diff --git a/ui/src/lib/api.ts b/ui/src/lib/api.ts
index a2115a95..f93fdd24 100644
--- a/ui/src/lib/api.ts
+++ b/ui/src/lib/api.ts
@@ -10,10 +10,6 @@ const API_BASE = (import.meta.env.VITE_DASHBOARD_API_BASE_URL ?? "").replace(
   ""
 )
 
-if (!API_BASE && typeof window !== "undefined") {
-  console.warn("VITE_DASHBOARD_API_BASE_URL is not set")
-}
-
 const GITHUB_IMAGE_HOST_RE =
   /^(?:www\.)?github\.com$|\.githubusercontent\.com$/i
 
diff --git a/ui/vite.config.ts b/ui/vite.config.ts
index f320b2e2..c179137b 100644
--- a/ui/vite.config.ts
+++ b/ui/vite.config.ts
@@ -92,6 +92,13 @@ function backendProxyRouteRules():
   // ship a build whose API calls 404, which is exactly how the previous approach broke.
   const onVercel =
     !!process.env.VERCEL || process.env.NITRO_PRESET === "vercel"
+  if (onVercel && process.env.VITE_DASHBOARD_API_BASE_URL) {
+    throw new Error(
+      "VITE_DASHBOARD_API_BASE_URL must not be set on Vercel builds - the API is " +
+        "same-origin via the LANGGRAPH_BACKEND_URL CDN proxy. A baked-in absolute " +
+        "API base sends OAuth logins to the backend host and breaks the state cookie.",
+    )
+  }
   if (!backend) {
     if (onVercel) {
       throw new Error(