refactor: improve conversation metadata generation logic

backend/internal/application/conversation/service_metadata.go

@@ -52,7 +52,7 @@ type conversationMetadataLLMResult struct {
 }
 
 func (s *Service) maybeGenerateConversationMetadataAsync(conversation model.Conversation, userMsg model.Message, assistantMsg model.Message) {
-	if conversation.MessageCount != 0 {
+	if !shouldGenerateConversationMetadata(conversation) {
 		return
 	}
 	if strings.TrimSpace(userMsg.Content) == "" && strings.TrimSpace(assistantMsg.Content) == "" {
@@ -117,7 +117,7 @@ func (s *Service) generateConversationMetadata(ctx context.Context, conversation
 		}()
 	}
 
-	if s.routeResolver != nil && s.llmClient != nil {
+	if s.routeResolver != nil && s.llmClient != nil && conversationLabelsEmpty(conversation.LabelsJSON) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
@@ -455,6 +455,15 @@ func shouldAutoReplaceConversationTitle(title string) bool {
 	}
 }
 
+func shouldGenerateConversationMetadata(conversation model.Conversation) bool {
+	return shouldAutoReplaceConversationTitle(conversation.Title) || conversationLabelsEmpty(conversation.LabelsJSON)
+}
+
+func conversationLabelsEmpty(labelsJSON string) bool {
+	value := strings.TrimSpace(strings.ToLower(labelsJSON))
+	return value == "" || value == "null" || value == "[]"
+}
+
 func truncateByEstimatedTokens(text string, maxTokens int64) string {
 	if maxTokens <= 0 || estimateTokens(text) <= maxTokens {
 		return text

backend/internal/application/conversation/service_metadata_test.go

@@ -71,3 +71,27 @@ func TestConversationTitleFromFirstUserMessage(t *testing.T) {
 		}
 	}
 }
+
+func TestShouldGenerateConversationMetadataAfterFailedFirstTurn(t *testing.T) {
+	conversation := model.Conversation{
+		Title:        "新会话",
+		LabelsJSON:   "[]",
+		MessageCount: 2,
+	}
+
+	if !shouldGenerateConversationMetadata(conversation) {
+		t.Fatal("expected placeholder metadata to be generated even when failed messages already exist")
+	}
+}
+
+func TestConversationLabelsEmpty(t *testing.T) {
+	emptyCases := []string{"", "null", "[]", "  []  "}
+	for _, value := range emptyCases {
+		if !conversationLabelsEmpty(value) {
+			t.Fatalf("expected labels %q to be empty", value)
+		}
+	}
+	if conversationLabelsEmpty(`["技术"]`) {
+		t.Fatal("expected non-empty labels to be preserved")
+	}
+}