feat: replace optimistic message removal with error response handling

- Updated error handling in `threadSlice` to add agent responses instead of removing user messages.
- Simplified Redux actions by consolidating error handling logic for failed message transmissions.
- Ensured conversation flow consistency by providing fallback error messages to users.

src/pages/Conversations.tsx

@@ -44,7 +44,6 @@
   setLastViewed,
   setPanelWidth,
   setSelectedThread,
-  updateMessagesForThread,
 } from '../store/threadSlice';
 import type { ThreadMessage } from '../types/thread';
 
@@ -344,31 +343,19 @@
       },
       onError: event => {
         if (event.thread_id !== selectedThreadIdRef.current) return;
-        if (event.error_type !== 'cancelled') {
-          setSendError(event.message);
-        }
         setIsSending(false);
         setActiveToolCall(null);
-        dispatch(setActiveThread(null));
 
-        // Remove the optimistic user message on error
-        dispatch((innerDispatch, getState) => {
-          const state = getState() as {
-            thread: { messagesByThreadId: Record<string, ThreadMessage[]> };
-          };
-          const persistedMessages = state.thread.messagesByThreadId[event.thread_id] || [];
-          const lastUserIdx = [...persistedMessages]
-            .reverse()
-            .findIndex(m => m.sender === 'user');
-          if (lastUserIdx !== -1) {
-            const actualIdx = persistedMessages.length - 1 - lastUserIdx;
-            const updated = persistedMessages.filter((_, i) => i !== actualIdx);
-            innerDispatch(updateMessagesForThread({ threadId: event.thread_id, messages: updated }));
-            if (event.thread_id === selectedThreadIdRef.current) {
-              innerDispatch(setSelectedThread(event.thread_id));
-            }
-          }
-        });
+        if (event.error_type !== 'cancelled') {
+          dispatch(
+            addInferenceResponse({
+              content: 'Something went wrong — please try again.',
+              threadId: event.thread_id,
+            })
+          );
+        } else {
+          dispatch(setActiveThread(null));
+        }
       },
     }).then(fn => {
       if (mounted) cleanup = fn;
@@ -419,7 +406,7 @@
   const handleSendMessageWeb = async (
     sendingThreadId: string,
     trimmed: string,
     userMessage: ThreadMessage,
     historySnapshot: ThreadMessage[]
   ) => {
     // Safety-net timeout: force-clear loading states if everything hangs
@@ -633,32 +620,13 @@
 
       // Pass the original sending thread ID to ensure response goes to correct thread
       dispatch(addInferenceResponse({ content: finalContent, threadId: sendingThreadId }));
-    } catch (err) {
-      // Remove the user message from persistent storage on error
-      // We'll use a thunk-like approach to access current state
-      dispatch((innerDispatch, getState) => {
-        const state = getState() as {
-          thread: { messagesByThreadId: Record<string, ThreadMessage[]> };
-        };
-        const persistedMessages = state.thread.messagesByThreadId[sendingThreadId] || [];
-        const currentMessages = persistedMessages.filter(m => m.id !== userMessage.id);
-        innerDispatch(
-          updateMessagesForThread({ threadId: sendingThreadId, messages: currentMessages })
-        );
-
-        // Also remove from current view if this is the selected thread
-        if (sendingThreadId === selectedThreadId) {
-          innerDispatch(setSelectedThread(sendingThreadId));
-        }
-      });
-
-      const msg =
-        err && typeof err === 'object' && 'error' in err
-          ? String((err as { error: unknown }).error)
-          : 'Failed to get response';
-      setSendError(msg);
-      // Clear active thread on error
-      dispatch(setActiveThread(null));
+    } catch {
+      dispatch(
+        addInferenceResponse({
+          content: 'Something went wrong — please try again.',
+          threadId: sendingThreadId,
+        })
+      );
     } finally {
       clearTimeout(safetyTimeout);
       setIsSending(false);

src/store/threadSlice.ts

@@ -83,7 +83,7 @@
   'thread/sendMessage',
   async (
     { threadId, message }: { threadId: string; message: string },
     { dispatch, getState, rejectWithValue }
   ) => {
     // 1. Add user message locally immediately (optimistic update)
     const userMessage: ThreadMessage = {
@@ -114,11 +114,13 @@
 
       return data;
     } catch (error) {
-      // Remove optimistic user message on failure
-      const state = (getState() as { thread: ThreadState }).thread;
-      const messages = state.messagesByThreadId[threadId] || [];
-      const filteredMessages = messages.filter(m => m.id !== userMessage.id);
-      dispatch(updateMessagesForThread({ threadId, messages: filteredMessages }));
+      // Add an error message as an agent response so the conversation flow continues
+      dispatch(
+        addInferenceResponse({
+          content: 'Something went wrong — please try again.',
+          threadId,
+        })
+      );
 
       const msg =
         error && typeof error === 'object' && 'error' in error