diff --git a/internal/errors/errors.go b/internal/errors/errors.go
deleted file mode 100644
index 15adeab..0000000
--- a/internal/errors/errors.go
+++ /dev/null
@@ -1,22 +0,0 @@
-package errors
-
-type ErrorCode int
-
-const (
-	ErrInvalidConfig ErrorCode = iota + 1
-	ErrRateLimit
-	ErrMetricsCollection
-)
-
-type MetricsError struct {
-	Code    ErrorCode
-	Message string
-	Err     error
-}
-
-func (e *MetricsError) Error() string {
-	if e.Err != nil {
-		return e.Message + ": " + e.Err.Error()
-	}
-	return e.Message
-}
diff --git a/internal/handler/proxy.go b/internal/handler/proxy.go
index 78382d7..f93e334 100644
--- a/internal/handler/proxy.go
+++ b/internal/handler/proxy.go
@@ -195,7 +195,7 @@ func (h *ProxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// 使用统一的路由选择逻辑
-	targetBase := utils.GetTargetURL(h.client, r, pathConfig, decodedPath)
+	targetBase, usedAltTarget := utils.GetTargetURL(h.client, r, pathConfig, decodedPath)
 
 	// 重新编码路径，保留 '/'
 	parts := strings.Split(decodedPath, "/")
@@ -277,6 +277,12 @@ func (h *ProxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				w.Header().Set("Content-Encoding", item.ContentEncoding)
 			}
 			w.Header().Set("Proxy-Go-Cache", "HIT")
+
+			// 如果使用了扩展名映射的备用目标，添加标记响应头
+			if usedAltTarget {
+				w.Header().Set("Proxy-Go-AltTarget", "true")
+			}
+
 			if notModified {
 				w.WriteHeader(http.StatusNotModified)
 				return
@@ -305,6 +311,11 @@ func (h *ProxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	copyHeader(w.Header(), resp.Header)
 	w.Header().Set("Proxy-Go-Cache", "MISS")
 
+	// 如果使用了扩展名映射的备用目标，添加标记响应头
+	if usedAltTarget {
+		w.Header().Set("Proxy-Go-AltTarget", "true")
+	}
+
 	// 设置响应状态码
 	w.WriteHeader(resp.StatusCode)
 
diff --git a/internal/utils/utils.go b/internal/utils/utils.go
index 5725c7a..c4569cb 100644
--- a/internal/utils/utils.go
+++ b/internal/utils/utils.go
@@ -183,9 +183,10 @@ func GetFileSize(client *http.Client, url string) (int64, error) {
 }
 
 // GetTargetURL 根据路径和配置决定目标URL
-func GetTargetURL(client *http.Client, r *http.Request, pathConfig config.PathConfig, path string) string {
+func GetTargetURL(client *http.Client, r *http.Request, pathConfig config.PathConfig, path string) (string, bool) {
 	// 默认使用默认目标
 	targetBase := pathConfig.DefaultTarget
+	usedAltTarget := false
 
 	// 如果配置了扩展名映射
 	if pathConfig.ExtensionMap != nil {
@@ -198,7 +199,7 @@ func GetTargetURL(client *http.Client, r *http.Request, pathConfig config.PathCo
 				contentLength, err := GetFileSize(client, targetBase+path)
 				if err != nil {
 					log.Printf("[Route] %s -> %s (error getting size: %v)", path, targetBase, err)
-					return targetBase
+					return targetBase, false
 				}
 
 				// 如果没有设置最小阈值，使用默认值 500KB
@@ -244,7 +245,7 @@ func GetTargetURL(client *http.Client, r *http.Request, pathConfig config.PathCo
 							log.Printf("[Route] %s -> %s (size: %s > %s and <= %s)",
 								path, altTarget, FormatBytes(contentLength),
 								FormatBytes(minThreshold), FormatBytes(maxThreshold))
-							return altTarget
+							return altTarget, true
 						}
 						log.Printf("[Route] %s -> %s (fallback: alternative target not accessible)",
 							path, targetBase)
@@ -269,7 +270,7 @@ func GetTargetURL(client *http.Client, r *http.Request, pathConfig config.PathCo
 		log.Printf("[Route] %s -> %s (no extension map)", path, targetBase)
 	}
 
-	return targetBase
+	return targetBase, usedAltTarget
 }
 
 // isTargetAccessible 检查目标URL是否可访问