diff --git a/backend/internal/service/payment_fulfillment.go b/backend/internal/service/payment_fulfillment.go
index 7bde03c8..423ed80f 100644
--- a/backend/internal/service/payment_fulfillment.go
+++ b/backend/internal/service/payment_fulfillment.go
@@ -25,9 +25,9 @@ func (s *PaymentService) HandlePaymentNotification(ctx context.Context, n *payme
 	// Look up order by out_trade_no (the external order ID we sent to the provider)
 	order, err := s.entClient.PaymentOrder.Query().Where(paymentorder.OutTradeNo(n.OrderID)).Only(ctx)
 	if err != nil {
-		// Fallback: try legacy format (sub2_N where N is DB ID)
-		trimmed := strings.TrimPrefix(n.OrderID, orderIDPrefix)
-		if oid, parseErr := strconv.ParseInt(trimmed, 10, 64); parseErr == nil {
+		// Fallback only for true legacy "sub2_N" DB-ID payloads when the
+		// current out_trade_no lookup genuinely did not find an order.
+		if oid, ok := parseLegacyPaymentOrderID(n.OrderID, err); ok {
 			return s.confirmPayment(ctx, oid, n.TradeNo, n.Amount, pk, n.Metadata)
 		}
 		return fmt.Errorf("order not found for out_trade_no: %s", n.OrderID)
@@ -35,6 +35,25 @@ func (s *PaymentService) HandlePaymentNotification(ctx context.Context, n *payme
 	return s.confirmPayment(ctx, order.ID, n.TradeNo, n.Amount, pk, n.Metadata)
 }
 
+func parseLegacyPaymentOrderID(orderID string, lookupErr error) (int64, bool) {
+	if !dbent.IsNotFound(lookupErr) {
+		return 0, false
+	}
+	orderID = strings.TrimSpace(orderID)
+	if !strings.HasPrefix(orderID, orderIDPrefix) {
+		return 0, false
+	}
+	trimmed := strings.TrimPrefix(orderID, orderIDPrefix)
+	if trimmed == "" || trimmed == orderID {
+		return 0, false
+	}
+	oid, err := strconv.ParseInt(trimmed, 10, 64)
+	if err != nil || oid <= 0 {
+		return 0, false
+	}
+	return oid, true
+}
+
 func (s *PaymentService) confirmPayment(ctx context.Context, oid int64, tradeNo string, paid float64, pk string, metadata map[string]string) error {
 	o, err := s.entClient.PaymentOrder.Get(ctx, oid)
 	if err != nil {
diff --git a/backend/internal/service/payment_fulfillment_test.go b/backend/internal/service/payment_fulfillment_test.go
index 8883d3b8..d70f8946 100644
--- a/backend/internal/service/payment_fulfillment_test.go
+++ b/backend/internal/service/payment_fulfillment_test.go
@@ -307,3 +307,17 @@ func TestValidateProviderNotificationMetadataAllowsLegacyOrdersWithoutSnapshotFi
 	})
 	assert.NoError(t, err)
 }
+
+func TestParseLegacyPaymentOrderID(t *testing.T) {
+	t.Parallel()
+
+	oid, ok := parseLegacyPaymentOrderID("sub2_42", &dbent.NotFoundError{})
+	assert.True(t, ok)
+	assert.EqualValues(t, 42, oid)
+
+	_, ok = parseLegacyPaymentOrderID("42", &dbent.NotFoundError{})
+	assert.False(t, ok)
+
+	_, ok = parseLegacyPaymentOrderID("sub2_42", errors.New("db down"))
+	assert.False(t, ok)
+}
diff --git a/backend/internal/service/payment_order_lifecycle.go b/backend/internal/service/payment_order_lifecycle.go
index a192f599..ccab7c11 100644
--- a/backend/internal/service/payment_order_lifecycle.go
+++ b/backend/internal/service/payment_order_lifecycle.go
@@ -292,10 +292,48 @@ func (s *PaymentService) getOrderProvider(ctx context.Context, o *dbent.PaymentO
 	if inst != nil {
 		return s.createProviderFromInstance(ctx, inst)
 	}
+	if !paymentOrderAllowsRegistryFallback(o) {
+		return nil, fmt.Errorf("order %d provider instance is unresolved", o.ID)
+	}
+	providerKey := paymentOrderFallbackProviderKey(s.registry, o)
+	if providerKey == "" {
+		return nil, fmt.Errorf("order %d provider fallback key is missing", o.ID)
+	}
+	if !s.webhookRegistryFallbackAllowed(ctx, providerKey) {
+		return nil, fmt.Errorf("order %d provider fallback is ambiguous for %s", o.ID, providerKey)
+	}
 	s.EnsureProviders(ctx)
 	return s.registry.GetProvider(o.PaymentType)
 }
 
