Bun 1.4: JavaScript runtime rewritten in Rust

Bun 1.4 is the first release built on a Rust rewrite of the runtime, which Bun says Claude Code has run in production for months. The release reports p99 CPU in Claude Code production falling from 24% to 10%, idle CPU down 5x for small apps, and Windows startup at 15.5 ms against 39.0 ms before. It adds built-in APIs including Bun.Image, Bun.WebView for headless browser automation without Puppeteer or Playwright, Bun.markdown, Bun.cron, and Bun.Terminal, plus bun test flags for parallel, isolated, and sharded runs. Node compatibility work added 1,517 tests from the Node suite, with node:http, node:fs, node:stream and other core modules passing 97% of them.