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docs-site/docs/developer/bun-bundle-and-feature-flags.md

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+# 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](../reproducibility.md) states you cannot rebuild the shipping binary from this mirror.
+
+## See also
+
+- [System design: layers](../system-design/layers.md)
+- [CLI entry reference](../reference/cli-entry.md)