Prevent oversized assistant messages from crashing the timeline (#4166)

* fix(app): cap oversized assistant messages

A single assistant message can expand into enough rendered Markdown nodes to crash the renderer. Limit the Markdown input while preserving the complete timeline value and report its full byte size.

* fix(app): bound oversized stream processing

Cap the render input before paced reveal and defer full-message byte counting until completion. Reuse the existing grapheme boundary policy so capped text never ends in a partial glyph.

* fix(app): preserve capped text without segmentation

Hermes lacks Intl.Segmenter, while the shared reveal boundary intentionally returns zero without it. Keep that contract and fall back locally to a surrogate-safe 32K prefix.

packages/app/e2e/browser/assistant-message-render-limit.spec.ts

@@ -0,0 +1,32 @@
+import { expect, test } from "../support/fixtures";
+import { getUtf8ByteLength } from "../../src/components/assistant-message-render-limit";
+import { openAgentRoute, seedMockAgentWorkspace } from "../support/helpers/mock-agent";
+
+test("caps a newline-heavy assistant message before rendering markdown", async ({ page }) => {
+  const hiddenTail = "HIDDEN_OVERSIZED_MESSAGE_TAIL";
+  const response = `${"x\n".repeat(16_000)}${hiddenTail}`;
+  const agent = await seedMockAgentWorkspace({
+    repoPrefix: "assistant-message-render-limit-",
+    title: "Assistant message render limit",
+    initialPrompt: "Render the configured oversized response.",
+    featureValues: { mockAssistantResponse: response },
+  });
+
+  try {
+    await agent.client.waitForFinish(agent.agentId, 30_000);
+    await openAgentRoute(page, agent);
+
+    const assistantMessage = page.getByTestId("assistant-message").last();
+    await expect(assistantMessage).toBeVisible({ timeout: 30_000 });
+    await expect(assistantMessage).not.toContainText(hiddenTail);
+
+    const notice = assistantMessage.getByTestId("assistant-message-capped-notice");
+    await expect(notice).toBeVisible();
+    await expect(notice).toHaveText(
+      `This message was capped (${getUtf8ByteLength(response)} bytes).`,
+    );
+    await expect(notice).toHaveCSS("font-style", "italic");
+  } finally {
+    await agent.cleanup();
+  }
+});

packages/app/src/components/assistant-message-render-limit.test.ts

@@ -0,0 +1,54 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import {
+  ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT,
+  capAssistantMessageForRender,
+  getUtf8ByteLength,
+} from "./assistant-message-render-limit";
+
+describe("assistant message render limit", () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.resetModules();
+  });
+
+  it("leaves messages at the limit unchanged", () => {
+    const message = "a".repeat(ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT);
+
+    expect(capAssistantMessageForRender(message)).toEqual({ text: message, capped: false });
+  });
+
+  it("caps oversized messages before markdown rendering", () => {
+    const message = "a".repeat(ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT + 1);
+
+    expect(capAssistantMessageForRender(message)).toEqual({
+      text: "a".repeat(ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT),
+      capped: true,
+    });
+  });
+
+  it("does not split a grapheme cluster at the boundary", () => {
+    const prefix = "a".repeat(ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT - 1);
+
+    expect(capAssistantMessageForRender(`${prefix}étail`)).toEqual({
+      text: prefix,
+      capped: true,
+    });
+  });
+
+  it("counts the complete message size in UTF-8 bytes", () => {
+    expect(getUtf8ByteLength("aé😀")).toBe(7);
+  });
+
+  it("keeps a bounded prefix when grapheme segmentation is unavailable", async () => {
+    vi.resetModules();
+    vi.stubGlobal("Intl", { ...Intl, Segmenter: undefined });
+    const { capAssistantMessageForRender: capWithoutSegmenter } =
+      await import("./assistant-message-render-limit");
+    const message = "a".repeat(ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT + 1);
+
+    expect(capWithoutSegmenter(message)).toEqual({
+      text: "a".repeat(ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT),
+      capped: true,
+    });
+  });
+});

