diff --git a/svc/product_series_auto_scheduler.go b/svc/product_series_auto_scheduler.go
index e888fce..20f5807 100644
--- a/svc/product_series_auto_scheduler.go
+++ b/svc/product_series_auto_scheduler.go
@@ -118,6 +118,38 @@ func productSeriesFallbackOverrideExisting() bool {
 	return raw == "1" || raw == "true" || raw == "on" || raw == "yes"
 }
 
+func productSeriesDebugMatch(v productSeriesAutoVariant) bool {
+	p := strings.TrimSpace(os.Getenv("PRODUCT_SERIES_DEBUG_PRODUCT"))
+	if p != "" && !strings.EqualFold(strings.TrimSpace(v.ProductCode), p) {
+		return false
+	}
+	c := strings.TrimSpace(os.Getenv("PRODUCT_SERIES_DEBUG_COLOR"))
+	if c != "" && !strings.EqualFold(strings.TrimSpace(v.ColorCode), c) {
+		return false
+	}
+	d := strings.TrimSpace(os.Getenv("PRODUCT_SERIES_DEBUG_DIM3"))
+	if d != "" && !strings.EqualFold(strings.TrimSpace(v.Dim3Code), d) {
+		return false
+	}
+	return p != "" || c != "" || d != ""
+}
+
+func productSeriesFormatSizeQty(m map[string]float64) string {
+	if len(m) == 0 {
+		return "{}"
+	}
+	keys := make([]string, 0, len(m))
+	for k := range m {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	parts := make([]string, 0, len(keys))
+	for _, k := range keys {
+		parts = append(parts, fmt.Sprintf("%s=%.0f", k, m[k]))
+	}
+	return "{" + strings.Join(parts, ",") + "}"
+}
+
 func productSeriesFallbackCode() string {
 	code := strings.TrimSpace(os.Getenv("PRODUCT_SERIES_FALLBACK_SERIES_CODE"))
 	if code == "" {
@@ -507,7 +539,42 @@ func productSeriesApplyVariant(ctx context.Context, pg *sql.DB, v productSeriesA
 		}
 		return 0, 1, nil
 	}
+	if productSeriesDebugMatch(v) {
+		log.Printf("[ProductSeriesDebug] variant product=%s color=%s dim3=%s mmitem_id=%d dim1=%d dim3_id=%v group=%s total=%.0f sizes=%s",
+			strings.TrimSpace(v.ProductCode),
+			strings.TrimSpace(v.ColorCode),
+			strings.TrimSpace(v.Dim3Code),
+			mmitemID,
+			dim1ID,
+			func() any {
+				if dim3ID.Valid {
+					return dim3ID.Int64
+				}
+				return nil
+			}(),
+			strings.TrimSpace(v.GroupKey),
+			math.Round(v.TotalQty),
+			productSeriesFormatSizeQty(v.SizeQty),
+		)
+	}
 	selected := productSeriesSelectRules(v, rules)
+	if productSeriesDebugMatch(v) {
+		if len(selected) == 0 {
+			log.Printf("[ProductSeriesDebug] selected=0 product=%s color=%s dim3=%s", strings.TrimSpace(v.ProductCode), strings.TrimSpace(v.ColorCode), strings.TrimSpace(v.Dim3Code))
+		} else {
+			ids := make([]string, 0, len(selected))
+			for _, r := range selected {
+				ids = append(ids, fmt.Sprintf("%d", r.SeriesID))
+			}
+			log.Printf("[ProductSeriesDebug] selected=%d product=%s color=%s dim3=%s series_ids=%s",
+				len(selected),
+				strings.TrimSpace(v.ProductCode),
+				strings.TrimSpace(v.ColorCode),
+				strings.TrimSpace(v.Dim3Code),
+				strings.Join(ids, ","),
+			)
+		}
+	}
 	if len(selected) == 0 {
 		fallbackID, fallbackCode, err := productSeriesResolveFallbackSeries(ctx, pg)
 		if err != nil {