Claude Code uses Bun written in Rust now
simonwillison.net
July 19, 2026
1 min read
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Summary
Claude Code v2.1.181 and later utilize the Rust port of Bun, resulting in a 10% faster startup on Linux. The command `strings ~/.local/bin/claude | grep -m1 'Bun v1'` confirms the use of Bun v1.4.0 on macOS.
Key Takeaways
- Claude Code v2.1.181, released on June 17th, uses the Rust port of Bun, resulting in a 10% faster startup on Linux.
- The version of Bun used in Claude Code is v1.4.0, which is a preview of a not-yet-released version.
- The Rust version of Bun has been released as Bun canary, and users can install it by running "bun upgrade --canary."
- Evidence shows that Bun in Rust is being utilized in production across millions of devices.
Community Sentiment
Positives
- The ability to build and use the new Bun version from the public PR shows some transparency in the development process, allowing developers to experiment.
- The integration of Rust could potentially lead to performance improvements in the long run, even if the initial rollout is rocky.
Concerns
- The switch to Rust feels like a death knell for Bun, with concerns about governance and transparency leaving users in the dark about the project's future.
- Many commenters question why anyone would switch to Bun, citing performance issues and a lack of compliance that make it a risky choice.
- The communication around the changes has been poor, damaging trust and creating divisions within the community.