diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index 92400fedd9b3..8c4f5875281e 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -2166,9 +2166,16 @@ export default function ChatView(props: ChatViewProps) {
     }
     return labels;
   }, [activeThreadKnownSessions]);
+  // Keyed on ids only: the thread object changes identity on every streamed
+  // update, and effects keyed on this ref (e.g. the diff scope reset) must not
+  // re-run mid-turn.
+  const activeThreadEnvironmentId = activeThread?.environmentId ?? null;
   const activeThreadRef = useMemo(
-    () => (activeThread ? scopeThreadRef(activeThread.environmentId, activeThread.id) : null),
-    [activeThread],
+    () =>
+      activeThreadEnvironmentId !== null && activeThreadId !== null
+        ? scopeThreadRef(activeThreadEnvironmentId, activeThreadId)
+        : null,
+    [activeThreadEnvironmentId, activeThreadId],
   );
   const activeThreadKey = activeThreadRef ? scopedThreadKey(activeThreadRef) : null;
   const previewPanelInlineSize = usePreviewPanelInlineSize(undefined, {