Bun's Rust rewrite hits 99.8% test compatibility on Linux

Bun's experimental Rust rewrite is now passing 99.8% of the existing Bun test suite on Linux x64 glibc, according to creator Jarred Sumner [jarredsumner].

── What shipped ──

The number refers to the Rust port of Bun's runtime — not the Zig codebase that ships today — running against the production test suite on the most common Linux target. Other platforms (macOS, ARM Linux, Windows) and other libcs (musl) aren't yet at that compatibility level. The announcement is platform-specific by design.

── Why it matters ──

Two things to read into this:

• Test compatibility is a harder bar than feature parity. Hitting 99.8% means the rewrite isn't just running Bun's APIs — it's matching Bun's edge-case behaviour. Error messages, stream timing, module resolution quirks. Switching runtimes only works if semantics line up at the failure modes, not just the happy path.
• Rust is a deliberate move down-level. Bun started in Zig — already a low-level language. The Rust track trades some compile-time tooling for ecosystem reach: more contributors, more libraries around io_uring, networking, parsers, and the wider Rust async stack.

For teams running Bun in production today, this isn't a "switch now" moment. The Rust port is still labeled experimental and only hits its compat number on one platform target. But the slope matters — most JavaScript-runtime forks never get past 50% test compat. Crossing 99% on the dominant target signals the rewrite is moving from research to plausible replacement.

── Editor's take ──

Bun has been the JavaScript-runtime story to watch for two years. Sumner crossing 99.8% test compat on Linux x64 means the Rust port has graduated from interesting experiment to credible production candidate within a single planning cycle. Worth bookmarking for 2027 infra refresh — not for today's deployment, but for the conversation in next quarter's engineering planning.