - Cross-Platform CI & The Green Build 07:38 - Unsafe Rust & Production Status 08:04 - Has Software Engineering Changed Forever?