Bun is joining Anthropic | Bun Blog

TLDR: Bun has been acquired by Anthropic. Anthropic is betting on Bun as the infrastructure powering Claude Code, Claude Agent SDK, and future AI coding products & tools.
Claude Code ships as a Bun executable to millions of users. If Bun breaks, Claude Code breaks. Anthropic has direct incentive to keep Bun excellent.
Almost five years ago, I was building a Minecraft-y voxel game in the browser. The codebase got kind of large, and the iteration cycle time took 45 seconds to test if changes worked. Most of that time was spent waiting for the Next.js dev server to hot reload.
This was frustrating, and I got really distracted trying to fix it.
I started porting esbuild's JSX & TypeScript transpiler from Go to Zig. Three weeks later, I had a somewhat working JSX & TypeScript transpiler.
Early benchmark from a new JavaScript bundler. It transpiles JSX files:- 3x faster than esbuild- 94x faster than swc- 197x faster than babel pic.twitter.com/NBRt9ESu2d
I spent much of that first year in a very cramped apartment in Oakland, just coding and tweeting about Bun.
To get Next.js server side rendering to work, we needed a JavaScript runtime. And JavaScript runtimes need an engine to interpret & JIT compile the code.
The start time difference between JavaScriptCore and V8 is interesting. JavaScriptCore seems to start around 4x faster.It's possible this is due to the specifics of their respective CLIs though (rather than about JavaScript execution) pic.twitter.com/xd5tSbWf6p
So after about a month of reading WebKit's source code trying to figure out how to embed JavaScriptCore with the same flexibility as what Safari does, I had the very initial version of Bun's JavaScript runtime.
Bun v0.1.0 was released in July of 2022. A bundler, a transpiler, a runtime (designed to be a drop-in replacement for Node.js), test runner, and a package manager - all in one. We ended up reaching 20k GitHub stars in the first week.
Introducing Bun - an incredibly fast all-in-one JavaScript runtime. https://t.co/Yt6tAcnBQs
Those first two weeks after the release were one of the craziest weeks of my life. My job switched from writing code all day to replying to people all day. We raised a $7 million seed round led by Kleiner Perkins (thanks Bucky & Leigh Marie! And also Shrav Mehta), I took a salary and convinced a handful of engineers to move to San Francisco and help build Bun.
got keys & desks for oven’s office today pic.twitter.com/bfTmRaF7Oh
Bun started to feel more stable, so we shipped Bun v1.0 in September of 2023.
Production usage started to pick up and we raised a $19 million Series A led by Khosla Ventures (thanks Nikita & Jon!), grew the team to 14 people and got a slightly larger office.
After all this time, we still didn't have Windows support. And every day, people asked us the same question: "when will Bun support Windows?"
So we added Windows support and called it Bun v1.1. Our Windows support was pretty rough at first, but we've made a lot of progress since then.
Bun v1.2 made big improvements to Node.js compatibility, added a builtin PostgreSQL client and S3 client. We also started seeing production usage from companies like X and Midjourney. Tailwind's standalone CLI is built with Bun.
Bun v1.3 added a builtin frontend dev server, a Redis client, a MySQL client, several improvements to bun install and improved Node.js compatibility. The real feature: continued increasing production usage.
In late 2024, AI coding tools went from "cool demo" to "actually useful." And a ton of them are built with Bun.
Bun's single-file executables turned out to be perfect for distributing CLI tools. You can compile any JavaScript project into a self-contained binary—runs anywhere, even if the user doesn't have Bun or Node installed. Works with native addons. Fast startup. Easy to distribute.
Claude Code, FactoryAI, OpenCode, and others are all built with Bun.
I started using Claude Code myself. I got kind of obsessed with it.
Over the last several months, the GitHub username with the most merged PRs in Bun's repo is now a Claude Code bot. We have it set up in our internal Discord and we mostly use it to help fix bugs. It opens PRs with tests that fail in the earlier system-installed version of Bun before the fix and pass in the fixed debug build of Bun. It responds to review comments. It does the whole thing.
This feels approximately a few months ahead of where things are going. Certainly not years.
Today, Bun makes $0 in revenue.
One of the most common questions I get is about sustainability. Questions like:
"How does Bun become a business?"
"If I bet my work project or company's tech stack on Bun, will it still be around in five or ten years?"
Our default answer was always some version of "we'll eventually build a cloud hosting product.", vertically integrated with Bun’s runtime & bundler.
But the world when I first started working on Bun is different from the world today. AI coding tools are this massive change to how developers do productive work, and the infrastructure layer matters more when agents are writing code.
Forcing ourselves down the prescribed path felt wrong when AI coding tools are getting this good, this fast.
We've been prioritizing issues from the Claude Code team for several months now. I have so many ideas all the time and it's really fun. Many of these ideas also help other AI coding products.
A few weeks ago, I went on a four hour walk with Boris from the Claude Code team. We talked about Bun. We talked about where AI coding is going. We talked about what it would look like for Bun's team to join Anthropic. Then we did that about 3 more times over the next few weeks. Then I did that with many of their competitors. I think Anthropic is going to win.
Betting on Anthropic sounded like a more interesting path. To be in the center of things. To work alongside the team building the best AI coding product.
At the time of writing, Bun's monthly downloads grew 25% last month (October, 2025), passing 7.2 mill