Bun 1.4 released, rewritten in Rust with major Node.js compatibility gains

Bun 1.4 is the first version to be rewritten in Rust, a move that directly addresses performance and stability. The rewrite was done with Claude Code over 11 days, a detail highlighted by the Bun team. The resulting fixes and optimizations are substantial, from a 5x reduction in idle CPU usage to a 35% cut in memory use. Startup time on Linux has also improved by up to 50%, bringing it to 4.8 milliseconds. These gains stem from the Rust port and memory-related stabilizations. The version also marks a significant push toward Node.js compatibility, adding over 1,500 tests from the official suite, which enabled frameworks like Playwright and vitest to function. However, Bun notes that full Node.js compatibility is still a work in progress. The memory leak fix for Next.js App Router SSR, caused by a GC retention cycle when fetch() is called, is another key improvement that could benefit developers using that framework.