Claude Code now runs on the Rust port of Bun and almost nobody noticed

Since June 17, every time you launch Claude Code you have been running a different runtime than before, and you most likely never noticed. Jarred Sumner, creator of Bun, dropped the detail in his announcement “Rewriting Bun in Rust”: Claude Code version 2.1.181 and later use the Rust port of Bun. According to Sumner, startup is 10% faster on Linux and, beyond that, “barely anyone noticed. Boring is good”.

Simon Willison did not settle for the claim and on July 19 published the results of inspecting his own installation looking for evidence.

How to verify it on your machine

The first command Willison found convincing searches for the Bun version string inside the Claude Code binary:

bash
strings ~/.local/bin/claude | grep -m1 'Bun v1'

On his machine the output is “Bun v1.4.0 (macOS arm64)”, and that detail is more interesting than it looks. The latest public release of Bun on GitHub is v1.3.14, published on May 12. The fact that Claude Code embeds a v1.4.0 confirms that Anthropic is shipping a preview of a Bun version that has not been officially released yet.

The second command extracts source file paths embedded in the executable:

bash
strings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\.rs'

The .rs extension gives away Rust files inside the binary, exactly what you would expect if the runtime bundled with Claude Code is already the new port.

Why it matters

Bun was originally written in Zig and became one of the fastest JavaScript runtimes on the market. The project's decision to rewrite itself in Rust was already big news in the runtime world; that the first large-scale production application on that port is Claude Code adds another layer. Anthropic's CLI ships as a self-contained executable that bundles the runtime together with the application code, and that packaging decision is what makes a change like this possible: the runtime travels inside the binary and gets swapped in a regular update, with nothing for the user to install.

There is also a reading about the relationship between the two companies. Shipping an unreleased preview implies a direct channel between the Bun and Anthropic teams: in practice, Claude Code works as a large-scale test bench for the new runtime ahead of its official release. For Bun it is validation money cannot easily buy; for Anthropic, early access to performance improvements. The reasonable caveat is the usual one with unreleased software: a preview has not gone through the full stabilization cycle, although running in production since June 17 with barely anyone noticing clearly works in its favor.

Who this is relevant for

For Claude Code users, the message is that there is nothing to do: same commands, same skills, same MCP servers, and the only measurable difference is that faster startup on Linux. Willison's two commands are, however, a cheap way to audit what exactly your machine is running, a healthy exercise with any self-updating binary.

For teams building and packaging JavaScript CLIs, the case is an example of a runtime migration with no apparent friction. And for those following the competition between Node, Deno and Bun, the signal is strong: the Bun rewrite in Rust is not a lab experiment, it already powers one of the most used CLIs in the AI development ecosystem.

At ElephantPink we work with Claude Code daily and we subscribe to Sumner's philosophy: boring is good. The best infrastructure changes are the ones you only discover by running strings on the binary.