Improve chat stream failure visibility and fallback response

src/app/api/chat/route.ts

@@ -5,6 +5,16 @@ import { ensureCronSchedulerStarted } from "@/lib/cron/runtime";
 
 export const maxDuration = 300; // 5 min max for long agent runs
 
+function formatChatStreamError(error: unknown): string {
+  const raw = error instanceof Error ? error.message : String(error);
+  const compact = raw.replace(/\s+/g, " ").trim();
+  if (!compact) {
+    return "Generation failed after tool execution. Please retry.";
+  }
+  const short = compact.length > 220 ? `${compact.slice(0, 220)}...` : compact;
+  return `Generation failed after tool execution: ${short}`;
+}
+
 export async function POST(req: NextRequest) {
   try {
     await ensureCronSchedulerStarted();
@@ -60,6 +70,10 @@ export async function POST(req: NextRequest) {
       headers: {
         "X-Chat-Id": resolvedChatId,
       },
+      onError: (error) => {
+        console.error("Chat stream response error:", error);
+        return formatChatStreamError(error);
+      },
     });
   } catch (error) {
     console.error("Chat API error:", error);

src/components/chat/chat-messages.tsx

@@ -8,9 +8,10 @@ import type { UIMessage } from "ai";
 interface ChatMessagesProps {
   messages: UIMessage[];
   isLoading: boolean;
+  errorMessage?: string | null;
 }
 
-export function ChatMessages({ messages, isLoading }: ChatMessagesProps) {
+export function ChatMessages({ messages, isLoading, errorMessage }: ChatMessagesProps) {
   const endRef = useRef<HTMLDivElement>(null);
 
   // Auto-scroll on new messages
@@ -69,6 +70,12 @@ export function ChatMessages({ messages, isLoading }: ChatMessagesProps) {
           </div>
         )}
 
+        {errorMessage ? (
+          <div className="rounded-md border border-destructive/30 bg-destructive/10 px-3 py-2 text-sm text-destructive">
+            {errorMessage}
+          </div>
+        ) : null}
+
         <div ref={endRef} />
       </div>
     </div>

src/components/chat/chat-panel.tsx

@@ -208,6 +208,15 @@ function areUIMessagesEquivalentById(
   return true;
 }
 
+function formatChatErrorMessage(error: unknown): string {
+  const raw = error instanceof Error ? error.message : String(error);
+  const compact = raw.replace(/\s+/g, " ").trim();
+  if (!compact) {
+    return "The model stopped before producing a final response. Please retry.";
+  }
+  return compact.length > 280 ? `${compact.slice(0, 280)}...` : compact;
+}
+
 export function ChatPanel() {
   const {
     activeChatId,
@@ -220,6 +229,7 @@ export function ChatPanel() {
     addChat,
   } = useAppStore();
   const [input, setInput] = useState("");
+  const [chatError, setChatError] = useState<string | null>(null);
 
   // Internal chatId that stays stable during a message send.
   // Pre-generate a UUID so useChat always has a consistent id.
@@ -247,6 +257,7 @@ export function ChatPanel() {
   useEffect(() => {
     if (activeChatId !== prevActiveChatId.current) {
       prevActiveChatId.current = activeChatId;
+      setChatError(null);
       if (activeChatId !== null) {
         setInternalChatId(activeChatId);
       } else {
@@ -275,6 +286,7 @@ export function ChatPanel() {
     transport,
     onError: (error) => {
       console.error("Chat error:", error);
+      setChatError(formatChatErrorMessage(error));
     },
   });
 
@@ -435,6 +447,7 @@ export function ChatPanel() {
 
   const onSubmit = useCallback(() => {
     if (!input.trim() || isLoading) return;
+    setChatError(null);
 
     pendingProjectSwitchRef.current = true;
     submissionStartCountRef.current = messagesRef.current.length;
@@ -473,7 +486,7 @@ export function ChatPanel() {
 
   return (
     <div className="flex flex-col h-full">
-      <ChatMessages messages={messages} isLoading={isLoading} />
+      <ChatMessages messages={messages} isLoading={isLoading} errorMessage={chatError} />
       <ChatInput
         input={input}
         setInput={setInput}

src/lib/agent/agent.ts

@@ -679,6 +679,53 @@ function getLastResponseToolText(messages: ModelMessage[]): string {
   return "";
 }
 
