diff --git a/common/util/billing.go b/common/util/billing.go
index 59fac22..b9dabe6 100644
--- a/common/util/billing.go
+++ b/common/util/billing.go
@@ -158,7 +158,15 @@ func ExtractRequestBilling(ctx context.Context, config map[string]any, requestPa
 	data := make(map[string]any)
 
 	for key, path := range fields {
-		data[key] = extractValue(requestPayload, gconv.String(path))
+		val := extractValue(requestPayload, gconv.String(path))
+		// TTS 模型：input_chars 自动计算字符数
+		if key == "input_chars" {
+			if s, ok := val.(string); ok {
+				data[key] = len([]rune(s))
+				continue
+			}
+		}
+		data[key] = val
 	}
 
 	if defaults, ok := config["defaults"].(map[string]any); ok {
@@ -169,7 +177,6 @@ func ExtractRequestBilling(ctx context.Context, config map[string]any, requestPa
 		}
 	}
 
-	// 处理 compute 字段
 	if compute, ok := config["compute"].(map[string]any); ok {
 		for targetField, rule := range compute {
 			r := rule.(map[string]any)