Bun (software)

• 1. Introduction

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  In September 2021, Jarred Sumner released the first version of Bun. It was built as a direct replacement for Node.js, the runtime that had long dominated server-side JavaScript. Sumner combined three tools that developers typically maintained separately: a runtime, a package manager, and a test runner. All three came in one program. What made Bun distinct at its core was not its feature list, but the engine it ran on.

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• 2. A Different Engine

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  JavaScriptCore is the engine Apple built for Safari. Sumner chose it as Bun's JavaScript engine, a departure from what both Node.js and Deno had done. Both of those runtimes ran on V8, the engine used by Chromium. By building on JavaScriptCore instead, Sumner gave Bun an engine that neither Node.js nor Deno had used. The practical capabilities Bun built on top of that engine reached well beyond executing JavaScript code.

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• 3. Everything Included

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  Svelte, Nuxt.js, and Vite each had server-side rendering support inside Bun from the start. Bundling was also part of the package, combining JavaScript, CSS, and HTML files into fewer files to reduce server requests and improve load times. Minifying stripped whitespace and comments from code to compress file sizes without changing how the code functioned. Bun provided an API for choosing how aggressively to minify, including the option to preserve some whitespace for readability. Alongside those tools, the runtime included foreign function interfaces, SQLite3, TLS 1.3, and DNS resolution. File editing, HTTP servers, WebSocket support, and hashing rounded out the standard installation. Oven, the company Sumner founded to support Bun's development, would announce its first major funding in August 2022.

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• 4. Oven and the Seven Million

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  On the 24th of August 2022, Oven announced it had raised seven million dollars. Kleiner Perkins led the round, with Guillermo Rauch and Y Combinator among the participants. The raise gave Oven capital to develop Bun further. Sumner had built the project on his own from the beginning. With the funding in place, work could continue toward the stable release the project needed. Bun 1.0 would arrive just over a year later.

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• 5. Version by Version

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  Bun 1.0, the project's first stable release, arrived on the 8th of September 2023. Bun 1.1 extended the runtime to Windows 10 and Windows 11. It also introduced the Bun Shell, a cross-platform tool for running Bash commands without any extra dependencies. Version 1.3 brought hot module replacement to Bun's local development server. Pages updated in real time as a developer changed code, without any manual refresh step. On the 2nd of December 2025, Bun announced a change that went well beyond any version update.

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• 6. Joining Anthropic

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  Jarred Sumner posted on his blog that December to announce that Bun had been acquired by Anthropic. He named two Anthropic tools as the purpose behind the deal: Claude Code and the Claude Agent SDK. In that post, Sumner specifically addressed the project's open-source status. Bun would remain open-source. It would stay under the MIT License. The financial terms of the acquisition were never disclosed. Anthropic's backing would allow the team to hire more engineers and release updates at a faster pace. Less than six months after the announcement, Sumner would begin the most ambitious change Bun had yet undergone.

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• 7. Rewriting in Rust

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  In May 2026, Sumner began rewriting Bun from Zig to Rust. He described the effort as experimental at first. Within days, he had merged the rewrite branch into the project's main codebase. The rewrite covered more than a million lines of code. Sumner made heavy use of AI tools throughout the process. The finished version passed Bun's test suite. Rust had replaced Zig. What the runtime would do next was now Anthropic's to direct.

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