In 11 days, Jarred Sumner, the creator of the JavaScript runtime Bun, rewrote Bun’s entire codebase from Zig to Rust using Claude Code and a pre-release version of Claude Fable 5. Sumner estimates the same project would have taken three engineers with full context on the codebase roughly a year by hand, during which Bun’s bug fixes, security patches and new features would have effectively been frozen. Bun was acquired by Anthropic in December 2025. Sumner’s post opens with a long list of memory-safety bugs the team fixed in a single recent release, including several use-after-free and double-free crashes, along with memory leaks. Sumner describes roughly 50 “dynamic workflows” running continuously in Claude Code over 11 days, each handling one phase of the port. Before touching the bulk of the codebase, Sumner spent about three hours talking through the port with Claude to produce a document mapping Zig idioms to Rust ones, plus a lifetimes reference. He then tested the approach on three files before scaling up to all 1,448.