diff --git a/web/frontend/src/components/app-header.tsx b/web/frontend/src/components/app-header.tsx
index 1a673f598..798ac8ad5 100644
--- a/web/frontend/src/components/app-header.tsx
+++ b/web/frontend/src/components/app-header.tsx
@@ -49,10 +49,12 @@ export function AppHeader() {
     state: gwState,
     loading: gwLoading,
     canStart,
+    startReason,
     restartRequired,
     start,
     restart,
     stop,
+    error: gwError,
   } = useGateway()
 
   const isRunning = gwState === "running"
@@ -196,37 +198,50 @@ export function AppHeader() {
                 <IconPower className="h-4 w-4 opacity-80" />
               </Button>
             </TooltipTrigger>
-            <TooltipContent>{t("header.gateway.action.stop")}</TooltipContent>
+            <TooltipContent>{gwError ?? t("header.gateway.action.stop")}</TooltipContent>
           </Tooltip>
         ) : (
-          <Button
-            variant={
-              isStarting || isRestarting || isStopping ? "secondary" : "default"
-            }
-            size="sm"
-            data-tour="gateway-button"
-            className={`h-8 gap-2 px-3 ${isStopped ? "bg-green-500 text-white hover:bg-green-600" : ""
-              }`}
-            onClick={handleGatewayToggle}
-            disabled={
-              gwLoading || isStarting || isRestarting || isStopping || !canStart
-            }
-          >
-            {gwLoading || isStarting || isRestarting || isStopping ? (
-              <IconLoader2 className="h-4 w-4 animate-spin opacity-70" />
-            ) : (
-              <IconPlayerPlay className="h-4 w-4 opacity-80" />
-            )}
-            <span className="text-xs font-semibold">
-              {isStopping
-                ? t("header.gateway.status.stopping")
-                : isRestarting
-                  ? t("header.gateway.status.restarting")
-                  : isStarting
-                    ? t("header.gateway.status.starting")
-                    : t("header.gateway.action.start")}
-            </span>
-          </Button>
+          <Tooltip delayDuration={(gwError || (!canStart && startReason)) ? 0 : 700}>
+            <TooltipTrigger asChild>
+              {/* Wrap in span so the tooltip still fires when the button is disabled */}
+              <span
+                className={!canStart && startReason ? "cursor-not-allowed" : undefined}
+                tabIndex={!canStart && startReason ? 0 : undefined}
+              >
+                <Button
+                  variant={
+                    isStarting || isRestarting || isStopping ? "secondary" : "default"
+                  }
+                  size="sm"
+                  data-tour="gateway-button"
+                  className={`h-8 gap-2 px-3 ${isStopped ? "bg-green-500 text-white hover:bg-green-600" : ""
+                    } ${!canStart ? "pointer-events-none" : ""}`}
+                  onClick={handleGatewayToggle}
+                  disabled={
+                    gwLoading || isStarting || isRestarting || isStopping || !canStart
+                  }
+                >
+                  {gwLoading || isStarting || isRestarting || isStopping ? (
+                    <IconLoader2 className="h-4 w-4 animate-spin opacity-70" />
+                  ) : (
+                    <IconPlayerPlay className="h-4 w-4 opacity-80" />
+                  )}
+                  <span className="text-xs font-semibold">
+                    {isStopping
+                      ? t("header.gateway.status.stopping")
+                      : isRestarting
+                        ? t("header.gateway.status.restarting")
+                        : isStarting
+                          ? t("header.gateway.status.starting")
+                          : t("header.gateway.action.start")}
+                  </span>
+                </Button>
+              </span>
+            </TooltipTrigger>
+            {(gwError || (!canStart && startReason)) ? (
+              <TooltipContent>{gwError ?? startReason}</TooltipContent>
+            ) : null}
+          </Tooltip>
         )}
 
         <Separator
diff --git a/web/frontend/src/hooks/use-gateway.ts b/web/frontend/src/hooks/use-gateway.ts
index b118b43da..31bee0e91 100644
--- a/web/frontend/src/hooks/use-gateway.ts
+++ b/web/frontend/src/hooks/use-gateway.ts
@@ -13,8 +13,9 @@ import {
 
 export function useGateway() {
   const gateway = useAtomValue(gatewayAtom)
-  const { status: state, canStart, restartRequired } = gateway
+  const { status: state, canStart, startReason, restartRequired } = gateway
   const [loading, setLoading] = useState(false)
+  const [error, setError] = useState<string | null>(null)
 
   useEffect(() => {
     return subscribeGatewayPolling()
@@ -23,6 +24,7 @@ export function useGateway() {
   const start = useCallback(async () => {
     if (!canStart) return
 
