Bun 1.4 ships the Rust rewrite, adds 1,517 Node.js test-suite passes, and cuts HTTP server memory by 13 to 48 percent

Sources: Bun 1.4 release post, HN discussion
Summary: The Zig to Rust port tracked since 2026-07-09 is now in a stable release rather than a canary, which closes the open question of whether it would ship. The reported changes are upgrade-visible: JavaScriptCore moves off libpas to mimalloc, ReadableStream, WritableStream and TransformStream become native and pass the Web Platform Tests, Linux startup halves to 5.1 ms, HTTP server memory falls by 13 to 48 percent, and Bun reports p99 CPU for Claude Code dropping from 24 percent to 10 percent. Every figure is Bun's own and no independent reproduction was located. The release is not performance work alone, because it also adds API surface, Bun.WebView, Bun.Image, Bun.markdown, Bun.cron and Bun.Terminal, parallel modes for bun run and bun test, bun audit fix, Windows ARM64 support and an opt-in global virtual store, and reports over 2,900 issues fixed.
Why it matters: A stable tag moves the rewritten runtime, a different allocator, and native stream implementations onto every machine that upgrades.
Follow-up: Track an independent reproduction of the memory, startup, and p99 CPU figures.