remove unnecessary internal middleware header from response

Author: ztanner

packages/next/src/server/lib/router-utils/resolve-routes.ts

@@ -585,6 +585,14 @@ export function getResolveRoutes(
               ) {
                 continue
               }
+
+              // for set-cookie, the header shouldn't be added to the response
+              // as it's only needed for the request to the middleware function.
+              if (key === 'x-middleware-set-cookie') {
+                req.headers[key] = value
+                continue
+              }
+
               if (value) {
                 resHeaders[key] = value
                 req.headers[key] = value

test/e2e/app-dir/app-middleware/app-middleware.test.ts

@@ -6,7 +6,7 @@ import { nextTestSetup, FileRef } from 'e2e-utils'
 import type { Response } from 'node-fetch'
 
 describe('app-dir with middleware', () => {
-  const { next } = nextTestSetup({
+  const { next, isNextDeploy } = nextTestSetup({
     files: __dirname,
   })
 
@@ -187,6 +187,15 @@ describe('app-dir with middleware', () => {
     await browser.deleteCookies()
   })
 
+  // TODO: Re-enable this test in deploy mode once Vercel has proper handling
+  if (!isNextDeploy) {
+    it('should omit internal headers for middleware cookies', async () => {
+      const response = await next.fetch('/rsc-cookies/cookie-options')
+      expect(response.status).toBe(200)
+      expect(response.headers.get('x-middleware-set-cookie')).toBeNull()
+    })
+  }
+
   it('should be possible to read cookies that are set during the middleware handling of a server action', async () => {
     const browser = await next.browser('/rsc-cookies')
     const initialRandom1 = await browser.elementById('rsc-cookie-1').text()