Vercel

To deploy to Vercel, use adapter-vercel.

This adapter will be installed by default when you use adapter-auto, but adding it to your project allows you to specify Vercel-specific options.

Usagepermalink

Install with npm i -D @sveltejs/adapter-vercel, then add the adapter to your svelte.config.js:

ts
import adapter from '@sveltejs/adapter-vercel';

export default {
	kit: {
		adapter: adapter({
			// see below for options that can be set here
		})
	}
};

Deployment configurationpermalink

To control how your routes are deployed to Vercel as functions, you can specify deployment configuration, either through the option shown above or with export const config inside +server.js, +page(.server).js and +layout(.server).js files.

For example you could deploy some parts of your app as Edge Functions...

ts
/** @type {import('@sveltejs/adapter-vercel').Config} */
export const config = {
	runtime: 'edge'
};

ts
import type { Config } from '@sveltejs/adapter-vercel';

export const config: Config = {
	runtime: 'edge',
};

...and others as Serverless Functions (note that by specifying config inside a layout, it applies to all child pages):

ts
/** @type {import('@sveltejs/adapter-vercel').Config} */
export const config = {
	runtime: 'nodejs18.x'
};

ts
import type { Config } from '@sveltejs/adapter-vercel';

export const config: Config = {
	runtime: 'nodejs18.x',
};

The following options apply to all functions:

• runtime: 'edge', 'nodejs18.x' or 'nodejs20.x'. By default, the adapter will select the 'nodejs<version>.x' corresponding to the Node version your project is configured to use on the Vercel dashboard
• regions: an array of edge network regions (defaulting to ["iad1"] for serverless functions) or 'all' if runtime is edge (its default). Note that multiple regions for serverless functions are only supported on Enterprise plans
• split: if true, causes a route to be deployed as an individual function. If split is set to true at the adapter level, all routes will be deployed as individual functions

Additionally, the following option applies to edge functions:

• external: an array of dependencies that esbuild should treat as external when bundling functions. This should only be used to exclude optional dependencies that will not run outside Node

And the following option apply to serverless functions:

• memory: the amount of memory available to the function. Defaults to 1024 Mb, and can be decreased to 128 Mb or increased in 64Mb increments up to 3008 Mb on Pro or Enterprise accounts
• maxDuration: maximum execution duration of the function. Defaults to 10 seconds for Hobby accounts, 15 for Pro and 900 for Enterprise
• isr: configuration Incremental Static Regeneration, described below

If your functions need to access data in a specific region, it's recommended that they be deployed in the same region (or close to it) for optimal performance.

Image Optimizationpermalink

You may set the images config to control how Vercel builds your images. See the image configuration reference for full details. As an example, you may set:

import adapter from '@sveltejs/adapter-vercel';

export default {
	kit: {
		adapter({
			images: {
				sizes: [640, 828, 1200, 1920, 3840],
				formats: ['image/avif', 'image/webp'],
				minimumCacheTTL: 300,
				domains: ['example-app.vercel.app'],
			}
		})
	}
};

Incremental Static Regenerationpermalink

Vercel supports Incremental Static Regeneration (ISR), which provides the performance and cost advantages of prerendered content with the flexibility of dynamically rendered content.

To add ISR to a route, include the isr property in your config object:

ts
import { BYPASS_TOKEN } from '$env/static/private';

export const config = {
	isr: {
		// Expiration time (in seconds) before the cached asset will be re-generated by invoking the Serverless Function.
		// Setting the value to `false` means it will never expire.
		expiration: 60,

		// Random token that can be provided in the URL to bypass the cached version of the asset, by requesting the asset
		// with a __prerender_bypass=<token> cookie.
		//
		// Making a `GET` or `HEAD` request with `x-prerender-revalidate: <token>` will force the asset to be re-validated.
		bypassToken: BYPASS_TOKEN,

