• Running strings on the claude binary and grepping for 'Bun v1' prints Bun v1.4.0, a version number ahead of any public Bun release. That gap is the tell that the agent embeds an unreleased build.
• Grepping the same binary for src/*.rs paths returns 563 Rust filenames, including src/bundler/bundle_v2.rs. That is direct evidence the embedded runtime is the Rust port, not just a version bump.
• A cleaner check than string-grepping: set BUN_OPTIONS to preload a script that logs Bun.version, then run claude --version. It prints the embedded runtime's own version.
• The rewrite reached millions of machines through Claude Code from v2.1.181 on, bought about 10% faster startup on Linux, and nobody noticed. For a runtime swap that invisibility is the success condition.

• AI agent — An AI system that doesn't just answer once but works toward a goal in a loop — taking actions, reading the results, and deciding what to do next.

You now know your Claude Code installs run on pre-release Rust Bun, and you have a repeatable recipe for inspecting what runtime any agent binary actually embeds.

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