Willison Insights

Simon Willison: Claude Code now runs on Bun rewritten in Rust — evidence found directly in the binary

Nedeľa 19. júla 2026 • Source: Simon Willison

Main idea

Willison technically verified Jarred Sumner's (Bun creator) claim about the Rust rewrite: using the strings command on the Claude Code binary, he found version Bun v1.4.0 (macOS arm64) — newer than the last public release v1.3.14 — and 563 references to Rust .rs source files.

Context

Jarred Sumner publicly announced that Bun had been rewritten in Rust and Anthropic was one of the first production adopters. Willison verified this methodically through production binary analysis, not just public statements. Post published July 19, 2026 (97 HN points).

Why it matters

Anthropica is distributing pre-release infrastructure software to millions of devices without public communication — an interesting pattern of silent infrastructure migration at scale. For developers, this signals that Claude Code is an active distribution channel for bleeding-edge tooling.

Details / arguments

Extracted version: Bun v1.4.0 (macOS arm64) — pre-release compared to public v1.3.14
Number of Rust source references: 563 .rs files
Example paths: src/runtime/bake/dev_server/mod.rs, src/bundler/bundle_v2.rs
Rust port delivers ~10% faster startup on Linux
Implication: Rust port runs in production on every device with Claude Code

Open original source Simon Willison