diff --git a/src/hooks/useSSEStream.ts b/src/hooks/useSSEStream.ts
index 79df5882..a37c7f3f 100644
--- a/src/hooks/useSSEStream.ts
+++ b/src/hooks/useSSEStream.ts
@@ -237,11 +237,12 @@ function handleSSEEvent(
 export async function consumeSSEStream(
   reader: ReadableStreamDefaultReader<Uint8Array>,
   callbacks: SSECallbacks,
-): Promise<{ accumulated: string; tokenUsage: TokenUsage | null }> {
+): Promise<{ accumulated: string; tokenUsage: TokenUsage | null; receivedDone: boolean }> {
   const decoder = new TextDecoder();
   let buffer = '';
   let accumulated = '';
   let tokenUsage: TokenUsage | null = null;
+  let receivedDone = false;
 
   const wrappedCallbacks: SSECallbacks = {
     ...callbacks,
@@ -264,6 +265,9 @@ export async function consumeSSEStream(
 
       try {
         const event: SSEEvent = JSON.parse(line.slice(6));
+        if (event.type === 'done') {
+          receivedDone = true;
+        }
         accumulated = handleSSEEvent(event, accumulated, wrappedCallbacks);
       } catch {
         // skip malformed SSE lines
@@ -271,7 +275,20 @@ export async function consumeSSEStream(
     }
   }
 
-  return { accumulated, tokenUsage };
+  // Flush any residual buffer in case the final chunk didn't end with \n
+  if (buffer.trim().startsWith('data: ')) {
+    try {
+      const event: SSEEvent = JSON.parse(buffer.trim().slice(6));
+      if (event.type === 'done') {
+        receivedDone = true;
+      }
+      accumulated = handleSSEEvent(event, accumulated, wrappedCallbacks);
+    } catch {
+      // skip malformed residual data
+    }
+  }
+
+  return { accumulated, tokenUsage, receivedDone };
 }
 
 /**
diff --git a/src/i18n/en.ts b/src/i18n/en.ts
index 479d50f5..24eeda06 100644
--- a/src/i18n/en.ts
+++ b/src/i18n/en.ts
@@ -57,6 +57,7 @@ const en = {
   'streaming.allowForSession': 'Allow for Session',
   'streaming.allowed': 'Allowed',
   'streaming.denied': 'Denied',
+  'streaming.connectionDrop': 'Connection lost — the server stream ended unexpectedly. Claude may still be running in the background. Please try sending your message again.',
 
   // ── Chat view / session page ────────────────────────────────
   'chat.newConversation': 'New Conversation',
diff --git a/src/i18n/zh.ts b/src/i18n/zh.ts
index c239873d..20537069 100644
--- a/src/i18n/zh.ts
+++ b/src/i18n/zh.ts
@@ -54,6 +54,7 @@ const zh: Record<TranslationKey, string> = {
   'streaming.allowForSession': '本次会话允许',
   'streaming.allowed': '已允许',
   'streaming.denied': '已拒绝',
+  'streaming.connectionDrop': '连接中断 — 服务器流意外结束。Claude 可能仍在后台运行，请尝试重新发送消息。',
 
   // ── Chat view / session page ────────────────────────────────
   'chat.newConversation': '新对话',
diff --git a/src/lib/claude-client.ts b/src/lib/claude-client.ts
index 9979e91e..4d6ed193 100644
--- a/src/lib/claude-client.ts
+++ b/src/lib/claude-client.ts
@@ -397,6 +397,8 @@ export function streamClaude(options: ClaudeStreamOptions): ReadableStream<strin
     generativeUI,
   } = options;
 
+  let heartbeatTimer: ReturnType<typeof setInterval> | null = null;
+
   return new ReadableStream<string>({
     async start(controller) {
       // Resolve provider via the unified resolver. The caller may pass an explicit
@@ -824,6 +826,22 @@ export function streamClaude(options: ClaudeStreamOptions): ReadableStream<strin
         let tokenUsage: TokenUsage | null = null;
         // Track pending TodoWrite tool_use_ids so we can sync after successful execution
         const pendingTodoWrites = new Map<string, Array<{ content: string; status: string; activeForm?: string }>>();
+
+        // Server-side heartbeat: send keep_alive every 30s to prevent
+        // transport-level idle connection drops (Electron, OS TCP, proxies).
+        // This is independent of the SDK's own keep_alive messages.
+        heartbeatTimer = setInterval(() => {
+          try {
+            controller.enqueue(formatSSE({ type: 'keep_alive', data: '' }));
+          } catch {
+            // Controller may be closed — stop heartbeat
+            if (heartbeatTimer) {
+              clearInterval(heartbeatTimer);
+              heartbeatTimer = null;
+            }
+          }
+        }, 30_000);
+
         for await (const message of conversation) {
           if (abortController?.signal.aborted) {
             break;
@@ -1059,9 +1077,11 @@ export function streamClaude(options: ClaudeStreamOptions): ReadableStream<strin
           }
         }
 
+        if (heartbeatTimer) { clearInterval(heartbeatTimer); heartbeatTimer = null; }
         controller.enqueue(formatSSE({ type: 'done', data: '' }));
         controller.close();
       } catch (error) {
+        if (heartbeatTimer) { clearInterval(heartbeatTimer); heartbeatTimer = null; }
         const rawMessage = error instanceof Error ? error.message : 'Unknown error';
         // Log full error details for debugging (visible in terminal / dev tools)
         const stderrContent = error instanceof Error ? (error as { stderr?: string }).stderr : undefined;
@@ -1124,6 +1144,7 @@ export function streamClaude(options: ClaudeStreamOptions): ReadableStream<strin
     },
 
     cancel() {
+      if (heartbeatTimer) { clearInterval(heartbeatTimer); heartbeatTimer = null; }
       abortController?.abort();
     },
   });
diff --git a/src/lib/stream-session-manager.ts b/src/lib/stream-session-manager.ts
index 8371f401..34115b90 100644
--- a/src/lib/stream-session-manager.ts
+++ b/src/lib/stream-session-manager.ts
@@ -358,6 +358,42 @@ async function runStream(stream: ActiveStream, params: StartStreamParams): Promi
       },
     });
 
+    // Detect premature stream end (connection drop without server 'done' event)
+    if (!result.receivedDone) {
+      cleanupTimers(stream);
+
+      const dropMsg = 'Connection lost — the server stream ended unexpectedly. Claude may still be running in the background. Please try sending your message again.';
+      const errContent = stream.accumulatedText.trim()
+        ? stream.accumulatedText.trim() + `\n\n**Error:** ${dropMsg}`
+        : `**Error:** ${dropMsg}`;
+
+      stream.snapshot = {
+        ...buildSnapshot(stream),
+        phase: 'error',
+        completedAt: Date.now(),
+        error: dropMsg,
+        finalMessageContent: errContent,
+        statusText: undefined,
+        pendingPermission: null,
+        permissionResolved: null,
+      };
+      stream.accumulatedText = '';
+      stream.toolUsesArray = [];
+      stream.toolResultsArray = [];
+      stream.toolOutputAccumulated = '';
+      emit(stream, 'completed');
+
+      // Clear stale SDK session so next message starts fresh
+      fetch(`/api/chat/sessions/${encodeURIComponent(stream.sessionId)}`, {
+        method: 'PATCH',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ sdk_session_id: '' }),
+      }).catch(() => {});
+
+      scheduleGC(stream);
+      return;
+    }
+
     // Stream completed successfully — build final message content
     const accumulated = result.accumulated;
     const finalToolUses = stream.toolUsesArray;