Claude Code uses Bun written in Rust now
Points and comments are a snapshot, not live.
Claude Code ships with a Rust-based Bun v1.4.0 preview before public release.
Simon Willison investigates Jarred Sumner's claim that Claude Code v2.1.181+ uses a Rust port of Bun. He finds evidence via `strings` commands on the Claude binary: output shows 'Bun v1.4.0' (public latest is v1.3.14) and lists 563 `.rs` source filenames like `src/runtime/bake/dev_server/mod.rs`. A trick using a Bun version script confirms the embedded version is 1.4.0. The commit updating `package.json` to 1.4.0 dates from May 17th and hasn't been tag-released outside canary.
What commenters are saying
Several commenters question Bun's governance after the acquisition, with one asking if Anthropic effectively controls what gets accepted. Others downplay the language change: 'Literally who cares what language a JS toolset is written in?' and note the rewrite merged publicly 60 days ago without measurable harm. A counterpoint observes that issues were mass-closed claiming 'zig is no longer relevant' without fixing them. Another defends the migration, calling pre-rewrite Bun's Zig code 'the prime example of How Not To Write Zig Code' per Andrew Kelley.
The most substantive criticism centers on Anthropic using a pre-release Bun version not available to the community, and on Jarred Sumner closing all pre-rewrite issues. A reply notes the 1.4 PR was merged publicly into main. Some commenters split between those who see this as a natural FOSS evolution and those who view it as a governance red flag.