Claude Code has switched to a Rust-rewritten Bun

Jarred Sumner, the author of the Bun runtime, reported in a piece about rewriting Bun in Rust that Claude Code version 2.1.181 (released on June 17) and later use precisely the Rust port of Bun. According to him, on Linux this yielded a 10% speedup in startup, while otherwise the change went almost unnoticed — “boring is good,” he noted.

Simon Willison decided to test this claim on his own installation of Claude Code. He found two ways to get confirmation via the strings command.

The first command — strings ~/.local/bin/claude | grep -m1 'Bun v1' — printed the string Bun v1.4.0 (macOS arm64). Since at the time of the check the latest official Bun release on GitHub was v1.3.14 from May 12, version 1.4.0 indicates that Anthropic is shipping a not-yet-released preview version of Bun.

The second command — strings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\.rs' — printed a list of 563 file names with the .rs extension, including paths such as src/runtime/bake/dev_server/mod.rs, src/runtime/bake/production.rs, and src/bundler/bundle_v2.rs.

In Willison's view, this is compelling confirmation that the Rust-rewritten Bun really is running in production on the millions of devices where Claude Code is installed.

Source: Simon Willison