diff --git a/.changesets/11718.md b/.changesets/11718.md
new file mode 100644
index 000000000000..09c40430f2b0
--- /dev/null
+++ b/.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
diff --git a/packages/auth-providers/dbAuth/api/src/shared.ts b/packages/auth-providers/dbAuth/api/src/shared.ts
index 342ad8ba9310..ba7417150767 100644
--- a/packages/auth-providers/dbAuth/api/src/shared.ts
+++ b/packages/auth-providers/dbAuth/api/src/shared.ts
@@ -295,12 +295,18 @@ export function getDbAuthResponseBuilder(
     const setCookieHeaders = response.headers?.getSetCookie() || []
 
     if (setCookieHeaders.length > 0) {
-      if ('multiValueHeaders' in event) {
+      delete headers['set-cookie']
+      delete headers['Set-Cookie']
+
+      if (supportsMultiValueHeaders(event)) {
         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
       }
     }
@@ -309,6 +315,16 @@ export function getDbAuthResponseBuilder(
   }
 }
 
+// `'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
+function supportsMultiValueHeaders(event: APIGatewayProxyEvent | Request) {
+  return (
+    'multiValueHeaders' in event &&
+    (!event.headers || !('x-vercel-id' in event.headers))
+  )
+}
+
 export const extractHashingOptions = (text: string): ScryptOptions => {
   const [_hash, ...options] = text.split('|')