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Cross-Origin Resource Sharing (CORS) is an HTTP‑header mechanism that lets a browser ask another origin for permission to read its responses. It softens the browser’s same‑origin policy so frontend code at https://app.example can fetch from https://api.example or any other domain when the server explicitly allows it.

Headers you typically need (and why)

Access-Control-Allow-Origin: which origins may read the response. Use * for public APIs or a specific origin such as https://app.example for stricter security.
Access-Control-Allow-Methods: HTTP verbs the client may use (e.g. GET, POST, PUT, PATCH, DELETE, OPTIONS).
Access-Control-Allow-Headers: custom request headers the browser may send, such as Authorization, Content-Type, or X-CSRF-Token.
Access-Control-Allow-Credentials: whether the browser may send cookies or HTTP‑auth headers. Must be true and used together with an explicit (non‑*) origin.
Access-Control-Max-Age: how long the browser can cache the pre‑flight response, in seconds (e.g. 86400 for 24 h).

Browsers issue an OPTIONS pre‑flight request whenever a request is “non‑simple” (has custom headers, a non‑GET/POST verb, etc.). The pre‑flight must return all of the headers above or the real request is never sent.

Implementation patterns on Vercel

Vercel Functions, when used standalone or through frameworks, do not add CORS headers automatically. If you are seeing CORS errors, here's how you can fix it.

Next.js Route Handler (App Router)

app/api/hello/route.ts

constALLOWED_ORIGIN=
 process.env.NODE_ENV==='production'
?'https://app.example'
:'*';
exportasyncfunctionOPTIONS(){
returnnewResponse(null,{
status:200,
headers:{
'Access-Control-Allow-Origin':ALLOWED_ORIGIN,
'Access-Control-Allow-Methods':'GET, POST, OPTIONS',
'Access-Control-Allow-Headers':'Content-Type, Authorization',
'Access-Control-Allow-Credentials':'true',
},
});
}
exportasyncfunctionGET(){
returnResponse.json({ok:true},{
headers:{'Access-Control-Allow-Origin':ALLOWED_ORIGIN},
});
}

Standalone Vercel Functions

This same code example works for standalone Vercel Functions without a framework, placed at api/hello.ts.

Framework‑level headers

You can also apply headers through different configuration patterns in frameworks:

Next.js : add a headers() async function in next.config.ts that matches /api/:path* and sets the five headers
SvelteKit : set headers in the global handle hook
Remix: return json(data, { headers }) from loaders or actions
Nuxt: use routeRules or set headers in server/api/*

Each method ends up setting the same headers as the examples above.

Global route headers via `vercel.json`

vercel.json

{
"headers":[
{
"source":"/api/(.*)",
"headers":[
{"key":"Access-Control-Allow-Origin","value":"https://app.example"},
{"key":"Access-Control-Allow-Methods","value":"GET, POST, OPTIONS"},
{"key":"Access-Control-Allow-Headers","value":"Content-Type, Authorization"},
{"key":"Access-Control-Allow-Credentials","value":"true"}
]
}
]
}

Placing headers in vercel.json pushes them to Vercel’s CDN so they apply before your function runs.

Using Routing Middleware for Dynamic CORS Headers

For more advanced CORS scenarios where you need dynamic header values based on request properties (like origin, user agent, or geolocation), you can use Vercel’s Routing Middleware. This approach is particularly useful when you need to:

Set different CORS policies based on the requesting origin
Apply CORS headers conditionally based on request properties
Implement more complex CORS logic that goes beyond static configuration

Basic Dynamic CORS with Middleware

Create a middleware.ts file at the root of your project:

middleware.ts

import{NextResponse}from'next/server';
import type {NextRequest}from'next/server';
exportconst config ={
matcher:'/api/:path*',// Apply only to API routes
};
exportdefaultfunctionmiddleware(request:NextRequest){
const origin = request.headers.get('origin');
// Define allowed origins dynamically
const allowedOrigins = process.env.NODE_ENV==='production'
?['https://app.example.com','https://admin.example.com']
:['http://localhost:3000','http://localhost:3001'];
const isAllowedOrigin = origin && allowedOrigins.includes(origin);
// Handle preflight requests
if(request.method==='OPTIONS'){
returnnewResponse(null,{
status:200,
headers:{
'Access-Control-Allow-Origin': isAllowedOrigin ? origin :'null',
'Access-Control-Allow-Methods':'GET, POST, PUT, DELETE, OPTIONS',
'Access-Control-Allow-Headers':'Content-Type, Authorization',
'Access-Control-Allow-Credentials':'true',
'Access-Control-Max-Age':'86400',
},
});
}
// Continue with the request and add CORS headers to the response
const response =NextResponse.next();
if(isAllowedOrigin){
 response.headers.set('Access-Control-Allow-Origin', origin);
 response.headers.set('Access-Control-Allow-Credentials','true');
}
return response;
}