packages/app/src/components/assistant-message-render-limit.ts

@@ -0,0 +1,52 @@
+import { clampToSafeRevealBoundary, isTextRevealPacingSupported } from "@/agent-stream/text-reveal";
+
+export const ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT = 32_000;
+
+export interface CappedAssistantMessage {
+  text: string;
+  capped: boolean;
+}
+
+export function getUtf8ByteLength(message: string): number {
+  let bytes = 0;
+
+  for (let index = 0; index < message.length; index += 1) {
+    const codeUnit = message.charCodeAt(index);
+    if (codeUnit < 0x80) {
+      bytes += 1;
+    } else if (codeUnit < 0x800) {
+      bytes += 2;
+    } else if (
+      codeUnit >= 0xd800 &&
+      codeUnit <= 0xdbff &&
+      index + 1 < message.length &&
+      message.charCodeAt(index + 1) >= 0xdc00 &&
+      message.charCodeAt(index + 1) <= 0xdfff
+    ) {
+      bytes += 4;
+      index += 1;
+    } else {
+      bytes += 3;
+    }
+  }
+
+  return bytes;
+}
+
+export function capAssistantMessageForRender(message: string): CappedAssistantMessage {
+  if (message.length <= ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT) {
+    return { text: message, capped: false };
+  }
+
+  let end = ASSISTANT_MESSAGE_RENDER_CHARACTER_LIMIT;
+  if (isTextRevealPacingSupported()) {
+    end = clampToSafeRevealBoundary(message, end);
+  } else {
+    const finalCodeUnit = message.charCodeAt(end - 1);
+    if (finalCodeUnit >= 0xd800 && finalCodeUnit <= 0xdbff) {
+      end -= 1;
+    }
+  }
+
+  return { text: message.slice(0, end), capped: true };
+}

packages/app/src/components/message.tsx

@@ -116,6 +116,7 @@ import {
   markdownCopyTableCellDataSet,
   type MarkdownCopyInlineTag,
 } from "@/assistant-selection-copy/markup";
+import { capAssistantMessageForRender, getUtf8ByteLength } from "./assistant-message-render-limit";
 export type { InlinePathTarget } from "@/assistant-file-links";
 export type { AssistantForkTarget };
 
@@ -763,6 +764,13 @@ export const assistantMessageStylesheet = StyleSheet.create((theme) => ({
   containerCompactBottom: {
     paddingBottom: 0,
   },
+  cappedNotice: {
+    marginTop: theme.spacing[3],
+    fontFamily: theme.fontFamily.ui,
+    fontSize: theme.fontSize.base,
+    fontStyle: "italic",
+    color: theme.colors.foregroundMuted,
+  },
   imageFrame: {
     width: "100%",
     minHeight: 160,
@@ -1488,10 +1496,16 @@ export const AssistantMessage = memo(function AssistantMessage({
   spacing = "default",
   phase,
 }: AssistantMessageProps) {
+  const { t } = useTranslation();
   const markdownParser = useMemo(createAssistantMarkdownParser, []);
+  const renderedMessage = useMemo(() => capAssistantMessageForRender(message), [message]);
   // Paint a paced prefix while the turn is streaming so text arrives at a steady
   // rate instead of in whatever lumps the daemon's coalescing window produced.
-  const revealedMessage = useRevealedText(message, phase);
+  const revealedMessage = useRevealedText(renderedMessage.text, phase);
+  const fullMessageByteLength = useMemo(
+    () => (renderedMessage.capped && phase === "complete" ? getUtf8ByteLength(message) : null),
+    [message, phase, renderedMessage.capped],
+  );
 
   const fileLinkActions = useAssistantFileLinkActions();
   const handleMarkdownLinkPress = useStableEvent((url: string) => {
@@ -1970,6 +1984,14 @@ export const AssistantMessage = memo(function AssistantMessage({
           />
         </AssistantMessageBlockContainer>
       ))}
+      {fullMessageByteLength !== null ? (
+        <Text
+          testID="assistant-message-capped-notice"
+          style={assistantMessageStylesheet.cappedNotice}
+        >
+          {t("agentStream.messageCapped", { bytes: fullMessageByteLength })}
+        </Text>
+      ) : null}
     </View>
   );
 });