		// List of valid query parameters. Other parameters (such as utm tracking codes) will be ignored,
		// ensuring that they do not result in content being regenerated unnecessarily
		allowQuery: ['search']
	}
};

ts
import { BYPASS_TOKEN } from '$env/static/private';

export const config = {
	isr: {
		// Expiration time (in seconds) before the cached asset will be re-generated by invoking the Serverless Function.
		// Setting the value to `false` means it will never expire.
		expiration: 60,

		// Random token that can be provided in the URL to bypass the cached version of the asset, by requesting the asset
		// with a __prerender_bypass=<token> cookie.
		//
		// Making a `GET` or `HEAD` request with `x-prerender-revalidate: <token>` will force the asset to be re-validated.
		bypassToken: BYPASS_TOKEN,

		// List of valid query parameters. Other parameters (such as utm tracking codes) will be ignored,
		// ensuring that they do not result in content being regenerated unnecessarily
		allowQuery: ['search'],
	},
};

The expiration property is required; all others are optional.

Environment variablespermalink

Vercel makes a set of deployment-specific environment variables available. Like other environment variables, these are accessible from $env/static/private and $env/dynamic/private (sometimes — more on that later), and inaccessible from their public counterparts. To access one of these variables from the client:

ts
import { VERCEL_COMMIT_REF } from '$env/static/private';

/** @type {import('./$types').LayoutServerLoad} */
export function load() {
	return {
		deploymentGitBranch: VERCEL_COMMIT_REF
	};
}

ts
import { VERCEL_COMMIT_REF } from '$env/static/private';
import type { LayoutServerLoad } from './$types';

export const load: LayoutServerLoad = () => {
	return {
		deploymentGitBranch: VERCEL_COMMIT_REF,
	};
};

<script>
	/** @type {import('./$types').LayoutServerData} */
	export let data;
</script>

<p>This staging environment was deployed from {data.deploymentGitBranch}.</p>

<script lang="ts">
	import type { LayoutServerData } from './$types';
	
	export let data: LayoutServerData;
</script>

<p>This staging environment was deployed from {data.deploymentGitBranch}.</p>

Since all of these variables are unchanged between build time and run time when building on Vercel, we recommend using $env/static/private — which will statically replace the variables, enabling optimisations like dead code elimination — rather than $env/dynamic/private.

Skew protectionpermalink

When a new version of your app is deployed, assets belonging to the previous version may no longer be accessible. If a user is actively using your app when this happens, it can cause errors when they navigate — this is known as version skew. SvelteKit mitigates this by detecting errors resulting from version skew and causing a hard reload to get the latest version of the app, but this will cause any client-side state to be lost. (You can also proactively mitigate it by observing the updated store value, which tells clients when a new version has been deployed.)

Skew protection is a Vercel feature that routes client requests to their original deployment. When a user visits your app, a cookie is set with the deployment ID, and any subsequent requests will be routed to that deployment for as long as skew protection is active. When they reload the page, they will get the newest deployment. (The updated store is exempted from this behaviour, and so will continue to report new deployments.) To enable it, visit the Advanced section of your project settings on Vercel.

Cookie-based skew protection comes with one caveat: if a user has multiple versions of your app open in multiple tabs, requests from older versions will be routed to the newer one, meaning they will fall back to SvelteKit's built-in skew protection.

Notespermalink

Vercel functionspermalink

If you have Vercel functions contained in the api directory at the project's root, any requests for /api/* will not be handled by SvelteKit. You should implement these as API routes in your SvelteKit app instead, unless you need to use a non-JavaScript language in which case you will need to ensure that you don't have any /api/* routes in your SvelteKit app.

Node versionpermalink

Projects created before a certain date may default to using an older Node version than what SvelteKit currently requires. You can change the Node version in your project settings.

Troubleshootingpermalink

Accessing the file systempermalink

You can't use fs in edge functions.

You can use it in serverless functions, but it won't work as expected, since files are not copied from your project into your deployment. Instead, use the read function from $app/server to access your files. read does not work inside routes deployed as edge functions (this may change in future).

Alternatively, you can prerender the routes in question.