Bun 1.4's Rust Rewrite: The New Builtins, the AI-Generated Internals, and the CLAUDE.md Rules to Pin Down First (2026)
Bun 1.4 replaced roughly a million lines of Zig with Rust, much of it AI-assisted, and shipped four new builtin APIs in the same release. What CLAUDE.md needs to say before Claude Code reaches for bun upgrade or swaps in a new builtin.





























































































































































































