diff --git a/apps/web/src/components/chat/MessagesTimeline.logic.ts b/apps/web/src/components/chat/MessagesTimeline.logic.ts
index afdfdaa3d2..2fdb044235 100644
--- a/apps/web/src/components/chat/MessagesTimeline.logic.ts
+++ b/apps/web/src/components/chat/MessagesTimeline.logic.ts
@@ -2285,12 +2285,11 @@ export function deriveMessagesTimelineRowsWithState(
   input: MessagesTimelineRowsInput,
   previous: MessagesTimelineRowsProjection | null = null,
 ): MessagesTimelineRowsProjection {
-  return {
-    input,
-    rows:
-      (previous === null ? null : replaceStreamingMessageRows(input, previous)) ??
-      deriveMessagesTimelineRows(input),
-  };
+  const reused = previous === null ? null : replaceStreamingMessageRows(input, previous);
+  const probe = Reflect.get(globalThis, "issue24120Rows") ?? [];
+  probe.push({ at: Date.now(), reused: reused !== null, entries: input.timelineEntries });
+  Reflect.set(globalThis, "issue24120Rows", probe);
+  return { input, rows: reused ?? deriveMessagesTimelineRows(input) };
 }
 
 export function computeStableMessagesTimelineRows(
diff --git a/apps/web/src/session-logic.ts b/apps/web/src/session-logic.ts
index c170ec73e3..b68fbd5a26 100644
--- a/apps/web/src/session-logic.ts
+++ b/apps/web/src/session-logic.ts
@@ -1086,6 +1086,9 @@ export function deriveTimelineEntriesFromVisibleTurnItemsWithState(
   previous: TimelineEntriesProjection | null = null,
 ): TimelineEntriesProjection {
   const reused = previous === null ? null : reuseTimelineEntries(input, previous);
+  const probe = Reflect.get(globalThis, "issue24120Entries") ?? [];
+  probe.push({ at: Date.now(), reused: reused !== null, items: input.visibleTurnItems, plans: input.plans });
+  Reflect.set(globalThis, "issue24120Entries", probe);
   if (reused !== null) return { input, entries: reused };
   const entries = deriveTimelineEntriesFromVisibleTurnItems(input);
   if (previous === null || !shallow(input.attachmentUrlById, previous.input.attachmentUrlById)) {