---
title: Bun v1.3.8
description: "Fixes 8 issues (addressing 6 👍). Bun.markdown is a builtin CommonMark-compliant Markdown parser written in Zig. Bun's bundler gets bun build --metafile-md to write LLM-friendly module graph metadata. Fixes a regression in 'npm install -g bun' on Windows."
date: "2026-01-29T10:04:52.476Z"
author: jarred
---

#### To install Bun

{% codetabs %}

```sh#curl
$ curl -fsSL https://bun.sh/install | bash
```

```sh#npm
$ npm install -g bun
```

```sh#powershell
$ powershell -c "irm bun.sh/install.ps1|iex"
```

```sh#scoop
$ scoop install bun
```

```sh#brew
$ brew tap oven-sh/bun
$ brew install bun
```

```sh#docker
$ docker pull oven/bun
$ docker run --rm --init --ulimit memlock=-1:-1 oven/bun
```

{% /codetabs %}

#### To upgrade Bun

```sh
$ bun upgrade
```

## `Bun.markdown` — Built-in Markdown Parser

Bun now includes a fast, CommonMark-compliant Markdown parser written in Zig. This is a Zig port of the popular `md4c` library.

The new `Bun.markdown` API provides three ways to render Markdown:

### `Bun.markdown.html()` — Render to HTML

```js
const html = Bun.markdown.html("# Hello **world**");
// "<h1>Hello <strong>world</strong></h1>\n"

// With options
Bun.markdown.html("## Hello", { headingIds: true });
// '<h2 id="hello">Hello</h2>\n'
```

### `Bun.markdown.render()` — Custom Callbacks

Render with JavaScript callbacks for each element, perfect for custom HTML, ANSI terminal output, or stripping formatting:

```js
// Custom HTML with classes
const html = Bun.markdown.render("# Title\n\nHello **world**", {
  heading: (children, { level }) =>
    `<h${level} class="title">${children}</h${level}>`,
  paragraph: (children) => `<p>${children}</p>`,
  strong: (children) => `<b>${children}</b>`,
});

// ANSI terminal output
const ansi = Bun.markdown.render("# Hello\n\n**bold**", {
  heading: (children) => `\x1b[1;4m${children}\x1b[0m\n`,
  paragraph: (children) => children + "\n",
  strong: (children) => `\x1b[1m${children}\x1b[22m`,
});

// Return null to omit elements
const result = Bun.markdown.render("# Title\n\n![logo](img.png)", {
  image: () => null,
  heading: (children) => children,
});
```

### `Bun.markdown.react()` — React Elements

Returns a React Fragment directly usable as a component return value:

```tsx
function Markdown({ text }: { text: string }) {
  return Bun.markdown.react(text);
}

// With custom components
const element = Bun.markdown.react("# Hello", {
  h1: ({ children }) => <h1 className="title">{children}</h1>,
});

// Server-side rendering
import { renderToString } from "react-dom/server";
const html = renderToString(Bun.markdown.react("# Hello **world**"));
```

For React 18 and older, pass `reactVersion: 18` since the default targets React 19's element format.

### GFM Extensions

GitHub Flavored Markdown extensions are enabled by default:

- Tables
- Strikethrough (`~~deleted~~`)
- Task lists (`- [x] done`)
- Permissive autolinks

Additional options include `wikiLinks`, `latexMath`, `headingIds`, and `autolinkHeadings`.

<!-- https://github.com/oven-sh/bun/commit/1bfe5c6b37e65995ac58e761ccbff7bb7b8dc954 -->

## `--metafile-md` CLI option for LLM-friendly bundle analysis

`bun build` now supports a `--metafile-md` option that generates a Markdown visualization of your bundle's module graph. This is particularly useful for analyzing bundle composition with LLMs like Claude—paste the output into a chat to identify bloat, understand dependency chains, and optimize your builds.

```bash
# Default filename (meta.md)
bun build entry.js --metafile-md --outdir=dist

# Custom filename
bun build entry.js --metafile-md=analysis.md --outdir=dist

# Both JSON and markdown
bun build entry.js --metafile=meta.json --metafile-md=meta.md --outdir=dist
```

The generated Markdown includes:

- **Quick Summary** — Module counts, sizes, ESM/CJS breakdown, output/input ratio
- **Largest Input Files** — Sorted by size to identify potential bloat
- **Entry Point Analysis** — Bundle size, exports, CSS bundles, and bundled modules
- **Dependency Chains** — Most commonly imported modules and reverse dependencies
- **Full Module Graph** — Complete import/export info for each module
- **Raw Data for Searching** — Grep-friendly markers like `[MODULE:]`, `[SIZE:]`, `[IMPORT:]`

The `Bun.build()` API also supports this via an expanded `metafile` option:

```js
const result = await Bun.build({
  entrypoints: ["./index.ts"],
  outdir: "./dist",
  // Can now be a string path or object with json/markdown paths
  metafile: {
    json: "meta.json",
    markdown: "meta.md",
  },
});
```

<!-- https://github.com/oven-sh/bun/commit/aded701d1d2f89663a114c22ef6550153c9b23e4 -->

## Bugfixes

- Updated mimalloc
- Fixed: `napi_typeof` incorrectly returning `napi_object` instead of `napi_function` for callbacks wrapped in `AsyncContextFrame`, which caused native addons like encore.dev to panic with "expect Function, got: Object" when using `AsyncLocalStorage.run()`
- Fixed: Crash during heap snapshot generation in certain cases
- Fixed: Crash in `node:vm` when using `SyntheticModule` with `node:async_hooks` enabled, such as when running the React Email preview server
- Fixed: HTTP/2 stream state handling that caused gRPC streaming calls to fail with `DEADLINE_EXCEEDED` errors in certain cases when using libraries like `@google-cloud/firestore` and `@grpc/grpc-js`
- Fixed: `npm i -g bun` failing on Windows due to npm's `cmd-shim` generating broken wrappers that referenced `/bin/sh` from placeholder scripts' shebang lines

### Thanks to 4 contributors!

- [@dylan-conway](https://github.com/dylan-conway)
- [@jarred-sumner](https://github.com/jarred-sumner)
- [@robobun](https://github.com/robobun)
- [@sosukesuzuki](https://github.com/sosukesuzuki)