diff --git a/api/src/routes/webhooks.ts b/api/src/routes/webhooks.ts
index 7029050..0391300 100644
--- a/api/src/routes/webhooks.ts
+++ b/api/src/routes/webhooks.ts
@@ -799,14 +799,24 @@ async function handlePaymentFailure(orderId: string, reason: string): Promise<vo
 // ═══════════════════════════════════════════════════════════════════════════════
 
 /**
- * Extract the Stripe Subscription id from an Invoice. As of API 2026-03-25 the
- * subscription link moved under invoice.parent.subscription_details.subscription
- * (the top-level invoice.subscription field was removed).
+ * Extract the Stripe Subscription id from an Invoice, tolerant of API version.
+ * As of API 2026-03-25 the subscription link moved under
+ * invoice.parent.subscription_details.subscription and the top-level
+ * invoice.subscription field was removed; older versions (incl. the account
+ * default 2024-12-18.acacia that the unpinned live endpoint follows) still send
+ * the top-level field. We read whichever is present.
  */
 function invoiceSubscriptionId(inv: Stripe.Invoice): string | null {
-  const sub = inv.parent?.subscription_details?.subscription;
-  if (!sub) return null;
-  return typeof sub === "string" ? sub : sub.id;
+  // New API (>= 2026-03-25.dahlia): invoice.parent.subscription_details.subscription
+  const modern = inv.parent?.subscription_details?.subscription;
+  if (modern) return typeof modern === "string" ? modern : modern.id;
+  // Legacy API (<= 2025): top-level invoice.subscription. The live webhook
+  // endpoint has NO pinned api_version, so it follows the account default
+  // (currently 2024-12-18.acacia) which delivers this older shape. We must read
+  // both or recurring renewals silently fail to map back to an order.
+  const legacy = (inv as unknown as { subscription?: string | { id: string } }).subscription;
+  if (legacy) return typeof legacy === "string" ? legacy : legacy.id;
+  return null;
 }
 
 /**
diff --git a/api/tests/recurring-subscription.test.ts b/api/tests/recurring-subscription.test.ts
index 32545c3..76d4725 100644
--- a/api/tests/recurring-subscription.test.ts
+++ b/api/tests/recurring-subscription.test.ts
@@ -80,14 +80,22 @@ check("recurring line item has recurring.interval=month", built[0].price_data.re
 check("recurring line item keeps $79 unit_amount", built[0].price_data.unit_amount === 7900);
 check("recurring line item defaults quantity=1", built[0].quantity === 1);
 
-// ── 7. invoiceSubscriptionId extraction (mirrors webhooks.ts; API 2026-03-25)
+// ── 7. invoiceSubscriptionId extraction (mirrors webhooks.ts; version-tolerant)
 function invoiceSubscriptionId(inv: any): string | null {
-  const sub = inv?.parent?.subscription_details?.subscription;
-  if (!sub) return null;
-  return typeof sub === "string" ? sub : sub.id;
+  // New API (>= 2026-03-25.dahlia)
+  const modern = inv?.parent?.subscription_details?.subscription;
+  if (modern) return typeof modern === "string" ? modern : modern.id;
+  // Legacy API (<= 2025, incl. account default 2024-12-18.acacia on the
+  // unpinned live endpoint): top-level invoice.subscription
+  const legacy = inv?.subscription;
+  if (legacy) return typeof legacy === "string" ? legacy : legacy.id;
+  return null;
 }
 check("subId from string field", invoiceSubscriptionId({ parent: { subscription_details: { subscription: "sub_123" } } }) === "sub_123");
 check("subId from object field", invoiceSubscriptionId({ parent: { subscription_details: { subscription: { id: "sub_456" } } } }) === "sub_456");
+check("subId from legacy top-level string (acacia)", invoiceSubscriptionId({ subscription: "sub_legacy" }) === "sub_legacy");
+check("subId from legacy top-level object", invoiceSubscriptionId({ subscription: { id: "sub_legacy2" } }) === "sub_legacy2");
+check("subId prefers modern over legacy", invoiceSubscriptionId({ subscription: "sub_old", parent: { subscription_details: { subscription: "sub_new" } } }) === "sub_new");
 check("subId null when no parent", invoiceSubscriptionId({ id: "in_1" }) === null);
 check("subId null for one-time invoice", invoiceSubscriptionId({ parent: { type: "quote_details" } }) === null);