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Rewriting Bun in Rust

Jarred Sumner

Last highlighted July 11, 2026 Source ↗ ← All highlights

I asked Claude to loop the workflow on all 1,448 .zig files, and about 2 minutes in, one Claude ran git stash before committing. Another ran git stash pop. And then git reset HEAD --hard. They were stepping on each other! And if I put each Claude into a separate worktree, I would run out of disk space because Bun’s git repository is too big and eventually the changes will need to be compiled and seen together. So, I asked Claude to edit the workflow to instruct Claude to never run git stash or git reset or any git command that doesn’t commit a specific file at once. No cargo either. No slow commands at all.
Claude interpreted “let’s get all the crates to compile” as “stub out the functions with compilation errors”. Claude also started adding suspiciously long explanatory comments to document workarounds, so I added this rule for the adversarial reviewers to reject: If you need a paragraph-long comment to justify why the workaround is OK, the code is wrong — fix the code. One prompt edit and a few hours later, these things stopped happening. Models love saying “smoke tests”
This Rust rewrite would’ve taken a team of engineers with full-context on the codebase a year of work. With 1 engineer using Fable & closely monitoring Claude Code, we went from start to 100% of the test suite passing on all platforms in 11 days. One engineer can do a lot more today than a year ago.