Claude Code shifts to Rust-based Bun runtime, claiming faster startup

Anthropic’s Claude Code platform has updated its underlying infrastructure to incorporate the Rust port of the Bun JavaScript runtime. The change was introduced with version 2.1.181, which was released on 17 June 2026, and is now present in subsequent updates to the software.

The transition marks a significant technical shift for the developer tool, moving away from previous runtime configurations. According to claims outlined by Jarred Sumner in his article "Rewriting Bun in Rust", the adoption of the Rust-based version of Bun has resulted in a reported 10 per cent reduction in startup times.

Details of this performance improvement were highlighted in a blog post published by Simon Willison on 19 July 2026. Willison’s entry, titled "Claude Code uses Bun written in Rust now", referenced Sumner’s original findings and served as the primary conduit for the news, which was subsequently circulated via the Hacker News platform.

The reported speed increase applies specifically to the initial launch phase of the application. However, the source material does not provide independent verification of these metrics, nor does it detail the specific operational conditions under which this 10 per cent improvement is realised.

As the update rolls out to users, the focus remains on the technical implications of integrating a Rust-compiled runtime into the Claude Code ecosystem. The move suggests a continued effort by Anthropic to optimise the performance and efficiency of its AI-driven coding assistants.