diff --git a/ui/src/components/AssistantPanel.tsx b/ui/src/components/AssistantPanel.tsx
index cb61420..36e8448 100644
--- a/ui/src/components/AssistantPanel.tsx
+++ b/ui/src/components/AssistantPanel.tsx
@@ -50,11 +50,23 @@ export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelP
   )
 
   // Fetch conversation details when we have an ID
-  const { data: conversationDetail, isLoading: isLoadingConversation } = useConversation(
+  const { data: conversationDetail, isLoading: isLoadingConversation, error: conversationError } = useConversation(
     projectName,
     conversationId
   )
 
+  // Clear stored conversation ID if it no longer exists (404 error)
+  useEffect(() => {
+    if (conversationError && conversationId) {
+      const message = conversationError.message.toLowerCase()
+      // Only clear for 404 errors, not transient network issues
+      if (message.includes('not found') || message.includes('404')) {
+        console.warn(`Conversation ${conversationId} not found, clearing stored ID`)
+        setConversationId(null)
+      }
+    }
+  }, [conversationError, conversationId])
+
   // Convert API messages to ChatMessage format for the chat component
   const initialMessages: ChatMessage[] | undefined = conversationDetail?.messages.map((msg) => ({
     id: `db-${msg.id}`,
diff --git a/ui/src/hooks/useConversations.ts b/ui/src/hooks/useConversations.ts
index 908b22d..c3b50de 100644
--- a/ui/src/hooks/useConversations.ts
+++ b/ui/src/hooks/useConversations.ts
@@ -26,6 +26,16 @@ export function useConversation(projectName: string | null, conversationId: numb
     queryFn: () => api.getAssistantConversation(projectName!, conversationId!),
     enabled: !!projectName && !!conversationId,
     staleTime: 30_000, // Cache for 30 seconds
+    retry: (failureCount, error) => {
+      // Don't retry on "not found" errors (404) - conversation doesn't exist
+      if (error instanceof Error && (
+        error.message.toLowerCase().includes('not found') ||
+        error.message === 'HTTP 404'
+      )) {
+        return false
+      }
+      return failureCount < 3
+    },
   })
 }