diff --git a/cloud/src/server/billing-rules.test.ts b/cloud/src/server/billing-rules.test.ts
index 1e72ebf..7667137 100644
--- a/cloud/src/server/billing-rules.test.ts
+++ b/cloud/src/server/billing-rules.test.ts
@@ -3,6 +3,7 @@ import {
   classifyCheckout,
   complimentaryAfterSubscription,
   instanceTransition,
+  isOwnCheckout,
   isReleasableReservation,
   isProcessedCheckout,
   isSubscriptionLive,
@@ -147,3 +148,16 @@ describe("complimentary instances", () => {
     expect(isReleasableReservation({ status: "running", complimentary: false, createdAt: old }, now, ttl)).toBe(false);
   });
 });
+
+describe("isOwnCheckout (shared Stripe account)", () => {
+  it("accepts checkouts created by AutoSEO Cloud", () => {
+    expect(isOwnCheckout({ metadata: { instanceId: "i1", userId: "u1", slug: "acme" } })).toBe(true);
+  });
+  it.each([
+    ["another product's checkout", { metadata: { plan: "monthly", license_key: "GM-XXXX" } }],
+    ["no metadata", { metadata: null }],
+    ["instance without user", { metadata: { instanceId: "i1" } }],
+  ])("ignores %s", (_, session) => {
+    expect(isOwnCheckout(session)).toBe(false);
+  });
+});
diff --git a/cloud/src/server/billing-rules.ts b/cloud/src/server/billing-rules.ts
index 1e2ed0c..d884ed5 100644
--- a/cloud/src/server/billing-rules.ts
+++ b/cloud/src/server/billing-rules.ts
@@ -113,3 +113,11 @@ export function isReleasableReservation(
 ): boolean {
   return instance.status === "pending_payment" && !instance.complimentary && now - instance.createdAt.getTime() > ttlMs;
 }
+
+/**
+ * The Stripe account is shared with other Codext products. Only checkouts created here carry an instanceId and
+ * userId; any other checkout belongs to another product and must be left alone (never canceled as an orphan).
+ */
+export function isOwnCheckout(session: { metadata?: Record<string, string> | null }): boolean {
+  return Boolean(session.metadata?.instanceId && session.metadata?.userId);
+}
diff --git a/cloud/src/server/stripe.ts b/cloud/src/server/stripe.ts
index 799168e..6461472 100644
--- a/cloud/src/server/stripe.ts
+++ b/cloud/src/server/stripe.ts
@@ -22,6 +22,7 @@ import {
   classifyCheckout,
   complimentaryAfterSubscription,
   instanceTransition,
+  isOwnCheckout,
   isProcessedCheckout,
   isSubscriptionLive,
   shouldApplySubscription,
@@ -801,6 +802,10 @@ async function recreateReservation(session: Stripe.Checkout.Session): Promise<In
  */
 export async function processCheckoutSession(session: Stripe.Checkout.Session, source: string): Promise<Instance | null> {
   if (session.mode !== "subscription" || session.status !== "complete") return null;
+  if (!isOwnCheckout(session)) {
+    await logEvent("stripe.checkout_foreign", { data: { sessionId: session.id, source } });
+    return null;
+  }
   const instanceId = session.metadata?.instanceId;
   const existing = instanceId ? await getInstance(instanceId) : null;
   const instance = existing && existing.status !== "deleted" ? existing : await recreateReservation(session);