docs(device-authorization): gate approval on owner-only request details

Approve stays disabled unless the lookup returned the client ID, which
GET /device includes only for the session that claimed the code. The
demo also discards stale lookups, reports invalid or expired codes, and
both examples show a registered client name instead of the raw ID.

demo/nextjs/app/(auth)/device/approve/page.tsx

@@ -9,6 +9,14 @@ import { Card } from "@/components/ui/card";
 import { useSessionQuery } from "@/data/user/session-query";
 import { authClient } from "@/lib/auth-client";
 
+/**
+ * Client IDs this app registered, mapped to names users recognize. A device
+ * chooses its own `client_id`, so never present an unknown ID as a trusted name.
+ */
+const KNOWN_CLIENTS: Record<string, string> = {
+	"demo-cli": "Better Auth demo CLI",
+};
+
 export default function Page() {
 	const router = useRouter();
 	const searchParams = useSearchParams();
@@ -18,15 +26,31 @@ export default function Page() {
 	const [isDenyPending, startDenyTransition] = useTransition();
 	const [error, setError] = useState<string | null>(null);
 	const [request, setRequest] = useState<{
-		client_id?: string;
+		client_id: string;
 		scope?: string;
 	} | null>(null);
 
 	useEffect(() => {
+		setRequest(null);
+		setError(null);
 		if (!session || !userCode) return;
-		authClient.device({ query: { user_code: userCode } }).then(({ data }) => {
-			if (data) setRequest(data);
-		});
+		let active = true;
+		void authClient
+			.device({ query: { user_code: userCode } })
+			.then(({ data, error }) => {
+				if (!active) return;
+				if (!data?.client_id) {
+					setError(
+						error?.message ||
+							"This code is invalid, expired, or was claimed by another session.",
+					);
+					return;
+				}
+				setRequest({ client_id: data.client_id, scope: data.scope });
+			});
+		return () => {
+			active = false;
+		};
 	}, [session, userCode]);
 
 	const handleApprove = () => {
@@ -86,7 +110,18 @@ export default function Page() {
 
 						<div className="rounded-lg bg-muted p-4">
 							<p className="text-sm font-medium">Requesting client</p>
-							<p className="font-mono">{request?.client_id ?? "Loading..."}</p>
+							{request ? (
+								<>
+									<p>
+										{KNOWN_CLIENTS[request.client_id] ?? "Unrecognized client"}
+									</p>
+									<p className="font-mono text-xs text-muted-foreground">
+										ID sent by the device: {request.client_id}
+									</p>
+								</>
+							) : (
+								<p>Loading...</p>
+							)}
 						</div>
 
 						<div className="rounded-lg bg-muted p-4">

docs/content/docs/plugins/device-authorization.mdx

@@ -462,6 +462,11 @@ Users must be authenticated to approve or deny device authorization requests:
 Avoid approval pages that show only the user code, or that enable Approve before the client and scopes appear. On such a page the user cannot tell a request from their own device apart from one started by someone else, for example a code sent to them in a message. The risk is higher when `validateClient` is not configured, because anyone can then start a request.
 
 ```tsx title="app/device/approve/page.tsx"
+// Client IDs you registered, mapped to names your users recognize.
+const knownClients: Record<string, string> = {
+  "acme-cli": "Acme CLI",
+};
+
 export default function DeviceApprovalPage() {
   const { user } = useAuth(); // Must be authenticated
   const searchParams = useSearchParams();
@@ -474,10 +479,16 @@ export default function DeviceApprovalPage() {
   } | null>(null);
 
   useEffect(() => {
+    setRequest(null);
     if (!user || !userCode) return;
+    let active = true;
     authClient.device({ query: { user_code: userCode } }).then(({ data }) => {
-      setRequest(data);
+      // Owner-only details are missing when another session claimed the code.
+      if (active && data?.client_id) setRequest(data);
     });
+    return () => {
+      active = false;
+    };
   }, [user, userCode]);
   
   const handleApprove = async () => {
@@ -519,7 +530,11 @@ export default function DeviceApprovalPage() {
   return (
     <div>
       <h2>Device Authorization Request</h2>
-      <p>Client: {request?.client_id}</p>
+      <p>
+        Client: {request?.client_id
+          ? knownClients[request.client_id] ?? `Unrecognized client (${request.client_id})`
+          : "Loading..."}
+      </p>
       <p>Scopes: {request?.scope || "None"}</p>
       <p>
         Resources: {Array.isArray(request?.resource)