diff --git a/frontend/src/pages/api/[...path].ts b/frontend/src/pages/api/[...path].ts
index bf9b125..b07a76f 100644
--- a/frontend/src/pages/api/[...path].ts
+++ b/frontend/src/pages/api/[...path].ts
@@ -42,8 +42,15 @@ export const ALL: APIRoute = async ({ request, params }) => {
 
     const responseHeaders = new Headers();
     response.headers.forEach((value, key) => {
+      // Set-Cookie can repeat and must NOT be merged. Handle it separately below.
+      if (key.toLowerCase() === 'set-cookie') return;
       responseHeaders.set(key, value);
     });
+    // @ts-ignore — getSetCookie is on Node fetch's Headers
+    const setCookies: string[] = response.headers.getSetCookie?.() ?? [];
+    for (const c of setCookies) {
+      responseHeaders.append('set-cookie', c);
+    }
 
     return new Response(response.body, {
       status: response.status,