+    setError(null)
     setLoading(true)
     try {
       await startGateway()
@@ -32,6 +34,7 @@ export function useGateway() {
       })
     } catch (err) {
       console.error("Failed to start gateway:", err)
+      setError(err instanceof Error ? err.message : String(err))
     } finally {
       await refreshGatewayState({ force: true })
       setLoading(false)
@@ -39,12 +42,14 @@ export function useGateway() {
   }, [canStart])
 
   const stop = useCallback(async () => {
+    setError(null)
     setLoading(true)
     beginGatewayStoppingTransition()
     try {
       await stopGateway()
     } catch (err) {
       console.error("Failed to stop gateway:", err)
+      setError(err instanceof Error ? err.message : String(err))
       cancelGatewayStoppingTransition()
     } finally {
       await refreshGatewayState({ force: true })
@@ -55,6 +60,7 @@ export function useGateway() {
   const restart = useCallback(async () => {
     if (state !== "running") return
 
+    setError(null)
     setLoading(true)
     try {
       await restartGateway()
@@ -64,11 +70,12 @@ export function useGateway() {
       })
     } catch (err) {
       console.error("Failed to restart gateway:", err)
+      setError(err instanceof Error ? err.message : String(err))
     } finally {
       await refreshGatewayState({ force: true })
       setLoading(false)
     }
   }, [state])
 
-  return { state, loading, canStart, restartRequired, start, stop, restart }
+  return { state, loading, canStart, startReason, restartRequired, start, stop, restart, error }
 }
diff --git a/web/frontend/src/routes/__root.tsx b/web/frontend/src/routes/__root.tsx
index 33ad8c698..d287ce801 100644
--- a/web/frontend/src/routes/__root.tsx
+++ b/web/frontend/src/routes/__root.tsx
@@ -42,8 +42,14 @@ const RootLayout = () => {
           globalThis.location.assign("/launcher-login")
         }
       })
-      .catch(() => {
-        // Network error or 401 — launcherFetch will handle redirect on real API calls.
+      .catch((err: unknown) => {
+        // For real HTTP errors (e.g. 503 when the auth store is unavailable),
+        // redirect to login so the user can re-authenticate rather than being
+        // silently stranded on the dashboard.  Network failures (no response)
+        // are left alone — launcherFetch handles the 401 redirect on real calls.
+        if (err instanceof Error && /^status \d+$/.test(err.message)) {
+          globalThis.location.assign("/launcher-login")
+        }
       })
   }, [isAuthPage])
 
diff --git a/web/frontend/src/store/gateway.ts b/web/frontend/src/store/gateway.ts
index 1bdec6220..5bf6f3897 100644
--- a/web/frontend/src/store/gateway.ts
+++ b/web/frontend/src/store/gateway.ts
@@ -14,6 +14,7 @@ export type GatewayState =
 export interface GatewayStoreState {
   status: GatewayState
   canStart: boolean
+  startReason?: string
   restartRequired: boolean
 }
 
@@ -57,6 +58,7 @@ function normalizeGatewayStoreState(
   if (
     next.status === prev.status &&
     next.canStart === prev.canStart &&
+    next.startReason === prev.startReason &&
     next.restartRequired === prev.restartRequired
   ) {
     return prev
@@ -108,7 +110,10 @@ export function applyGatewayStatusToStore(
   data: Partial<
     Pick<
       GatewayStatusResponse,
-      "gateway_status" | "gateway_start_allowed" | "gateway_restart_required"
+      | "gateway_status"
+      | "gateway_start_allowed"
+      | "gateway_start_reason"
+      | "gateway_restart_required"
     >
   >,
 ) {
@@ -121,6 +126,10 @@ export function applyGatewayStatusToStore(
       prev.status === "stopping" && data.gateway_status === "running"
         ? false
         : (data.gateway_start_allowed ?? prev.canStart),
+    startReason:
+      prev.status === "stopping" && data.gateway_status === "running"
+        ? prev.startReason
+        : (data.gateway_start_reason ?? prev.startReason),
     restartRequired:
       prev.status === "stopping" && data.gateway_status === "running"
         ? false