An early preview of experimental Bun + JSC with ahead-of-time compilation For Claude Code's CLI: 24% faster startup, 48% lower memory, 46% lower CPU, and nearly 2x larger binary size
Bun AoT VS Bun JIT Claude Code, same source, same 5-turn session. AoT runs with the JIT off. Time to interactive 165 ms 216 ms -24% Wall time launch + 5 turns 322 ms 529 ms -39% CPU time Memory at interactive 395 ms 738 ms -46% 56 MB 108 MB -48% Memory after 5 turns 82 MB 159 MB -48% 101 MB Peak memory Executable size target: +100 MB at most 194 MB -48% 430 MB 229 MB +88% Early prototype, macoS arm64. Medians of 8 runs against a fake API server on one MacBook Pro. Memory is phys_footprint. CPU is user + system, all threads. Turns in the JIT build are not CPU bound, so part of the wall time gap is waiting. 28 Sep 2026.

ALT Bun AoT VS Bun JIT Claude Code, same source, same 5-turn session. AoT runs with the JIT off. Time to interactive 165 ms 216 ms -24% Wall time launch + 5 turns 322 ms 529 ms -39% CPU time Memory at interactive 395 ms 738 ms -46% 56 MB 108 MB -48% Memory after 5 turns 82 MB 159 MB -48% 101 MB Peak memory Executable size target: +100 MB at most 194 MB -48% 430 MB 229 MB +88% Early prototype, macoS arm64. Medians of 8 runs against a fake API server on one MacBook Pro. Memory is phys_footprint. CPU is user + system, all threads. Turns in the JIT build are not CPU bound, so part of the wall time gap is waiting. 28 Sep 2026.

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Most JavaScript usage is just-in-time compiled. As code runs, types of values are watched and when a function runs frequently enough, a faster specialized version of that function is compiled on-demand. Ahead of time compilation bakes that work into the executable itself.
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The cost of ahead-of-time compilation is less information about the types is available. In native languages like Rust or Go, numbers specify double or int, a sign, and a size. TypeScript is more flexible and less specific, which makes performance optimizations harder.
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In this prototype, a limited amount of TypeScript syntax is used to propagate type information.

Sep 29, 2026 · 1:18 AM UTC

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Generally, ahead of time compilation results in slower peak JavaScript execution than just-in-time compilation. We think a stricter flavor of TypeScript and locking down modifications to builtins like Array & Object can close most of the gap.
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There are a lot of optimizations that aren’t implemented yet. I want the binary size growth to go down a lot more (it started at 1 GB). It needs to support sized types like uint32_t and propagate more detailed types. There’s no separate checker to run and no diagnostics
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AoT uses Bun’s bundler & parser to parse typescript, tags types to JavaScriptCore via a typecheck bytecode intrinsic, and compiles the bytecode to machine code via B3 (what JSC’s JITs use) This keeps the JavaScriptCore changes well-contained, so that syncing with upstream WebKit isn’t complicated
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