agegr · northwang-lucky · Sep 12, 2026 · Sep 16, 2026 · Sep 16, 2026
diff --git a/components/ChatWindow.tsx b/components/ChatWindow.tsx
@@ -6,7 +6,7 @@ import { createPortal } from "react-dom";
 import type { AgentMessage, AssistantContentBlock, AssistantMessage, BashExecutionMessage, BlockingExtensionUiRequest, ExtensionUiRequest, SessionInfo, SessionTreeNode, ToolResultMessage, UserMessage } from "@/lib/types";
 import { normalizeCustomPanelLines } from "@/lib/ansi";
 import { asBracketedPaste, toTerminalKeyData } from "@/lib/terminal-input";
-import { countToolCallBlocks, getAssistantErrorMessage, getDisplayableAssistantBlocks, isMessageGroupAnchor, splitFinalAssistantBlocks } from "@/lib/message-display";
+import { countToolCallBlocks, getAssistantErrorMessage, getDisplayableAssistantBlocks, isAssistantTruncated, isMessageGroupAnchor, splitFinalAssistantBlocks } from "@/lib/message-display";
 import { extractTurnWrittenFiles, type WrittenFile } from "@/lib/turn-written-files";
 import { buildQuotedSelection } from "@/lib/quoted-selection";
 import { MessageView } from "./MessageView";
@@ -1103,7 +1103,7 @@ export function ChatWindow({ session, searchTarget, onSearchTargetHandled, initi
 
                 const finalAssistant = messages[finalAssistantIdx] as AssistantMessage;
                 const finalSplit = splitFinalAssistantBlocks(finalAssistant);
-                const finalAnswerMessage = finalSplit.answerBlocks.length > 0 || getAssistantErrorMessage(finalAssistant)
+                const finalAnswerMessage = finalSplit.answerBlocks.length > 0 || getAssistantErrorMessage(finalAssistant) || isAssistantTruncated(finalAssistant)
                   ? withAssistantBlocks(finalAssistant, finalSplit.answerBlocks)
                   : null;
 

diff --git a/components/MessageView.test.mjs b/components/MessageView.test.mjs
@@ -205,6 +205,33 @@ test("renders a provider error when the assistant message has no content", () =>
   assert.match(html, /&lt;html&gt;request forbidden&lt;\/html&gt;/);
 });
 
+test("renders a truncation notice for stopReason length", () => {
+  const html = renderMessage({
+    role: "assistant",
+    provider: "anthropic",
+    model: "claude-test",
+    content: [{ type: "thinking", thinking: "Long reasoning chain" }],
+    stopReason: "length",
+  });
+
+  assert.match(html, /role="alert"/);
+  assert.match(html, /output limit/i);
+  assert.match(html, /follow-up/i);
+});
+
+test("renders a truncation notice for thinking-only messages with stopReason length", () => {
+  const html = renderMessage({
+    role: "assistant",
+    provider: "anthropic",
+    model: "claude-test",
+    content: [],
+    stopReason: "length",
+  });
+
+  assert.match(html, /role="alert"/);
+  assert.match(html, /output limit/i);
+});
+
 test("renders partial assistant content before the provider error", () => {
   const html = renderMessage({
     role: "assistant",

diff --git a/components/MessageView.tsx b/components/MessageView.tsx
@@ -8,7 +8,7 @@ import { ThinkingIcon } from "./ThinkingIcon";
 import { copyText } from "@/lib/clipboard";
 import { useI18n } from "@/hooks/useI18n";
 import { parseCompactionSummary } from "@/lib/compaction-summary";
-import { getAssistantErrorMessage, getThinkingPreview, isEmptyThinkingBlock } from "@/lib/message-display";
+import { getAssistantErrorMessage, getThinkingPreview, isAssistantTruncated, isEmptyThinkingBlock } from "@/lib/message-display";
 import { parseUnifiedPatch, type SplitDiffCell } from "@/lib/patch";
 import { isEditToolName } from "@/lib/tool-names";
 import { isThinkingExpandedByDefault, THINKING_EXPANDED_EVENT } from "@/lib/thinking-expansion-preference";
@@ -629,6 +629,7 @@ function AssistantMessageView({
     .filter(({ block }) => !isEmptyThinkingBlock(block, { isStreaming })), [message.content, isStreaming]);
   const blocks = useMemo(() => blockItems.map(({ block }) => block), [blockItems]);
   const providerError = getAssistantErrorMessage(message, { isStreaming });
+  const truncated = isAssistantTruncated(message, { isStreaming });
   const [hovered, setHovered] = useState(false);
   const [copied, setCopied] = useState(false);
   const streamStartRef = useRef<number | null>(null);
@@ -746,7 +747,7 @@ function AssistantMessageView({
     return () => clearInterval(id);
   }, [isStreaming]);
 