+func paymentOrderAllowsRegistryFallback(order *dbent.PaymentOrder) bool {
+	if order == nil {
+		return false
+	}
+	if psOrderProviderSnapshot(order) != nil {
+		return false
+	}
+	if strings.TrimSpace(psStringValue(order.ProviderInstanceID)) != "" {
+		return false
+	}
+	if strings.TrimSpace(psStringValue(order.ProviderKey)) != "" {
+		return false
+	}
+	return true
+}
+
+func paymentOrderFallbackProviderKey(registry *payment.Registry, order *dbent.PaymentOrder) string {
+	if order == nil {
+		return ""
+	}
+	if registry != nil {
+		if key := strings.TrimSpace(registry.GetProviderKey(payment.PaymentType(order.PaymentType))); key != "" {
+			return key
+		}
+	}
+	return strings.TrimSpace(payment.GetBasePaymentType(strings.TrimSpace(order.PaymentType)))
+}
+
 func (s *PaymentService) createProviderFromInstance(ctx context.Context, inst *dbent.PaymentProviderInstance) (payment.Provider, error) {
 	if inst == nil {
 		return nil, fmt.Errorf("payment provider instance is missing")
diff --git a/backend/internal/service/payment_order_lifecycle_test.go b/backend/internal/service/payment_order_lifecycle_test.go
index 3c6c65a5..39993a2f 100644
--- a/backend/internal/service/payment_order_lifecycle_test.go
+++ b/backend/internal/service/payment_order_lifecycle_test.go
@@ -323,6 +323,43 @@ func TestVerifyOrderByOutTradeNoUsesOutTradeNoWhenPaymentTradeNoAlreadyExistsFor
 	require.Equal(t, "upstream-trade-existing", got.PaymentTradeNo)
 }
 
+func TestPaymentOrderAllowsRegistryFallbackOnlyForLegacyOrdersWithoutPinnedProviderState(t *testing.T) {
+	t.Parallel()
+
+	require.True(t, paymentOrderAllowsRegistryFallback(&dbent.PaymentOrder{
+		PaymentType: payment.TypeAlipay,
+	}))
+
+	instanceID := "12"
+	require.False(t, paymentOrderAllowsRegistryFallback(&dbent.PaymentOrder{
+		PaymentType:        payment.TypeAlipay,
+		ProviderInstanceID: &instanceID,
+	}))
+
+	require.False(t, paymentOrderAllowsRegistryFallback(&dbent.PaymentOrder{
+		PaymentType: payment.TypeAlipay,
+		ProviderSnapshot: map[string]any{
+			"schema_version":       2,
+			"provider_instance_id": "12",
+		},
+	}))
+}
+
+func TestPaymentOrderQueryReferenceUsesOutTradeNoForOfficialProviders(t *testing.T) {
+	t.Parallel()
+
+	order := &dbent.PaymentOrder{
+		PaymentType:    payment.TypeWxpay,
+		OutTradeNo:     "sub2_out_trade_no",
+		PaymentTradeNo: "wx-transaction-id",
+	}
+
+	require.Equal(t, "sub2_out_trade_no", paymentOrderQueryReference(order, &paymentOrderLifecycleQueryProvider{}))
+	require.Equal(t, "sub2_out_trade_no", paymentOrderQueryReference(order, paymentFulfillmentTestProvider{
+		key: payment.TypeWxpay,
+	}))
+}
+
 func newPaymentOrderLifecycleTestClient(t *testing.T) *dbent.Client {
 	t.Helper()