fix: prevent SSE connection drops from silently truncating responses (#7831)

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

ui/desktop/src/hooks/useChatStream.ts

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useReducer, useRef } from 'react';
 import { AppEvents } from '../constants/events';
 import { ChatState } from '../types/chatState';
+import { toastError } from '../toasts';
 
 import {
   getSession,
@@ -206,7 +207,8 @@ async function streamFromResponse(
   initialMessages: Message[],
   dispatch: React.Dispatch<StreamAction>,
   onFinish: (error?: string) => void,
-  sessionId: string
+  sessionId: string,
+  signal?: AbortController['signal']
 ): Promise<void> {
   let currentMessages = initialMessages;
   const reduceMotion = prefersReducedMotion();
@@ -309,8 +311,22 @@ async function streamFromResponse(
       }
     }
 
+    // If we reach here, the stream ended without a Finish or Error event.
+    // This happens when the connection drops and retries are exhausted — the
+    // generator exits its loop without yielding a terminal event. We call
+    // onFinish() without an error to keep the conversation visible (passing
+    // an error would trigger a full-page error screen via sessionLoadError),
+    // then show a toast so the user knows the response may be incomplete.
+    // If the signal was aborted, the user intentionally stopped streaming,
+    // so we skip the toast.
     flushBatchedUpdates();
     onFinish();
+    if (!signal?.aborted) {
+      toastError({
+        title: 'Connection lost',
+        msg: 'The response may be incomplete. You can try sending your message again.',
+      });
+    }
   } catch (error) {
     flushBatchedUpdates();
     if (error instanceof Error && error.name !== 'AbortError') {
@@ -593,9 +609,10 @@ export function useChatStream({
           },
           throwOnError: true,
           signal: abortControllerRef.current.signal,
+          sseMaxRetryAttempts: 0,
         });
 
-        await streamFromResponse(stream, currentMessages, dispatch, onFinish, sessionId);
+        await streamFromResponse(stream, currentMessages, dispatch, onFinish, sessionId, abortControllerRef.current.signal);
       } catch (error) {
         // AbortError is expected when user stops streaming
         if (error instanceof Error && error.name === 'AbortError') {
@@ -634,9 +651,10 @@ export function useChatStream({
           },
           throwOnError: true,
           signal: abortControllerRef.current.signal,
+          sseMaxRetryAttempts: 0,
         });
 
-        await streamFromResponse(stream, currentMessages, dispatch, onFinish, sessionId);
+        await streamFromResponse(stream, currentMessages, dispatch, onFinish, sessionId, abortControllerRef.current.signal);
       } catch (error) {
         if (error instanceof Error && error.name === 'AbortError') {
           // Silently handle abort
@@ -773,9 +791,10 @@ export function useChatStream({
                 },
                 throwOnError: true,
                 signal: abortControllerRef.current.signal,
+                sseMaxRetryAttempts: 0,
               });
 
-              await streamFromResponse(stream, messagesForUI, dispatch, onFinish, targetSessionId);
+              await streamFromResponse(stream, messagesForUI, dispatch, onFinish, targetSessionId, abortControllerRef.current.signal);
             } catch (error) {
               if (error instanceof Error && error.name === 'AbortError') {
                 dispatch({ type: 'SET_CHAT_STATE', payload: ChatState.Idle });