Bun
JavaScript runtime, bundler, package manager, and test runner, founded by jarred-sumner in 2021, built in Zig until 2026. Acquired by anthropic on 2025-12-03, positioned as infrastructure for Claude Code https://www.anthropic.com/news/anthropic-acquires-bun-as-claude-code-reaches-usd1b-milestone. On 2026-07-08 Sumner announced the port of Bun from Zig to Rust: roughly 535,000 lines (excluding comments), executed over days (6 to 11 depending on how you count, per his post and The Register’s) with Claude agent fleets, test-anchored, zero skipped tests, 6,502 commits at a peak near 1,300 lines per minute, 128 open bugs fixed, 19 regressions introduced after merge and all subsequently fixed, about 20 percent smaller binary https://bun.com/blog/bun-in-rust.
That port is the strongest single piece of evidence in the rust-for-agents argument, an agent-written migration at systems scale with measurable outcomes, and simultaneously the best counterexample, because the Zig creator’s critique of it as “unreviewed slop” https://www.theregister.com/devops/2026-07-14/zig-creator-calls-buns-claude-rust-rewrite-unreviewed-slop/5270743 shows compilation plus green tests still letting 19 behaviors through. Both readings are filed, on the same page, deliberately. anthropic’s migration guide treats the port as its flagship case study https://claude.com/blog/ai-code-migration.
Sources
- Bun joins Anthropic, 2025-12-02: https://bun.com/blog/bun-joins-anthropic
- Rewriting Bun in Rust, 2026-07-08: https://bun.com/blog/bun-in-rust
- The Register coverage, 2026-05-05 and 2026-07-13: https://assets.theregister.com/2026-05-05/bun_rust_port