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Adapter for SvelteKit apps that generates a standalone Bun server.

⚡ Usage

Install with bun add -d svelte-adapter-bun, then add the adapter to your svelte.config.js:

// svelte.config.js
import adapter from "svelte-adapter-bun";

export default {
  kit: {
    adapter: adapter(),

After building the server (vite build), use the following command to start:

# go to build directory
cd build/

# run Bun
bun run start

⚙️ Options

The adapter can be configured with various options:

// svelte.config.js
import adapter from "svelte-adapter-bun";
export default {
  kit: {
    adapter: adapter({
      out: "build",
      assets: true,
      envPrefix: "MY_CUSTOM_",
      development: true,
      // precompress: true,
      precompress: {
        brotli: true,
        gzip: true,
        files: ["htm", "html"],
      dynamic_origin: true,
      xff_depth: 1,


The directory to build the server to. It defaults to build — i.e. bun run start would start the server locally after it has been created.


Browse a static assets. Default: true


Enables precompressing using gzip and brotli for assets and prerendered pages. It defaults to false.


Enable brotli precompressing. It defaults to false.


Enable gzip precompressing. It defaults to false.


file extensions to compress.It defaults to ['html','js','json','css','svg','xml','wasm'].


If you need to change the name of the environment variables used to configure the deployment (for example, to deconflict with environment variables you don't control), you can specify a prefix:

envPrefix: "MY_CUSTOM_";
bun build/index.js


This enables bun's error page. Default: false


If enabled use PROTOCOL_HEADER HOST_HEADER like origin. Default: false


The default value of XFF_DEPTH if environment is not set. Default: 1

🕸️ WebSocket Server

// hooks.server.js

/** @type {import("svelte-adapter-bun").WebSocketHandler} */
export const handleWebsocket = {
  open(ws) {
    console.log("WebSocket opened");
    ws.send("Slava Ukraїni");
   * @param {Request} request
   * @param {Function} upgrade
  upgrade(request, upgrade) {
    const url = new URL(request.url);
    if (url.pathname.startsWith("/ws")) {
      return upgrade(request);


If you need to use polyfills in your app, you can add them to the src/polyfills.js file:

class Polifill {
  constructor() {

const globals = {

🖥️ Environment variables

Bun automatically reads configuration from .env.local, .env.development and .env


By default, the server will accept connections on using port 3000. These can be customized with the PORT and HOST environment variables:

HOST= PORT=4000 bun build/index.js


HTTP doesn't give SvelteKit a reliable way to know the URL that is currently being requested. The simplest way to tell SvelteKit where the app is being served is to set the ORIGIN environment variable:

ORIGIN= bun build/index.js

With this, a request for the /stuff pathname will correctly resolve to Alternatively, you can specify headers that tell SvelteKit about the request protocol and host, from which it can construct the origin URL:

PROTOCOL_HEADER=x-forwarded-proto HOST_HEADER=x-forwarded-host bun build/index.js

x-forwarded-proto and x-forwarded-host are de facto standard headers that forward the original protocol and host if you're using a reverse proxy (think load balancers and CDNs). You should only set these variables if your server is behind a trusted reverse proxy; otherwise, it'd be possible for clients to spoof these headers.


The RequestEvent object passed to hooks and endpoints includes an event.clientAddress property representing the client's IP address. Bun.js haven't got functionality to get client's IP address, so SvelteKit will receive or if your server is behind one or more proxies (such as a load balancer), you can get an IP address from headers, so we need to specify an ADDRESS_HEADER to read the address from:

ADDRESS_HEADER=True-Client-IP bun build/index.js

Headers can easily be spoofed. As with PROTOCOL_HEADER and HOST_HEADER, you should know what you're doing before setting these. If the ADDRESS_HEADER is X-Forwarded-For, the header value will contain a comma-separated list of IP addresses. The XFF_DEPTH environment variable should specify how many trusted proxies sit in front of your server. E.g. if there are three trusted proxies, proxy 3 will forward the addresses of the original connection and the first two proxies:

<client address>, <proxy 1 address>, <proxy 2 address>

Some guides will tell you to read the left-most address, but this leaves you vulnerable to spoofing:

<spoofed address>, <client address>, <proxy 1 address>, <proxy 2 address>

Instead, we read from the right, accounting for the number of trusted proxies. In this case, we would use XFF_DEPTH=3.

If you need to read the left-most address instead (and don't care about spoofing) — for example, to offer a geolocation service, where it's more important for the IP address to be real than trusted, you can do so by inspecting the x-forwarded-for header within your app.


MIT © Volodymyr Palamar