Claude Code now uses a Rust-based version of Bun

Anthropic has updated its Claude Code tool to run on a new version of the Bun runtime written in the Rust programming language. This transition, which began with version 2.1.181, marks a significant shift in the underlying infrastructure of the software.

Technical analysis of the Claude Code binary confirms this change. The software now includes references to Bun version 1.4.0, a release that has not yet been made public on GitHub, and contains hundreds of file paths pointing to Rust source code. These findings indicate that the Rust-based version of Bun is already being deployed to millions of user devices.

The primary performance benefit observed from this update is a 10 percent increase in startup speed on Linux systems. Despite the major architectural change, the transition has been seamless for users, with the software maintaining its expected functionality. This update demonstrates the successful integration of a new, high-performance runtime into a widely used production tool.