July 19, 2026
Bun in the oven, comments on fire
Claude Code uses Bun written in Rust now
Claude got a secret engine swap, and the comments are having a full meltdown
TLDR: Claude Code quietly switched to a new Bun engine written in Rust, and users found clues showing it’s already running in the wild. The update itself seems smooth, but the comments split hard between “this is the future” and “this is how open source loses its soul.”
A quiet little software update has turned into full-on comment section theater. The basic news: Anthropic’s Claude Code appears to be running on a new version of Bun, a tool under the hood that helps power the app, and that new version is now written in Rust instead of its older form. The payoff sounds almost comically modest: startup is about 10% faster on Linux, and, as Bun creator Jarred Sumner put it, “barely anyone noticed.” Naturally, people noticed immediately.
That’s where the real show begins. One camp saw this as a jaw-dropping sign that AI-assisted rewrites are no longer science fiction. A commenter admitted they expected the whole thing to be a disaster, then flipped after seeing it apparently ship to millions of users without exploding. To them, this is the future arriving in boring packaging.
But the backlash crowd was not buying the victory lap. One of the harshest reactions basically framed it as the moment the open-source project Bun “dies in darkness,” with critics upset about ownership, transparency, and the messy vibe around how the rewrite was handled. Another commenter called the whole approach not “serious” or “adult,” which is internet code for: the drama is now personal.
And then there were the jokers, tossing out lines like “Turns out it was just a build step after all”—a dry, nerdy punchline for a story that somehow managed to be both boring and wildly inflammatory. In other words: the software got faster, but the comments got way hotter.
Key Points
- •Jarred Sumner said Claude Code v2.1.181, released June 17, and later versions use Bun’s Rust port.
- •The article’s binary inspection of `~/.local/bin/claude` returned `Bun v1.4.0 (macOS arm64)`.
- •At the time of inspection, Bun’s latest GitHub release was v1.3.14, making the embedded v1.4.0 build appear to be a preview or canary version.
- •A second `strings` command found 563 `.rs` source file paths inside the Claude binary, supporting the conclusion that Rust-based Bun code was included.
- •An update showed that `BUN_OPTIONS="--preload=/tmp/bun-version.ts" claude --version` also reported Bun version 1.4.0, and the article linked this to a May 17 commit updating `package.json` to 1.4.0.