Anthropic updated Claude Code v2.1.181 to run on the Rust port of Bun, a move that increased startup speed on Linux by ten percent while remaining invisible to most users. Simon Willison confirmed the change by extracting strings from his local installation, which revealed a Bun version number of 1.4.0. This specific build predates the official public release of v1.3.14, indicating the software is already running a preview version across millions of devices.
The investigation also showed the application is executing hundreds of Rust source files directly within the CLI binary. This confirms the backend infrastructure has shifted away from JavaScript for its core runtime components. The primary outcome is a faster initial load time without requiring users to change their workflow or install new dependencies.
* Startup performance improved by 10% on Linux systems
* The build includes 563 Rust source files for the runtime
* Version 1.4.0 is a preview ahead of the official release
