diff --git a/src/app/(app)/settings/billing-usage/subscription/components/subscription-submit-button.tsx b/src/app/(app)/settings/billing-usage/subscription/components/subscription-submit-button.tsx
index d0657372..f0270b35 100644
--- a/src/app/(app)/settings/billing-usage/subscription/components/subscription-submit-button.tsx
+++ b/src/app/(app)/settings/billing-usage/subscription/components/subscription-submit-button.tsx
@@ -17,7 +17,7 @@ interface SubscriptionSubmitButtonProps {
   packageUpdates: PackageUpdateInput[];
   /** Desired end-state for the checkout flow. */
   checkoutProducts: ProductCheckoutInput[];
-  /** True when a Custom Amount has an empty/invalid quantity (update flow only). */
+  /** True when a Custom Amount has an empty/invalid quantity. */
   hasInvalidCustom: boolean;
   /**
    * The AI spending cap to store, when the user changed it: a USD figure, or
@@ -41,7 +41,8 @@ interface SubscriptionSubmitButtonProps {
  *
  * The ACTION still splits on the subscription state:
  * - no active paid subscription → `createCheckoutSession`, which redirects to
- *   Stripe. No diff gating: there is nothing to compare against.
+ *   Stripe. No diff gating: there is nothing to compare against — but disabled
+ *   while there is nothing to buy, and validated on click.
  * - active paid subscription → `updateSubscription`, a mutation that applies the
  *   plan change in place and does NOT redirect to a payment page (an upgrade may
  *   raise an invoice afterwards). Disabled when the selection equals the current
@@ -87,30 +88,32 @@ export function SubscriptionSubmitButton({
   };
 
   if (needsCheckout) {
+    const handleCheckout = () => {
+      // Checkout has no diff to gate on, but an out-of-range quantity is still
+      // one: it would be sent as a plan nobody can be billed for.
+      if (hasInvalidCustom) {
+        rejectInvalidAmount();
+        return;
+      }
+      if (!checkoutProducts.length) return;
+      // Opened from the click itself, and carried through both mutations:
+      // Stripe's URL only exists once the second answers, and a tab opened
+      // then has lost the user gesture that lets it through (see
+      // `openDeferredTab`). It is closed again if either step fails.
+      const tab = openDeferredTab();
+      withAiSpendCap(
+        () => createCheckout.mutate({ products: checkoutProducts }, { target: tab }),
+        () => tab.cancel(),
+      );
+    };
+
     return (
       <Button
         variant="accent"
         className={className}
-        onClick={() => {
-          // Checkout has no diff to gate on, but an out-of-range quantity is still
-          // one: it would be sent as a plan nobody can be billed for.
-          if (hasInvalidCustom) {
-            rejectInvalidAmount();
-            return;
-          }
-          if (!checkoutProducts.length) return;
-          // Opened from the click itself, and carried through both mutations:
-          // Stripe's URL only exists once the second answers, and a tab opened
-          // then has lost the user gesture that lets it through (see
-          // `openDeferredTab`). It is closed again if either step fails.
-          const tab = openDeferredTab();
-          withAiSpendCap(
-            () => createCheckout.mutate({ products: checkoutProducts }, { target: tab }),
-            () => tab.cancel(),
-          );
-        }}
+        onClick={handleCheckout}
         loading={isPending}
-        disabled={isPending}
+        disabled={isPending || checkoutProducts.length === 0}
       >
         {SUBMIT_LABEL}
       </Button>
diff --git a/src/lib/relay/environment.ts b/src/lib/relay/environment.ts
index 1e1d0be4..4d1b09da 100644
--- a/src/lib/relay/environment.ts
+++ b/src/lib/relay/environment.ts
@@ -424,6 +424,23 @@ const fetchRelay: FetchFunction = async (request, variables, cacheConfig, upload
         return fetchRelay(request, variables, cacheConfig, uploadables);
       }
     }
+
+    // The server returned errors and no data. Relay would otherwise reject with
+    // its internal "No data returned for operation ..." message, which reaches
+    // the user verbatim through a mutation's `onError` toast (e.g. checkout on a
+    // mutation like `createCheckoutSession`, whose `CheckoutResult` payload has
+    // no `errors` field). Throw the readable server message so `onError` shows
+    // that instead. Partial responses (data present with field errors) fall
+    // through unchanged.
+    if (json.data == null) {
+      const message = json.errors
+        .map((error: { message?: string }) => error?.message)
+        .filter(Boolean)
+        .join('. ');
+      if (message) {
+        throw new Error(message);
+      }
+    }
   }
 
   return json;