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Public story · 2026-07-19 · high
Simon Willison confirmed the swap on July 19 by pulling 563 Rust filenames from the Claude binary, three days after critics called it unreviewed AI slop.
Why now: Willison verified the swap on July 19, three days after a backlash that had already been running on top of the code it criticized for a month.
Claude Code has quietly run a Rust port of Bun since June 17, Simon Willison confirmed on July 19.
The rewrite that critics called unreviewed AI slop on July 16 had already been carrying a month of production traffic. Some of the people making that argument were probably running it themselves.
Jarred Sumner said Claude Code v2.1.181 and later ship the Rust version. Willison didn't take that on faith. He ran strings on the Claude binary and pulled out 563 Rust source filenames. A TypeScript preload query then returned an embedded Bun v1.4.0, ahead of the v1.3.14 public release. Startup got about 10% faster on Linux. Nothing else visibly changed, per Willison's post.
Willison is careful about what this proves, and he's right to be. It doesn't mean AI rewrites are safe in general. It means this one, on a codebase with strong test coverage, verified against the full suite before merge, held up under a month of load. Bun is the proof of method behind Anthropic's migration playbook: rulebook, throwaway pilot, parallel small-model translation, compiler as referee, full suite green before anything ships.
The test suite saved the Bun rewrite, not the model. If your own coverage sits around 30%, this precedent doesn't transfer, no matter how good the model gets.
Each link below shares sources, entities, or timing with this story.
Anthropic released Claude / Shared entities / Same source domain / Shared topic / Earlier coverage
Linked by a graph relationship (Anthropic released Claude); both cover Anthropic, Claude, Jarred Sumner, Linux; reported by the same outlet (simonwillison.net).
Simon Willison uses Claude / Shared entities / Same source domain / Shared topic / Earlier coverage
Linked by a graph relationship (Simon Willison uses Claude); both cover Claude, Jarred Sumner, Rust, Simon Willison; reported by the same outlet (simonwillison.net).
Anthropic released Claude / Shared entities / Shared topic
Linked by a graph relationship (Anthropic released Claude); both cover Anthropic, Claude Code, July, Rulebook; overlapping topics (suite, test).
Cursor supports Claude / Shared entities / Shared topic / Earlier coverage / Tension
Linked by a graph relationship (Cursor supports Claude); both cover Anthropic, CLAUDE, Claude Code, Rust; overlapping topics (claude, tool).
Anthropic released Claude / Shared entities / Shared topic / Earlier coverage
Linked by a graph relationship (Anthropic released Claude); both cover Anthropic, Claude Code, Jarred Sumner, Rust; overlapping topics (rust, suite, test).
Anthropic released Claude / Shared entities / Same source domain / Earlier coverage
Linked by a graph relationship (Anthropic released Claude); both cover Anthropic, CLAUDE, Claude Code, Simon Willison; reported by the same outlet (simonwillison.net).
Simon Willison uses Claude / Shared entities / Same source domain / Shared topic / Earlier coverage
Linked by a graph relationship (Simon Willison uses Claude); both cover Claude Code, July, Simon Willison, Willison; reported by the same outlet (simonwillison.net).
Anthropic released Claude / Shared entities / Same source domain / Shared topic / Earlier coverage
Linked by a graph relationship (Anthropic released Claude); both cover Anthropic, CLAUDE, Simon Willison, Then; reported by the same outlet (simonwillison.net).