fix(dbAuth): Don't use Multi Value Headers on Vercel

.changesets/11718.md

@@ -0,0 +1,3 @@
+- fix(dbAuth): Don't use Multi Value Headers on Vercel (#11718) by @Tobbe
+
+Fixes a regression regarding dbAuth on Vercel introduced in RW 8

packages/auth-providers/dbAuth/api/src/shared.ts

@@ -295,12 +295,21 @@ export function getDbAuthResponseBuilder(
     const setCookieHeaders = response.headers?.getSetCookie() || []
 
     if (setCookieHeaders.length > 0) {
-      if ('multiValueHeaders' in event) {
+      delete headers['set-cookie']
+      delete headers['Set-Cookie']
+
+      // `'multiValueHeaders' in event` is true for both Netlify and Vercel
+      // but only Netlify actually supports it. Vercel will just ignore it
+      // https://github.com/vercel/vercel/issues/7820
+      if ('multiValueHeaders' in event && !('x-vercel-id' in headers)) {
         dbAuthResponse.multiValueHeaders = {
+          // Netlify wants 'Set-Cookie' headers to be capitalized
+          // https://github.com/redwoodjs/redwood/pull/10889
           'Set-Cookie': setCookieHeaders,
         }
-        delete headers['set-cookie']
       } else {
+        // If we do this for Netlify the lambda function will throw an error
+        // https://github.com/redwoodjs/redwood/pull/10889
         headers['set-cookie'] = setCookieHeaders
       }
     }