Geolocation-Based CORS

You can also use Vercel’s geolocation data to implement region-specific CORS policies:

middleware.ts

import{NextResponse}from'next/server';
import type {NextRequest}from'next/server';
exportconst config ={
matcher:'/api/:path*',
};
exportdefaultfunctionmiddleware(request:NextRequest){
const origin = request.headers.get('origin');
const country = request.geo?.country ||'US';
// Different CORS policies based on country
constgetCorsPolicy=(country: string)=>{
switch(country){
case'US':
case'CA':
return{
allowedOrigins:['https://us.example.com','https://ca.example.com'],
allowCredentials:true,
};
case'GB':
case'DE':
return{
allowedOrigins:['https://eu.example.com'],
allowCredentials:true,
};
default:
return{
allowedOrigins:['https://global.example.com'],
allowCredentials:false,
};
}
};
const corsPolicy =getCorsPolicy(country);
const isAllowedOrigin = origin && corsPolicy.allowedOrigins.includes(origin);
if(request.method==='OPTIONS'){
returnnewResponse(null,{
status:200,
headers:{
'Access-Control-Allow-Origin': isAllowedOrigin ? origin :'null',
'Access-Control-Allow-Methods':'GET, POST, PUT, DELETE, OPTIONS',
'Access-Control-Allow-Headers':'Content-Type, Authorization',
'Access-Control-Allow-Credentials': corsPolicy.allowCredentials.toString(),
'Access-Control-Max-Age':'86400',
},
});
}
const response =NextResponse.next();
if(isAllowedOrigin){
 response.headers.set('Access-Control-Allow-Origin', origin);
 response.headers.set('Access-Control-Allow-Credentials', corsPolicy.allowCredentials.toString());
}
return response;
}

Combining with API Route Handlers

When using middleware for CORS, your API route handlers become simpler since the CORS headers are handled at the middleware level:

api/users/route.ts

exportasyncfunctionGET(){
// No need to set CORS headers here - middleware handles it
returnResponse.json({users:[]});
}
exportasyncfunctionPOST(request:Request){
const data =await request.json();
// Process the data
returnResponse.json({success:true});
}

Handling the OPTIONS pre‑flight with Deployment Protection

If you have Deployment Protection turned on your preview or production Vercel deployments, you can use OPTIONS Allowlist to allow CORS to work on a list of paths that you define.

When Vercel Authentication, Password Protection, or Trusted IPs is active, unauthenticated pre‑flight requests would normally be blocked.
OPTIONS Allowlist lets you exempt specific paths from Deployment Protection only for OPTIONS requests. /api/* is on the allowlist by default for new projects.

Here's an example of the typical flow:

Keep Vercel Authentication enabled for /api/*.
Ensure /api is in the OPTIONS Allowlist (default).
Browser pre‑flight succeeds; the real POST /api/... request still requires auth.

Test your setup

Terminal

# Pre‑flight
curl-i-X OPTIONS https://your-domain.vercel.app/api/hello \
-H"Origin: https://app.example"\
-H"Access-Control-Request-Method: POST"
# Simple request
curl-i https://your-domain.vercel.app/api/hello \
-H"Origin: https://app.example"

Look for a 200 status on the OPTIONS call and the correct CORS headers in both responses.

Common pitfalls and fixes

Forgetting to add headers on error paths: wrap all return points (including 4xx/5xx) in a helper that sets CORS, or apply rules globally through configuration
Using * with Access-Control-Allow-Credentials: true: the spec forbids this; send a specific origin instead
Excessive pre‑flight traffic: raise Access-Control-Max-Age (up to 86400 s = 24 h) so browsers cache the response

URL: https://vercel.com/kb/guide/how-to-enable-cors?query=cors

⇱ How can I enable CORS on Vercel? | Vercel Knowledge Base

How can I enable CORS on Vercel?