September 29, 2026
Claude Code Launches Faster in Preview: Bun Cuts Startup Time by 24%
A Sep 29 early preview of Bun + JSC sped up Claude Code CLI startup by 24%. It uses 48% less memory and 46% less CPU. The binary is nearly twice as large.
Jarred Sumner
@jarredsumner
An early preview of an experimental Bun + JSC setup with ahead-of-time compilation. For the Claude Code CLI: startup is 24% faster, it uses 48% less memory and 46% less CPU, while the binary is nearly twice as large.
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Bun embeds bytecode in a standalone binary before startup. In the example, Bun PGO-layout cut a large CLI's cold start from 1.0 to 0.53 seconds and reduced memory use by 40 MB.
For its own CLI, Bun provides the command `bun build --compile --minify --sourcemap --bytecode ./path/to/my/app.ts --outfile myapp`. `--bytecode-depth=0` compiles the top level of modules, while `=1` adds nested functions. For Claude Code, this is still an experimental build rather than a new app setting.
After updating Bun, the bytecode binary must be rebuilt, otherwise the runtime will fall back to parsing source files.
