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Last updated on June 6, 2023
15 min read

Build Output Configuration

Learn about the Build Output Configuration file, which is used to configure the behavior of a Deployment.

.vercel/output/config.json

Schema (as TypeScript):

type Config = {
  version: 3;
  routes?: Route[];
  images?: ImagesConfig;
  wildcard?: WildcardConfig;
  overrides?: OverrideConfig;
  cache?: string[];
  crons?: CronsConfig;
};

Config Types:

The config.json file contains configuration information and metadata for a Deployment. The individual properties are described in greater detail in the sub-sections below.

At a minimum, a config.json file with a "version" property is required.

Supported Properties

version

.vercel/output/config.json

The version property indicates which version of the Build Output API has been implemented. The version described in this document is version 3.

Example

  "version": 3

routes

.vercel/output/config.json
vercel/examples/build-output-api/routes

The routes property describes the routing rules that will be applied to the Deployment. It uses the same syntax as the routes property of the vercel.json file.

Routes may be used to point certain URL paths to others on your Deployment, attach response headers to paths, and various other routing-related use-cases.

type Route = Source | Handler;

Source Route

type Source = {
  src: string;
  dest?: string;
  headers?: Record<string, string>;
  methods?: string[];
  continue?: boolean;
  caseSensitive?: boolean;
  check?: boolean;
  status?: number;
  has?: Array<HostHasField | HeaderHasField | CookieHasField | QueryHasField>;
  missing?: Array<
    HostHasField | HeaderHasField | CookieHasField | QueryHasField
  >;
  locale?: Locale;
  middlewareRawSrc?: string[];
  middlewarePath?: string;
};
Key
Required
Description
src
Yes
A PCRE-compatible regular expression that matches each incoming pathname (excluding querystring).
dest
No
A destination pathname or full URL, including querystring, with the ability to embed capture groups as $1, $2, or named capture value $name.
headers
No
A set of headers to apply for responses.
methods
No
A set of HTTP method types. If no method is provided, requests with any HTTP method will be a candidate for the route.
continue
No
A boolean to change matching behavior. If true, routing will continue even when the src is matched.
caseSensitive
No
Specifies whether or not the route src should match with case sensitivity.
check
No
If true, the route triggers handle: 'filesystem' and handle: 'rewrite'
status
No
A status code to respond with. Can be used in tandem with Location: header to implement redirects.
has
Array<HostHasField | HeaderHasField | CookieHasField | QueryHasField>
No
Conditions of the HTTP request that must exist to apply the route.
missing
Array<HostHasField | HeaderHasField | CookieHasField | QueryHasField>
No
Conditions of the HTTP request that must NOT exist to match the route.
locale
Locale
No
Conditions of the Locale of the requester that will redirect the browser to different routes.
middlewareRawSrc
No
A list containing the original routes used to generate the middlewarePath.
middlewarePath
No
Path to an Edge Runtime function that should be invoked as middleware.
Source Route: Locale
type Locale = {
  redirect?: Record<string, string>;
  cookie?: string;
};
Key
Required
Description
redirect
Yes
An object of keys that represent locales to check for (en, fr, etc.) that map to routes to redirect to (/, /fr, etc.).
cookie
No
Cookie name that can override the Accept-Language header for determing the current locale.
Source Route: HostHasField
type HostHasField = {
  type: 'host';
  value: string;
};
Key
Required
Description
type
"host"
Yes
Determines the HasField type.
value
Yes
Host name that must match the request URL's host to cause this route to match.
Source Route: HeaderHasField
type HeaderHasField = {
  type: 'header';
  key: string;
  value?: string;
};
Key
Required
Description
type
"header"
Yes
Determines the HasField type.
key
Yes
Header name that must exist on the request for this route to match.
value
No
Header value (or regex) that must match for this route to match.
Source Route: CookieHasField
type CookieHasField = {
  type: 'cookie';
  key: string;
  value?: string;
};
Key
Required
Description
type
"cookie"
Yes
Determines the HasField type.
key
Yes
Cookie name that must exist on the request for this route to match.
value
No
Cookie value (or regex) that must match for this route to match.
Source Route: QueryHasField
type QueryHasField = {
  type: 'query';
  key: string;
  value?: string;
};
Key
Required
Description
type
"query"
Yes
Determines the HasField type.
key
Yes
Querystring key to look for.
value
No
Querystring value (or regex) that must match for this route to match.