packages/app/src/i18n/resources/ar.ts

@@ -203,6 +203,7 @@ export const ar: TranslationResources = {
     empty: "ابدأ الدردشة مع هذا الوكيل...",
     scrollToBottom: "قم بالتمرير إلى الأسفل",
     historyLoadFailed: "تعذر تحميل سجل الوكيل",
+    messageCapped: "تم اقتطاع هذه الرسالة ({{bytes}} بايت).",
     permission: {
       plan: "يخطط",
       required: "الإذن مطلوب",

packages/app/src/i18n/resources/en.ts

@@ -200,6 +200,7 @@ export const en = {
     empty: "Start chatting with this agent...",
     scrollToBottom: "Scroll to bottom",
     historyLoadFailed: "Couldn't load agent history",
+    messageCapped: "This message was capped ({{bytes}} bytes).",
     permission: {
       plan: "Plan",
       required: "Permission Required",

packages/app/src/i18n/resources/es.ts

@@ -203,6 +203,7 @@ export const es: TranslationResources = {
     empty: "Comience a chatear con este agente...",
     scrollToBottom: "Desplazarse hacia abajo",
     historyLoadFailed: "No se pudo cargar el historial del agente",
+    messageCapped: "Este mensaje fue truncado ({{bytes}} bytes).",
     permission: {
       plan: "Plan",
       required: "Permiso requerido",

packages/app/src/i18n/resources/fr.ts

@@ -205,6 +205,7 @@ export const fr: TranslationResources = {
     empty: "Commencez à discuter avec cet agent...",
     scrollToBottom: "Faire défiler vers le bas",
     historyLoadFailed: "Impossible de charger l’historique de l’agent",
+    messageCapped: "Ce message a été tronqué ({{bytes}} octets).",
     permission: {
       plan: "Plan",
       required: "Autorisation requise",

packages/app/src/i18n/resources/ja.ts

@@ -203,6 +203,7 @@ export const ja: TranslationResources = {
     empty: "このエージェントとチャットを始めましょう...",
     scrollToBottom: "下にスクロール",
     historyLoadFailed: "エージェントの履歴を読み込めませんでした",
+    messageCapped: "このメッセージは上限で切り詰められました（{{bytes}}バイト）。",
     permission: {
       plan: "プラン",
       required: "権限が必要です",

packages/app/src/i18n/resources/ko.ts

@@ -203,6 +203,7 @@ export const ko: TranslationResources = {
     empty: "이 에이전트와 대화를 시작하세요...",
     scrollToBottom: "맨 아래로 스크롤",
     historyLoadFailed: "에이전트 기록을 로드할 수 없습니다.",
+    messageCapped: "이 메시지는 길이 제한으로 잘렸습니다({{bytes}}바이트).",
     permission: {
       plan: "계획",
       required: "권한 필요",

packages/app/src/i18n/resources/pt-BR.ts

@@ -203,6 +203,7 @@ export const ptBR: TranslationResources = {
     empty: "Comece a conversar com este agente...",
     scrollToBottom: "Rolar para o fim",
     historyLoadFailed: "Não foi possível carregar o histórico do agente",
+    messageCapped: "Esta mensagem foi truncada ({{bytes}} bytes).",
     permission: {
       plan: "Plano",
       required: "Permissão necessária",

packages/app/src/i18n/resources/ru.ts

@@ -203,6 +203,7 @@ export const ru: TranslationResources = {
     empty: "Начните общаться с этим агентом...",
     scrollToBottom: "Прокрутить вниз",
     historyLoadFailed: "Не удалось загрузить историю агента",
+    messageCapped: "Это сообщение было обрезано ({{bytes}} байт).",
     permission: {
       plan: "План",
       required: "Требуется разрешение",

packages/app/src/i18n/resources/zh-CN.ts

@@ -203,6 +203,7 @@ export const zhCN: TranslationResources = {
     empty: "开始和这个 Agent 对话...",
     scrollToBottom: "滚动到底部",
     historyLoadFailed: "无法加载智能体历史记录",
+    messageCapped: "此消息已被截断（{{bytes}} 字节）。",
     permission: {
       plan: "Plan",
       required: "需要权限",