Bun's $165K Rust Rewrite Looks Far From Done — and Far More Expensive

A skeptical developer dug into the widely-shared claim that Bun’s JavaScript runtime was rewritten in Rust by Anthropic’s Claude in 11 days for $165,000 in API calls. His conclusion: the headline number tells only part of the story. Six weeks after the rewrite was merged to main, there is still no release tag — an 11-week gap since the last release (bun-v1.3.14), the longest drought since late 2022. Meanwhile, open pull requests from the ‘robobun’ bot (a proxy for Claude-generated PRs) nearly doubled from 1,277 to 2,475 in under three weeks, and at roughly 40-plus minutes per merge, clearing the backlog would take almost three months of continuous pipeline runs.

The author argues the true cost is largely off the books. The quoted figure excludes the constantly-running Buildkite CI/CD cluster and the labor of Anthropic employees who are directly committing Rust code alongside the bot. If the effort is still burning even $10,000 a day, he estimates spending is approaching $800,000. Notably, the Bun team never claimed the rewrite was ‘done’ for $165K — that framing came from breathless observers treating it as proof that AI can replace open-source maintainers cheaply and quickly.

The broader point is a warning about incentives: Anthropic acquired Bun and is dogfooding its own tooling on the rewrite, so the project doubles as marketing for coding capabilities that underpin sky-high AI valuations. The author, who describes himself as pro-AI and proud of his own ML work, says this is the worst hype cycle he has seen. He closes by noting that two other showcase AI projects — Anthropic’s C compiler and Cursor’s FastRender browser — have gone quiet with no commits for months.