> ## Documentation Index
> Fetch the complete documentation index at: https://aixyz.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# Why Bun?

> Why aixyz uses Bun as its runtime

## Software is Being Rewritten — by Agents

We are entering a world where humans won't be writing most software. AI agents will. And agents will be creating other agents. When Claude Code scaffolds a project, installs dependencies, writes tests, and iterates on failures — all in a single session — the bottleneck isn't the model. It's the runtime.

In December 2025, Anthropic made their first-ever acquisition: [they bought Bun](https://www.anthropic.com/news/anthropic-acquires-bun-as-claude-code-reaches-usd1b-milestone). Not a model company. Not a data company. A JavaScript runtime. That tells you everything about where this is going.

We're making the same bet.

### The Runtime is the Agent's Operating System

Every tool an AI agent calls is a runtime operation — HTTP requests, file I/O, code execution, JSON parsing. An agent making dozens of tool calls per session doesn't experience latency the way a human does. For a human, 500ms is imperceptible. For an agent in a tight loop, [it's the difference between fast iteration and a stall](https://dev.to/inboryn_99399f96579fcd705/anthropic-acquires-bun-why-ai-agents-need-a-lightweight-runtime-228d).

Node.js was designed for a world where humans start a server and it runs for hours. Bun was designed for a world where processes spin up, execute, and terminate in milliseconds. That's the world agents live in — generate code, run it, check the output, fix the errors, repeat. The runtime needs to keep up.

Anthropic understood this before anyone else. Claude Code [ships as a Bun executable](https://betterstack.com/community/guides/scaling-nodejs/anthropic-acquires-bun/). So does [FactoryAI. So does OpenCode](https://betterstack.com/community/guides/scaling-nodejs/anthropic-acquires-bun/). The pattern is clear: [the runtime is becoming the agent's operating system](https://jimmysong.io/blog/bun-anthropic-runtime-shift/).

### One Binary, Zero Configuration

The Node.js ecosystem asks you to choose: which package manager, which bundler, which test runner, which TypeScript compiler. Five tools, five configurations, five places where things break.

Bun is one binary that does all of it:

```bash theme={null}
bun install        # Package manager
bun run dev        # Runtime
bun run build      # Bundler
bun test           # Test runner
```

This isn't just developer convenience. [For an AI agent that needs to spin up projects, test, bundle, run, and iterate at speed, that fragmentation is friction](https://dev.to/axrisi/anthropic-just-acquired-bun-and-it-signals-the-beginning-of-ai-native-software-engineering-3cd9). Every tool choice is a decision point. Every configuration file is a place where an automated workflow can break. Bun eliminates the entire category of problem.

Jarred Sumner, Bun's creator, said it directly: [Bun's job is to be the best place to build, run, and test AI-driven software](https://bun.com/blog/bun-joins-anthropic).

### TypeScript Without the Ceremony

Bun runs TypeScript natively. No compilation step. No `tsconfig.json`. No build pipeline between writing code and running it.

aixyz packages ship raw `.ts` files and Bun handles them directly. When an agent generates TypeScript, it can execute it immediately — the generate-run-verify loop has zero gap. This is the kind of simplicity that [makes agent-driven development practical at scale](https://dev.to/axrisi/anthropic-just-acquired-bun-and-it-signals-the-beginning-of-ai-native-software-engineering-3cd9).

### Compile Once, Run Anywhere

Bun compiles projects into [single-file executables](https://bun.com/docs/bundler/executables). No runtime installation required. No dependency management on the target machine. This is how Claude Code distributes to millions of developers — one binary, every platform.

For aixyz agents deploying to serverless platforms like Vercel, Bun's cold starts are fast enough that your agent responds quickly even on first invocation:

```json title="vercel.json" theme={null}
{
  "buildCommand": "bun run build",
  "bunVersion": "1.x"
}
```

### Built-in Testing

Bun includes a Jest-compatible test runner with near-instant startup. aixyz uses `bun:test` for agent testing:

```typescript title="app/agent.test.ts" theme={null}
import { describe, expect, test } from "bun:test";
import { ToolLoopAgent } from "ai";
import agent from "./agent";

test("default export is a ToolLoopAgent", () => {
  expect(agent).toBeInstanceOf(ToolLoopAgent);
});
```