diff --git a/internal/proxy/http.go b/internal/proxy/http.go
index 2bf052460..54049f6cf 100644
--- a/internal/proxy/http.go
+++ b/internal/proxy/http.go
@@ -347,13 +347,14 @@ func (v *httpFlvTsConnection) serveByBackend(ctx context.Context, w http.Respons
 		return errors.Errorf("proxy stream to %v failed, status=%v", backendURL, resp.Status)
 	}
 
-	// Copy all headers from backend to client.
-	w.WriteHeader(resp.StatusCode)
+	// Copy all headers from backend to client before WriteHeader,
+	// because headers set after WriteHeader are silently ignored.
 	for k, v := range resp.Header {
 		for _, vv := range v {
 			w.Header().Add(k, vv)
 		}
 	}
+	w.WriteHeader(resp.StatusCode)
 
 	logger.Debug(ctx, "HTTP start streaming")
 
@@ -476,13 +477,14 @@ func (v *hlsPlayStream) serveByBackend(ctx context.Context, w http.ResponseWrite
 		return errors.Errorf("proxy stream to %v failed, status=%v", backendURL, resp.Status)
 	}
 
-	// Copy all headers from backend to client.
-	w.WriteHeader(resp.StatusCode)
+	// Copy all headers from backend to client before WriteHeader,
+	// because headers set after WriteHeader are silently ignored.
 	for k, v := range resp.Header {
 		for _, vv := range v {
 			w.Header().Add(k, vv)
 		}
 	}
+	w.WriteHeader(resp.StatusCode)
 
 	// For TS file, directly copy it.
 	if !strings.HasSuffix(r.URL.Path, ".m3u8") {
@@ -502,7 +504,7 @@ func (v *hlsPlayStream) serveByBackend(ctx context.Context, w http.ResponseWrite
 
 	m3u8 := string(b)
 	if strings.Contains(m3u8, ".ts?") {
-		m3u8 = strings.ReplaceAll(m3u8, ".ts?", fmt.Sprintf(".ts?spbhid=%v&&", v.SRSProxyBackendHLSID))
+		m3u8 = strings.ReplaceAll(m3u8, ".ts?", fmt.Sprintf(".ts?spbhid=%v&", v.SRSProxyBackendHLSID))
 	} else {
 		m3u8 = strings.ReplaceAll(m3u8, ".ts", fmt.Sprintf(".ts?spbhid=%v", v.SRSProxyBackendHLSID))
 	}
diff --git a/internal/proxy/http_test.go b/internal/proxy/http_test.go
index fa64225c4..0025daeb8 100644
--- a/internal/proxy/http_test.go
+++ b/internal/proxy/http_test.go
@@ -370,7 +370,7 @@ func TestHLSPlayStream_ServeByBackend_M3U8RewritesTSWithQuery(t *testing.T) {
 		&lb.OriginServer{IP: host, HTTP: []string{port}}); err != nil {
 		t.Fatalf("unexpected err: %v", err)
 	}
-	if want := "live-0.ts?spbhid=ABC&&token=foo"; !strings.Contains(rec.Body.String(), want) {
+	if want := "live-0.ts?spbhid=ABC&token=foo"; !strings.Contains(rec.Body.String(), want) {
 		t.Fatalf("missing %q in body: %q", want, rec.Body.String())
 	}
 }
@@ -396,6 +396,61 @@ func TestHLSPlayStream_ServeByBackend_AppendsRawQueryOnTS(t *testing.T) {
 	}
 }
 