Handler Route

The routing system has multiple phases. The handle value indicates the start of a phase. All following routes are only checked in that phase.

type HandleValue =
  | 'rewrite'
  | 'filesystem' // check matches after the filesystem misses
  | 'resource'
  | 'miss' // check matches after every filesystem miss
  | 'hit'
  | 'error'; //  check matches after error (500, 404, etc.)
 
type Handler = {
  handle: HandleValue;
  src?: string;
  dest?: string;
  status?: number;
};
Key
Required
Description
handle
HandleValue
Yes
The phase of routing when all subsequent routes should apply.
src
No
A PCRE-compatible regular expression that matches each incoming pathname (excluding querystring).
dest
No
A destination pathname or full URL, including querystring, with the ability to embed capture groups as $1, $2.
status
No
A status code to respond with. Can be used in tandem with Location: header to implement redirects.

Example

The following example shows a routing rule that will cause the /redirect path to perform an HTTP redirect to an external URL:

  "routes": [
    {
      "src": "/redirect",
      "status": 308,
      "headers": { "Location": "https://example.com/" }
    }
  ]

images

.vercel/output/config.json
vercel/examples/build-output-api/image-optimization

The images property defines the behavior of Vercel's native Image Optimization API, which allows on-demand optimization of images at runtime.

type ImageFormat = 'image/avif' | 'image/webp';
 
type RemotePattern = {
  protocol?: 'http' | 'https';
  hostname: string;
  port?: string;
  pathname?: string;
};
 
type ImagesConfig = {
  sizes: number[];
  domains: string[];
  remotePatterns?: RemotePattern[];
  minimumCacheTTL?: number; // seconds
  formats?: ImageFormat[];
  dangerouslyAllowSVG?: boolean;
  contentSecurityPolicy?: string;
};
Key
Required
Description
sizes
Yes
Supported image widths.
domains
Yes
Allowed external domains that can use Image Optimization. Leave empty for only allowing the deployment domain to use Image Optimization.
remotePatterns
RemotePattern[]
No
Allowed external patterns that can use Image Optimization. Similar to domains but provides more control with RegExp.
minimumCacheTTL
No
Cache duration (in seconds) for the optimized images.
formats
ImageFormat[]
No
Supported output image formats
dangerouslyAllowSVG
No
Allow SVG input image URLs. This is disabled by default for security purposes.
contentSecurityPolicy
No
Change the Content Security Policy of the optimized images.

Example

The following example shows an image optimization configuration that specifies allowed image size dimensions, external domains, caching lifetime and file formats:

  "images": {
    "sizes": [640, 750, 828, 1080, 1200],
    "domains": [],
    "minimumCacheTTL": 60,
    "formats": ["image/avif", "image/webp"],
    "remotePatterns": [{
      "protocol": "https",
      "hostname": "^via\\.placeholder\\.com$",
      "pathname": "^/1280x640/.*$"
    }]
  }

API

When the images property is defined, the Image Optimization API will be available by visiting the /_vercel/image path. When the images property is undefined, visiting the /_vercel/image path will respond with 404 Not Found.

The API accepts the following query string parameters:

Key
Required
Example
Description
url
Yes
/assets/me.png
The URL of the source image that should be optimized. Absolute URLs must match a pattern defined in the remotePatterns configuration.
w
Yes
200
The width (in pixels) that the source image should be resized to. Must match a value defined in the sizes configuration.
q
Yes
75
The quality that the source image should be reduced to. Must be between 1 (lowest quality) to 100 (highest quality).

wildcard

.vercel/output/config.json
vercel/examples/build-output-api/wildcard

The wildcard property relates to Vercel's Internationalization feature. The way it works is the domain names listed in this array are mapped to the $wildcard routing variable, which can be referenced by the routes configuration.

