claude-code-2026-04-02/docs-site/docs/developer/bun-bundle-and-feature-flags.md

Bun bundle and feature flags

The shipping Claude Code CLI is built with Bun and internal bun:bundle integration. In the recovered source, src/main.tsx uses compile-time feature gates such as:

• feature('KAIROS') — lazy require of assistant/ (assistant / Agent SDK–oriented paths).
• feature('COORDINATOR_MODE') — lazy require of coordinator/coordinatorMode.
• feature('VOICE_MODE') — conditional useVoiceIntegration in REPL.tsx.

What feature() means here

In Anthropic’s pipeline, feature('…') is almost certainly a constant resolved at bundle time: dead branches are stripped from the published cli.js. The reconstruction contains all branches as TypeScript, so you can read “internal” code paths that might not ship in every npm artifact.

Implications for “building from source”

A normal tsc or bun build without Anthropic’s bundler will not reproduce:

• The same dead-code elimination.
• The same bun:bundle API semantics.
• The same proprietary dependency closure.

That is one reason Reproducibility and limits states you cannot rebuild the shipping binary from this mirror.

See also

• System design: layers
• CLI entry reference