-  if (blocks.length === 0 && !isStreaming && !providerError) return null;
+  if (blocks.length === 0 && !isStreaming && !providerError && !truncated) return null;
 
   return (
     <div
@@ -825,6 +826,27 @@ function AssistantMessageView({
         </div>
       )}
 
+      {truncated && (
+        <div
+          role="alert"
+          style={{
+            marginTop: blocks.length > 0 || providerError ? 8 : 0,
+            padding: "7px 10px",
+            border: "1px solid rgba(234,179,8,0.3)",
+            borderRadius: 6,
+            background: "rgba(234,179,8,0.07)",
+            color: "#ca8a04",
+            fontFamily: "var(--font-mono)",
+            fontSize: 12,
+            lineHeight: 1.5,
+            whiteSpace: "pre-wrap",
+            overflowWrap: "anywhere",
+          }}
+        >
+          {t("chat.truncatedByOutputLimit")}
+        </div>
+      )}
+
       {writtenFiles && writtenFiles.length > 0 && (
         <TurnWrittenFiles files={writtenFiles} onOpenFile={onOpenFile} />
       )}

diff --git a/lib/i18n/messages/en.ts b/lib/i18n/messages/en.ts
@@ -301,6 +301,7 @@ export const enLocale: LocalePlugin = {
     "chat.loadingSession": "Loading session...",
     "chat.runningTool": "Running tool...",
     "chat.generatingToolInput": "Generating parameters...",
+    "chat.truncatedByOutputLimit": "This response was cut off after reaching the model’s output limit. Send a follow-up to continue.",
     "chat.runningNamedTool": "Running {name}...",
     "chat.runningTools": "Running {names}...",
     "chat.runningToolsMore": "Running {names} (+{count})...",

diff --git a/lib/i18n/messages/zh-CN.ts b/lib/i18n/messages/zh-CN.ts
@@ -301,6 +301,7 @@ export const zhCNLocale: LocalePlugin = {
     "chat.loadingSession": "正在加载会话...",
     "chat.runningTool": "正在运行工具...",
     "chat.generatingToolInput": "正在生成参数...",
+    "chat.truncatedByOutputLimit": "回复因达到模型输出长度上限而被截断。发送一条后续消息以继续。",
     "chat.runningNamedTool": "正在运行 {name}...",
     "chat.runningTools": "正在运行 {names}...",
     "chat.runningToolsMore": "正在运行 {names}（另有 {count} 个）...",

diff --git a/lib/i18n/messages/zh-TW.ts b/lib/i18n/messages/zh-TW.ts
@@ -301,6 +301,7 @@ export const zhTWLocale: LocalePlugin = {
     "chat.loadingSession": "正在載入工作階段...",
     "chat.runningTool": "正在執行工具...",
     "chat.generatingToolInput": "正在產生參數...",
+    "chat.truncatedByOutputLimit": "回覆因達到模型輸出長度上限而被截斷。傳送一則後續訊息以繼續。",
     "chat.runningNamedTool": "正在執行 {name}...",
     "chat.runningTools": "正在執行 {names}...",
     "chat.runningToolsMore": "正在執行 {names}（另有 {count} 個）...",

diff --git a/lib/message-display.test.mjs b/lib/message-display.test.mjs
@@ -149,3 +149,18 @@ test("treats compaction summaries as turn anchors", async () => {
   }), true);
   assert.equal(isMessageGroupAnchor(assistant([])), false);
 });
+
+test("isAssistantTruncated is true only for a finished stopReason length", async () => {
+  const { isAssistantTruncated } = await loadSubject();
+
+  const truncated = {
+    ...assistant([{ type: "thinking", thinking: " lengthy reasoning " }]),
+    stopReason: "length",
+  };
+  assert.equal(isAssistantTruncated(truncated), true);
+  // Still streaming: the final stopReason is not known yet.
+  assert.equal(isAssistantTruncated(truncated, { isStreaming: true }), false);
+  assert.equal(isAssistantTruncated({ ...assistant([]), stopReason: "stop" }), false);
+  assert.equal(isAssistantTruncated({ ...assistant([]), stopReason: "error", errorMessage: "oops" }), false);
+  assert.equal(isAssistantTruncated({ ...assistant([]) }), false);
+});
diff --git a/lib/message-display.ts b/lib/message-display.ts
@@ -32,6 +32,18 @@ export function getAssistantErrorMessage(
   return message.errorMessage?.trim() || "Unknown provider error";
 }
 
+/**
+ * A turn that ended on `stopReason: "length"` spent its whole output budget
+ * (often on reasoning alone) and produced no final answer; without a notice it
+ * looks like a hung session. The copy lives in i18n (`chat.truncatedByOutputLimit`).
+ */
+export function isAssistantTruncated(
+  message: AssistantMessage,
+  options: DisplayOptions = {},
+): boolean {
+  return !options.isStreaming && message.stopReason === "length";
+}
+
 function isFinalAnswerBlock(block: AssistantContentBlock): boolean {
   return block.type === "text" || block.type === "image";
 }