The Bun JavaScript runtime has been acquired by Anthropic, which already uses the product for deploying Claude Code.

Anthropic said Bun “will remain open source and MIT-licensed” and that its development for general use by JavaScript and TypeScript developers will continue. Claude Code uses Bun for its native installer. According to Anthropic Bun has become “essential infrastructure for AI-led software engineering,” thanks to its fast performance and combination of features, which include a runtime, package manager, bundler and test runner. Bun also has the ability to compile to a single-file executable.

Bun was invented by Jarred Sumner, first previewed in May 2021, and released as version 1.0 in September 2023. Sumner has given his own overview of Bun’s history and why he agreed to the acquisition. Sumner admitted to being “kind of obsessed” with Claude Code and said: “The GitHub username with the most merged PRs in Bun’s repo is now a Claude Code bot.” He speculated that most new code might in future be written, tested and deployed by AI agents, and argued that in this scenario the runtime and tooling around the code becomes more important.

The business model for Bun is also critical. The company behind Bun has raised $26 million in venture capital, Sumner said, but “today, Bun makes $0 in revenue.” He said the original monetization plan was a future hosting offer, but being part of Anthropic gives Bun long-term stability and the team would be able to hire more engineers. 

We note though that Anthropic is also funded by venture capital, so although Bun is now in the hands of a much larger entity, future financial stability is not necessarily assured.

Unlike Bun’s competitors Node.js and Deno, Bun does not use Chromium’s V8 JavaScript engine, but rather JavaScriptCore, part of the Apple-sponsored WebKit project. Another distinctive technology choice is that the language used for native code development in Bun is Zig, designed as a small language that competes with C. Zig is far from mature, with the current version being 0.15.2, and Zig inventor Andrew Kelley is known for making breaking changes as the language evolves.

Kelley also has a “strict no LLM [large language model]/No AI policy” in the official code of conduct for Zig contributors. There could be friction ahead if developers now at Anthropic turn their attention to contributing to the Zig language on which Bun depends.  

There are a number of other implications. AI coding tools already tend to default to a TypeScript/JavaScript stack if not instructed otherwise, and the Bun acquisition may further cement that default, even though JavaScript has many flaws. The inventor of JSON (JavaScript Object Notation) said “the best thing we can do today to JavaScript is to retire it,” a possibility that now seems more remote than ever.

Another key question is how Anthropic will shape the future of Bun. Many acquisitions begin with those involved stating that nothing will change, but this rarely proves to be the case a few years later. Developer views vary. “Anthropic will pump some resources towards platform stability, which could then drive a large migration to Bun,” commented one, while another worried about the “potential for either a rug pull or all the goodies going into a paid version”.

Until now, Bun has been driven by Sumner’s intense drive to develop the product, and under new ownership with different goals some change in direction seems inevitable.