fix: streaming

backend/internal/api/handlers.go

@@ -323,10 +323,13 @@ func (a *API) PostChat(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Send Message
-	if err := a.sendMessage(r.Context(), w, chat.ID, genReq.Model, genReq.Prompt); err != nil {
+	responseStarted, err := a.sendMessage(r.Context(), w, chat.ID, genReq.Model, genReq.Prompt)
+	if err != nil {
 		log.WithError(err).WithField("chat_id", chat.ID).Error("failed to send message")
+		if !responseStarted {
 			http.Error(w, "Failed to send message", http.StatusInternalServerError)
 		}
+	}
 }
 
 func (a *API) DeleteChat(w http.ResponseWriter, r *http.Request) {
@@ -421,10 +424,14 @@ func (a *API) PostChatMessage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := a.sendMessage(r.Context(), w, chatID, genReq.Model, genReq.Prompt); err != nil {
+	// Send Message
+	responseStarted, err := a.sendMessage(r.Context(), w, chatID, genReq.Model, genReq.Prompt)
+	if err != nil {
 		log.WithError(err).WithField("chat_id", chatID).Error("failed to send message")
+		if !responseStarted {
 			http.Error(w, "Failed to send message", http.StatusInternalServerError)
 		}
+	}
 }
 
 func (a *API) getClient() (*client.Client, error) {
@@ -449,10 +456,10 @@ func (a *API) getClient() (*client.Client, error) {
 	return a.client, nil
 }
 
-func (a *API) sendMessage(ctx context.Context, w http.ResponseWriter, chatID uuid.UUID, chatModel, userMessage string) error {
+func (a *API) sendMessage(ctx context.Context, w http.ResponseWriter, chatID uuid.UUID, chatModel, userMessage string) (bool, error) {
 	apiClient, err := a.getClient()
 	if err != nil {
-		return fmt.Errorf("failed to get client: %w", err)
+		return false, fmt.Errorf("failed to get client: %w", err)
 	}
 
 	// Detach Request Context
@@ -462,19 +469,20 @@ func (a *API) sendMessage(ctx context.Context, w http.ResponseWriter, chatID uui
 	// Create User Message
 	userMsg := &store.Message{ChatID: chatID, Role: "user", Content: userMessage}
 	if err := a.store.SaveChatMessage(userMsg); err != nil {
-		return fmt.Errorf("failed to add user message to chat: %w", err)
+		return false, fmt.Errorf("failed to add user message to chat: %w", err)
+	}
+
+	// Get Chat History - Fetch before creating the in-progress assistant message so the
+	// LLM request does not include an empty assistant response prefill.
+	chat, err := a.store.GetChat(chatID)
+	if err != nil {
+		return false, fmt.Errorf("failed to get chat: %w", err)
 	}
 
 	// Add Assistant Response - TODO: Ensure InProgress Flag?
 	assistantMsg := &store.Message{ChatID: chatID, Role: "assistant"}
 	if err := a.store.SaveChatMessage(assistantMsg); err != nil {
-		return fmt.Errorf("failed to add assistant message to chat: %w", err)
-	}
-
-	// Get Chat
-	chat, err := a.store.GetChat(chatID)
-	if err != nil {
-		return fmt.Errorf("failed to get chat: %w", err)
+		return false, fmt.Errorf("failed to add assistant message to chat: %w", err)
 	}
 
 	// Set Headers
@@ -491,35 +499,52 @@ func (a *API) sendMessage(ctx context.Context, w http.ResponseWriter, chatID uui
 		Chat:        toChatNoMessages(chat),
 		UserMessage: userMsg,
 	}); err != nil {
-		return fmt.Errorf("failed to send initial chunk: %w", err)
+		return false, fmt.Errorf("failed to send initial chunk: %w", err)
 	}
+	responseStarted := true
+	streamToClient := true
 
 	// Send Message
 	if _, err := apiClient.SendMessage(ctx, chat.Messages, chatModel, func(m *client.MessageChunk) error {
 		var apiMsgChunk MessageChunk
+		messageChanged := false
 
 		if m.Stats != nil {
+			messageChanged = true
 			assistantMsg.Stats = m.Stats
 		}
 
 		if m.Message != nil {
+			messageChanged = true
 			assistantMsg.Content += *m.Message
-			apiMsgChunk.AssistantMessage = assistantMsg
 		}
 
 		if m.Thinking != nil {
+			messageChanged = true
 			assistantMsg.Thinking += *m.Thinking
+		}
+
+		// Save Assistant Progress - Persist each streamed update so partial content
+		// survives client disconnects or upstream stream failures.
+		if messageChanged {
+			if err := a.store.SaveChatMessage(assistantMsg); err != nil {
+				return fmt.Errorf("failed to save assistant progress: %w", err)
+			}
 			apiMsgChunk.AssistantMessage = assistantMsg
 		}
 
-		// Send Progress Chunk
+		// Send Progress Chunk - If the browser disconnects, keep the detached
+		// generation running and continue saving streamed content to the store.
+		if streamToClient {
 			if err := json.NewEncoder(flushWriter).Encode(apiMsgChunk); err != nil {
-			return fmt.Errorf("failed to send progress chunk: %w", err)
+				streamToClient = false
+				a.logger.WithError(err).WithField("chat_id", chat.ID).Warn("client stream disconnected")
+			}
 		}
 
 		return nil
 	}); err != nil {
-		return fmt.Errorf("failed to generate text stream: %w", err)
+		return responseStarted, fmt.Errorf("failed to generate text stream: %w", err)
 	}
 
 	// Summarize & Update Chat Title
@@ -534,16 +559,18 @@ func (a *API) sendMessage(ctx context.Context, w http.ResponseWriter, chatID uui
 
 	// Update Assistant Message
 	if err := a.store.SaveChatMessage(assistantMsg); err != nil {
-		return fmt.Errorf("failed to save assistant message to chat: %w", err)
+		return responseStarted, fmt.Errorf("failed to save assistant message to chat: %w", err)
 	}
 
 	// Send Final Chunk
+	if streamToClient {
 		if err := json.NewEncoder(flushWriter).Encode(&MessageChunk{
 			Chat:             toChatNoMessages(chat),
 			AssistantMessage: assistantMsg,
 		}); err != nil {
-		return fmt.Errorf("failed to send final chunk: %w", err)
+			return responseStarted, fmt.Errorf("failed to send final chunk: %w", err)
+		}
 	}
 
-	return nil
+	return responseStarted, nil
 }