diff --git a/frontend/src/pages/AssistantChatPage.tsx b/frontend/src/pages/AssistantChatPage.tsx
index 4edc528a..7e4d27b0 100644
--- a/frontend/src/pages/AssistantChatPage.tsx
+++ b/frontend/src/pages/AssistantChatPage.tsx
@@ -272,12 +272,18 @@ export default function AssistantChatPage() {
     }
   }
 
-  // Single source of truth for "wipe every per-session task-lane state field"
-  // before switching to a different chat. Called from selectChat, handleNewChat,
-  // sendPrefill, and handleResumeNew so adding new lane-scoped state in future
+  // Single source of truth for "wipe every per-chat UI state field" before
+  // switching to a different chat. Called from selectChat, handleNewChat,
+  // sendPrefill, and handleResumeNew so adding new chat-scoped state in future
   // phases only requires touching this one helper. Forgetting to clear a field
-  // leaks the previous session's data into the new one (Phase 5 regression).
+  // leaks the previous chat's data into the new one — first noticed as a task
+  // lane regression (Phase 5), surfaced again as the conversation pane showing
+  // the previous chat's messages while the sidebar entry said "0 messages".
+  // `messages` belongs in here too: selectChat clears it asynchronously when
+  // getSession returns, but the gap between switching and that response is
+  // exactly when the previous chat's content stays visible.
   const resetSessionDerivedState = useCallback(() => {
+    setMessages([])
     setShowTaskLane(false)
     setActiveQuestions([])
     setActiveActions([])