diff --git a/packages/frontend/editor-ui/src/features/ai/instanceAi/__tests__/instanceAi.threadRuntime.test.ts b/packages/frontend/editor-ui/src/features/ai/instanceAi/__tests__/instanceAi.threadRuntime.test.ts
index 17517cd06b4..6039771ad29 100644
--- a/packages/frontend/editor-ui/src/features/ai/instanceAi/__tests__/instanceAi.threadRuntime.test.ts
+++ b/packages/frontend/editor-ui/src/features/ai/instanceAi/__tests__/instanceAi.threadRuntime.test.ts
@@ -790,6 +790,24 @@ describe('createThreadRuntime - SSE and hydration', () => {
 		expect(mockPostMessage).toHaveBeenCalledTimes(1);
 	});
 
+	test('sendMessage tracks whether this is the first user message in the thread', async () => {
+		mockPostMessage.mockResolvedValue({ runId: 'run-1' });
+
+		await activeRuntime(registry).sendMessage('first');
+		await activeRuntime(registry).sendMessage('second');
+
+		expect(mockTelemetryTrack).toHaveBeenNthCalledWith(1, 'User sent builder message', {
+			thread_id: activeThreadId,
+			instance_id: 'instance-1',
+			is_first_message: true,
+		});
+		expect(mockTelemetryTrack).toHaveBeenNthCalledWith(2, 'User sent builder message', {
+			thread_id: activeThreadId,
+			instance_id: 'instance-1',
+			is_first_message: false,
+		});
+	});
+
 	test('sendMessage forwards pushRef to postMessage', async () => {
 		mockPostMessage.mockResolvedValue({ runId: 'run-1' });
 
diff --git a/packages/frontend/editor-ui/src/features/ai/instanceAi/instanceAi.threadRuntime.ts b/packages/frontend/editor-ui/src/features/ai/instanceAi/instanceAi.threadRuntime.ts
index 661a96ba934..3dfc111713b 100644
--- a/packages/frontend/editor-ui/src/features/ai/instanceAi/instanceAi.threadRuntime.ts
+++ b/packages/frontend/editor-ui/src/features/ai/instanceAi/instanceAi.threadRuntime.ts
@@ -755,15 +755,11 @@ export function createThreadRuntime(threadId: string, hooks: ThreadRuntimeHooks)
 		}
 	}
 
-	function trackUserMessageSent(optimistic: InstanceAiMessage): void {
-		// The first user message in the array is the first the thread has ever
-		// seen. `find` short-circuits at the first match, so this is O(1) once
-		// the user has sent more than one message.
-		const firstUser = messages.value.find((m) => m.role === 'user');
+	function trackUserMessageSent(isFirstMessage: boolean): void {
 		telemetry.track('User sent builder message', {
 			thread_id: threadId,
 			instance_id: rootStore.instanceId,
-			is_first_message: firstUser === optimistic,
+			is_first_message: isFirstMessage,
 		});
 	}
 
@@ -815,8 +811,9 @@ export function createThreadRuntime(threadId: string, hooks: ThreadRuntimeHooks)
 		pendingMessageCount.value += 1;
 		try {
 			ensureSSEConnected();
+			const isFirstMessage = !messages.value.some((m) => m.role === 'user');
 			const optimistic = pushOptimisticUserMessage(message, attachments);
-			trackUserMessageSent(optimistic);
+			trackUserMessageSent(isFirstMessage);
 
 			if (!(await dispatchUserMessage(message, attachments, pushRef))) {
 				removeOptimisticMessage(optimistic);