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Claude Code uses Bun written in Rust now
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- DeepHorus
Anthropic's Claude Code, the AI-powered coding assistant, has silently transitioned to using a Rust-based port of the Bun JavaScript runtime, according to a recent analysis by developer Simon Willison. Jarred Sumner, the creator of Bun, announced on July 19, 2026, that Claude Code version 2.1.181, released June 17, now leverages the Rust implementation of Bun. This shift resulted in a 10% improvement in startup time on Linux, though the change went largely unnoticed by users.
Bun was originally written in Zig, a low-level systems programming language, but the project began rewriting core components in Rust to improve performance and reliability. The Rust port aims to maintain full compatibility while offering better memory safety and faster execution. Claude Code, which relies on Bun for executing JavaScript and TypeScript code, appears to be one of the first major production deployments of this new Rust-based version.
Willison discovered evidence of the migration by examining the Claude Code binary. Running strings ~/.local/bin/claude | grep -m1 'Bun v1' revealed "Bun v1.4.0 (macOS arm64)", a version number higher than the latest public release of Bun (v1.3.14 from May 12). Additionally, strings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\.rs' listed 563 Rust source file paths, confirming the presence of Rust code in the runtime. These findings indicate that Claude is shipping a preview of an unreleased Bun version.
The integration of Rust-based Bun into Claude Code has significant implications for both performance and ecosystem stability. The 10% faster startup time, while modest, can accumulate savings across millions of sessions, especially in CI/CD pipelines and developer workflows. More importantly, the seamless transition underscores Rust's growing role in production environments where reliability and speed are critical. Users experienced no breaking changes, aligning with Sumner's philosophy that "boring is good" when it comes to runtime upgrades.
Developer reactions have been largely positive, with many appreciating the invisible improvement. Sumner emphasized that the migration was designed to be uneventful, prioritizing stability over fanfare. "Barely anyone noticed," he noted, highlighting that successful infrastructure changes often go unnoticed by end users. This approach contrasts with more disruptive updates common in the JavaScript ecosystem, where breaking changes can frustrate developers.
Looking ahead, the adoption of Rust-based Bun in Claude Code may pave the way for broader usage of the port across other applications. The fact that Anthropic trusted this version for production use suggests that the Rust implementation has reached a high level of maturity. As more developers and companies evaluate Bun for their projects, the Rust version could become the standard, offering improved performance without sacrificing compatibility. For now, Claude Code users can enjoy a subtly faster experience, with the reassurance that their runtime is built on more robust foundations.