+function getLastNonResponseToolResult(messages: ModelMessage[]): {
+  toolName: string;
+  text: string;
+} | null {
+  for (let i = messages.length - 1; i >= 0; i -= 1) {
+    const msg = messages[i];
+    if (msg.role !== "tool" || !Array.isArray(msg.content)) {
+      continue;
+    }
+
+    for (let j = msg.content.length - 1; j >= 0; j -= 1) {
+      const part = msg.content[j];
+      if (!(typeof part === "object" && part !== null)) continue;
+      if (!("type" in part) || part.type !== "tool-result") continue;
+
+      const toolName =
+        "toolName" in part && typeof (part as { toolName?: unknown }).toolName === "string"
+          ? (part as { toolName: string }).toolName
+          : "";
+      if (!toolName || toolName === "response") continue;
+
+      const output =
+        "output" in part ? (part as { output?: unknown }).output : (part as { result?: unknown }).result;
+      const text = extractToolResultOutputText(output).trim();
+      return {
+        toolName,
+        text,
+      };
+    }
+  }
+
+  return null;
+}
+
+function truncateForFallback(value: string, maxChars: number): string {
+  if (value.length <= maxChars) {
+    return value;
+  }
+  return `${value.slice(0, maxChars)}\n...[truncated]`;
+}
+
+function formatStreamErrorForUser(errorMessage: string): string {
+  const compact = errorMessage.replace(/\s+/g, " ").trim();
+  if (!compact) return "";
+  return compact.length > 220 ? `${compact.slice(0, 220)}...` : compact;
+}
+
 function shouldAutoContinueAssistant(
   text: string,
   finishReason?: string
@@ -783,6 +830,8 @@ export async function runAgent(options: {
     label: "LLM Request (stream)",
   });
 
+  let streamErrorMessage = "";
+
   // Run the agent with streaming
   const result = streamText({
     model,
@@ -793,6 +842,10 @@ export async function runAgent(options: {
     stopWhen: [stepCountIs(MAX_TOOL_STEPS_PER_TURN), hasToolCall("response")],
     temperature: settings.chatModel.temperature ?? 0.7,
     maxOutputTokens: settings.chatModel.maxTokens ?? 4096,
+    onError: async ({ error }) => {
+      streamErrorMessage = error instanceof Error ? error.message : String(error);
+      console.error("Agent stream error:", error);
+    },
     onFinish: async (event) => {
       const finishReason =
         typeof (event as unknown as { finishReason?: unknown }).finishReason === "string"
@@ -801,7 +854,9 @@ export async function runAgent(options: {
 
       const responseMessages = event.response.messages;
       const lastAssistantText = getLastAssistantText(responseMessages);
+      const responseToolText = getLastResponseToolText(responseMessages).trim();
       let continuationText = "";
+      let fallbackText = "";
 
       if (shouldAutoContinueAssistant(lastAssistantText, finishReason)) {
         try {
@@ -828,6 +883,40 @@ export async function runAgent(options: {
         }
       }
 
+      if (
+        !lastAssistantText.trim() &&
+        !responseToolText &&
+        !continuationText.trim()
+      ) {
+        const lastToolResult = getLastNonResponseToolResult(responseMessages);
+        const streamErrorText = formatStreamErrorForUser(streamErrorMessage);
+        const fallbackLines: string[] = [
+          "Tool execution finished, but I could not produce a final response for this turn.",
+        ];
+
+        if (streamErrorText) {
+          fallbackLines.push(`Reason: ${streamErrorText}`);
+        }
+
+        if (lastToolResult?.toolName) {
+          fallbackLines.push(`Last tool: \`${lastToolResult.toolName}\``);
+        }
+
+        if (lastToolResult?.text) {
+          fallbackLines.push(
+            [
+              "Last tool output (truncated):",
+              "```text",
+              truncateForFallback(lastToolResult.text, 1200),
+              "```",
+            ].join("\n")
+          );
+        }
+
+        fallbackLines.push("Send `continue` and I will finish the answer.");
+        fallbackText = fallbackLines.join("\n\n");
+      }
+
       if (mcpCleanup) {
         try {
           await mcpCleanup();
@@ -853,11 +942,11 @@ export async function runAgent(options: {
           for (const msg of responseMessages) {
             chat.messages.push(...convertModelMessageToChatMessages(msg, now));
           }
-          if (continuationText) {
+          if (continuationText || fallbackText) {
             chat.messages.push({
               id: crypto.randomUUID(),
               role: "assistant",
-              content: continuationText,
+              content: continuationText || fallbackText,
               createdAt: now,
             });
           }
@@ -880,7 +969,11 @@ export async function runAgent(options: {
         topic: "chat",
         projectId: options.projectId ?? null,
         chatId: options.chatId,
-        reason: continuationText ? "agent_turn_auto_continued" : "agent_turn_finished",
+        reason: continuationText
+          ? "agent_turn_auto_continued"
+          : fallbackText
+            ? "agent_turn_fallback_response"
+            : "agent_turn_finished",
       });
       publishUiSyncEvent({
         topic: "files",