diff --git a/svc/product_series_auto_scheduler.go b/svc/product_series_auto_scheduler.go
index f739e73..9775d44 100644
--- a/svc/product_series_auto_scheduler.go
+++ b/svc/product_series_auto_scheduler.go
@@ -576,6 +576,22 @@ func productSeriesApplyVariant(ctx context.Context, pg *sql.DB, v productSeriesA
 		}
 	}
 	if len(selected) == 0 {
+		// Apply fallback based on per-size availability, not total.
+		// If none of the sizes has qty >= 1 (default), do nothing.
+		minSizeQty := envFloat64("PRODUCT_SERIES_FALLBACK_MIN_SIZE_QTY", 1)
+		if !productSeriesHasAnySizeAtLeast(v.SizeQty, minSizeQty) {
+			if productSeriesFallbackLogEnabled() {
+				log.Printf("[ProductSeriesFallback] skip_insufficient_stock product=%s color=%s dim3=%s min_size_qty=%.2f sizes=%s",
+					strings.TrimSpace(v.ProductCode),
+					strings.TrimSpace(v.ColorCode),
+					strings.TrimSpace(v.Dim3Code),
+					minSizeQty,
+					productSeriesFormatSizeQty(v.SizeQty),
+				)
+			}
+			return 0, 1, nil
+		}
+
 		fallbackID, fallbackCode, err := productSeriesResolveFallbackSeries(ctx, pg)
 		if err != nil {
 			return 0, 1, err
@@ -1409,6 +1425,27 @@ func envIntRange(key string, fallback int, min int, max int) int {
 	return n
 }
 
+func envFloat64(key string, fallback float64) float64 {
+	raw := strings.TrimSpace(os.Getenv(key))
+	if raw == "" {
+		return fallback
+	}
+	v, err := strconv.ParseFloat(raw, 64)
+	if err != nil {
+		return fallback
+	}
+	return v
+}
+
+func productSeriesHasAnySizeAtLeast(sizeQty map[string]float64, min float64) bool {
+	for _, q := range sizeQty {
+		if q >= min {
+			return true
+		}
+	}
+	return false
+}
+
 func envHHMM(key string, fallbackHH int, fallbackMM int) (int, int) {
 	raw := strings.TrimSpace(os.Getenv(key))
 	if raw == "" {