Each of the domain names specified in the wildcard configuration will need to be assigned as Production Domains in the Project Settings.

type WildCard = {
  domain: string;
  value: string;
};
 
type WildcardConfig = Array<WildCard>;

Supported Properties

Objects contained within the wildcard configuration support the following properties:

Key
Required
Description
domain
Yes
The domain name to match for this wildcard configuration.
value
Yes
The value of the $wildcard match that will be available for routes to utilize.

Example

The following example shows a wildcard configuration where the matching domain name will be served the localized version of the blog post HTML file:

  "wildcard": [
    {
      "domain": "example.com",
      "value": "en-US"
    },
    {
      "domain": "example.nl",
      "value": "nl-NL"
    },
    {
      "domain": "example.fr",
      "value": "fr"
    }
  ],
  "routes": [
    { "src": "/blog", "dest": "/blog.$wildcard.html" }
  ]

overrides

.vercel/output/config.json
vercel/examples/build-output-api/overrides

The overrides property allows for overriding the output of one or more static files contained within the .vercel/output/static directory.

The main use-cases are to override the Content-Type header that will be served for a static file, and/or to serve a static file in the Vercel Deployment from a different URL path than how it is stored on the file system.

type Override = {
  path?: string;
  contentType?: string;
};
 
type OverrideConfig = Record<string, Override>;

Supported Properties

Objects contained within the overrides configuration support the following properties:

Key
Required
Description
path
No
The URL path where the static file will be accessible from.
contentType
No
The value of the Content-Type HTTP response header that will be served with the static file.

Example

The following example shows an override configuration where an HTML file can be accessed without the .html file extension:

  "overrides": {
    "blog.html": {
      "path": "blog"
    }
  }

cache

.vercel/output/config.json

The cache property is an array of file paths and/or glob patterns that should be re-populated within the build sandbox upon subsequent Deployments.

Note that this property is only relevant when Vercel is building a Project from source code, meaning it is not relevant when building locally or when creating a Deployment from "prebuilt" build artifacts.

type Cache = string[];

Example

  "cache": [
    ".cache/**",
    "node_modules/**"
  ]

framework

.vercel/output/config.json

The optional framework property is an object describing the framework of the built outputs.

This value is used for display purposes only.

type Framework = {
  version: string;
};

Example

  "framework": {
    "version": "1.2.3"
  }

crons

.vercel/output/config.json

The optional crons property is an object describing the cron jobs for the production deployment of a project.

type Cron = {
  path: string;
  schedule: string;
};
 
type CronsConfig = Cron[];

Example

  "crons": [{
    "path": "/api/cron",
    "schedule": "0 0 * * *"
  }]

Full Example

{
  "version": 3,
  "routes": [
    {
      "src": "/redirect",
      "status": 308,
      "headers": { "Location": "https://example.com/" }
    },
    {
      "src": "/blog",
      "dest": "/blog.$wildcard.html"
    }
  ],
  "images": {
    "sizes": [640, 750, 828, 1080, 1200],
    "domains": [],
    "minimumCacheTTL": 60,
    "formats": ["image/avif", "image/webp"],
    "remotePatterns": [
      {
        "protocol": "https",
        "hostname": "^via\\.placeholder\\.com$",
        "pathname": "^/1280x640/.*$"
      }
    ]
  },
  "wildcard": [
    {
      "domain": "example.com",
      "value": "en-US"
    },
    {
      "domain": "example.nl",
      "value": "nl-NL"
    },
    {
      "domain": "example.fr",
      "value": "fr"
    }
  ],
  "overrides": {
    "blog.html": {
      "path": "blog"
    }
  },
  "cache": [".cache/**", "node_modules/**"],
  "framework": {
    "version": "1.2.3"
  },
  "crons": [
    {
      "path": "/api/cron",
      "schedule": "* * * * *"
    }
  ]
}