In isolation, yes, that does seem like it would have been the better decision for Claude Code, but it also would have severely diminished the value of their Bun/oven.sh acquisition.

Claude Code isn't the only user of Bun (it's probably not even the only user of it internally at Anthropic), and this way they get to keep the Javascript runtime (and other tooling) that their coding models may one day, if not already, prefer to use when given the opportunity. For those kind of apps, you'll probably also eventually find that--what do you know!--Anthropic also has their own cloud offering (instead of Bun being the one to build one) that specializes in running and managing those applications for you. Even if all they get is a community of developers that choose to use Bun over other options, that gives them power and seats at tables they wouldn't have if they just rewrote Claude Code in Rust.

That'd be my best guess, anyhow.

So they had to rewrite Bun instead of Claud Code because they “already had bought Bun.”

This thread is filled with non-sequiturs.