+func TestHLSPlayStream_ServeByBackend_HeadersCopiedFromBackend(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/vnd.apple.mpegurl")
+		w.Header().Set("Cache-Control", "no-cache")
+		w.Header().Set("X-Custom-Header", "custom-value")
+		_, _ = io.WriteString(w, "#EXTM3U\nlive-0.ts\n")
+	}))
+	defer ts.Close()
+	host, port := httptestHostPort(t, ts)
+
+	v := newHLSPlayStream()
+	req := httptest.NewRequest(http.MethodGet, "http://example.com/live.m3u8", nil)
+	rec := httptest.NewRecorder()
+	if err := v.serveByBackend(context.Background(), rec, req,
+		&lb.OriginServer{IP: host, HTTP: []string{port}}); err != nil {
+		t.Fatalf("unexpected err: %v", err)
+	}
+
+	// Verify headers are properly copied (not lost due to WriteHeader order)
+	if got := rec.Header().Get("Content-Type"); got != "application/vnd.apple.mpegurl" {
+		t.Errorf("Content-Type = %q, want application/vnd.apple.mpegurl", got)
+	}
+	if got := rec.Header().Get("Cache-Control"); got != "no-cache" {
+		t.Errorf("Cache-Control = %q, want no-cache", got)
+	}
+	if got := rec.Header().Get("X-Custom-Header"); got != "custom-value" {
+		t.Errorf("X-Custom-Header = %q, want custom-value", got)
+	}
+}
+
+func TestHLSPlayStream_ServeByBackend_TSHeadersCopiedFromBackend(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "video/mp2t")
+		w.Header().Set("Cache-Control", "max-age=3600")
+		_, _ = w.Write([]byte{0x47, 0x00, 0x01})
+	}))
+	defer ts.Close()
+	host, port := httptestHostPort(t, ts)
+
+	v := newHLSPlayStream()
+	req := httptest.NewRequest(http.MethodGet, "http://example.com/live.ts", nil)
+	rec := httptest.NewRecorder()
+	if err := v.serveByBackend(context.Background(), rec, req,
+		&lb.OriginServer{IP: host, HTTP: []string{port}}); err != nil {
+		t.Fatalf("unexpected err: %v", err)
+	}
+
+	if got := rec.Header().Get("Content-Type"); got != "video/mp2t" {
+		t.Errorf("Content-Type = %q, want video/mp2t", got)
+	}
+	if got := rec.Header().Get("Cache-Control"); got != "max-age=3600" {
+		t.Errorf("Cache-Control = %q, want max-age=3600", got)
+	}
+}
+
 // =============================================================================
 // httpFlvTsConnection
 // =============================================================================
@@ -666,6 +721,36 @@ func TestHTTPFlvTsConn_ServeByBackend_PreservesMethod(t *testing.T) {
 	}
 }
 
+func TestHTTPFlvTsConn_ServeByBackend_HeadersCopiedFromBackend(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "video/x-flv")
+		w.Header().Set("Cache-Control", "no-store")
+		w.Header().Set("X-Custom-Header", "flv-value")
+		_, _ = w.Write([]byte("FLV\x01\x05\x00\x00\x00\x09"))
+	}))
+	defer ts.Close()
+	host, port := httptestHostPort(t, ts)
+
+	v := newHTTPFlvTsConnection()
+	req := httptest.NewRequest(http.MethodGet, "http://example.com/live.flv", nil)
+	rec := httptest.NewRecorder()
+	if err := v.serveByBackend(context.Background(), rec, req,
+		&lb.OriginServer{IP: host, HTTP: []string{port}}); err != nil {
+		t.Fatalf("unexpected err: %v", err)
+	}
+
+	// Verify headers are properly copied (not lost due to WriteHeader order)
+	if got := rec.Header().Get("Content-Type"); got != "video/x-flv" {
+		t.Errorf("Content-Type = %q, want video/x-flv", got)
+	}
+	if got := rec.Header().Get("Cache-Control"); got != "no-store" {
+		t.Errorf("Cache-Control = %q, want no-store", got)
+	}
+	if got := rec.Header().Get("X-Custom-Header"); got != "flv-value" {
+		t.Errorf("X-Custom-Header = %q, want flv-value", got)
+	}
+}
+
 // =============================================================================
 // httpStreamProxyServer
 // =============================================================================