Stabilize live run activity updates

frontend/app/app/api/chat/route.ts

@@ -525,10 +525,6 @@ function writeRuntimeEvent(
 	}
 	if (event.type === "content_delta" && event.delta) {
 		if (!streamState?.textStarted) {
-			if (streamState && !streamState.reasoningEnded) {
-				writer.write({ type: "reasoning-end", id: "reasoning-1" });
-				streamState.reasoningEnded = true;
-			}
 			writer.write({ type: "text-start", id: "text-1" });
 			if (streamState) streamState.textStarted = true;
 		}
@@ -556,20 +552,12 @@ function writeRuntimeEvent(
 	}
 	if (event.type === "final_text_start") {
 		if (!streamState?.textStarted) {
-			if (streamState && !streamState.reasoningEnded) {
-				writer.write({ type: "reasoning-end", id: "reasoning-1" });
-				streamState.reasoningEnded = true;
-			}
 			writer.write({ type: "text-start", id: "text-1" });
 			if (streamState) streamState.textStarted = true;
 		}
 	}
 	if (event.type === "final_text_delta" && event.delta) {
 		if (!streamState?.textStarted) {
-			if (streamState && !streamState.reasoningEnded) {
-				writer.write({ type: "reasoning-end", id: "reasoning-1" });
-				streamState.reasoningEnded = true;
-			}
 			writer.write({ type: "text-start", id: "text-1" });
 			if (streamState) streamState.textStarted = true;
 		}

frontend/app/components/assistant-ui/activity-panel.tsx

@@ -156,10 +156,6 @@ export function ActivityPanel() {
 		readActiveSessionId(),
 	);
 	const liveRunStatus = useLiveRunStatus(activeSessionId, mode);
-	const replayedSessionId = useMemo(
-		() => findLatestSessionIdInMessages(messages),
-		[messages],
-	);
 	const messageItems = useMemo(
 		() => collectActivityItems(messages),
 		[messages],
@@ -169,22 +165,20 @@ export function ActivityPanel() {
 		[liveRunStatus],
 	);
 	const selectedMessageId = selectedId ? activityMessageId(selectedId) : null;
+	const liveRunActive = hasActiveRun(liveRunStatus) && liveItems.length > 0;
 	const selectedMessageItems = selectedMessageId
 		? messageItems.filter(
 				(item) => activityMessageId(item.id) === selectedMessageId,
 			)
 		: [];
-	const shouldUseLiveItems =
+	const shouldUseLiveItems = Boolean(activeSessionId && liveRunActive);
-		!selectedMessageId &&
-		liveItems.length > 0 &&
-		activeSessionId !== null &&
-		(activeSessionId !== replayedSessionId || hasActiveRun(liveRunStatus));
 	const items = shouldUseLiveItems ? liveItems : messageItems;
 	const selectedItems = selectedMessageId
 		? items.filter((item) => activityMessageId(item.id) === selectedMessageId)
 		: [];
-	const visibleItems =
+	const visibleItems = shouldUseLiveItems
-		selectedMessageId && selectedMessageItems.length > 0
+		? liveItems
+		: selectedMessageId && selectedMessageItems.length > 0
 			? selectedMessageItems
 			: selectedMessageId && selectedItems.length > 0
 				? selectedItems
@@ -764,23 +758,6 @@ function hasActiveRun(
 	return status?.status === "queued" || status?.status === "running";
 }
 
-function findLatestSessionIdInMessages(messages: readonly MessageState[]) {
-	for (const message of [...messages].reverse()) {
-		const metadata = message.metadata;
-		const custom =
-			metadata && typeof metadata === "object" && "custom" in metadata
-				? (metadata as { custom?: unknown }).custom
-				: null;
-		if (custom && typeof custom === "object" && "sessionId" in custom) {
-			const sessionId = (custom as { sessionId?: unknown }).sessionId;
-			if (typeof sessionId === "string" && sessionId.trim()) {
-				return sessionId.trim();
-			}
-		}
-	}
-	return null;
-}
-
 function ActivityPanelItem({
 	item,
 	open,