diff --git a/web/src/hooks/useChat.test.ts b/web/src/hooks/useChat.test.ts
index 3384bf3..a7dab48 100644
--- a/web/src/hooks/useChat.test.ts
+++ b/web/src/hooks/useChat.test.ts
@@ -523,4 +523,56 @@ describe('useChat - handleServerMessage characterization', () => {
     const s1 = result.current.messages.find((m) => m.id === 's1');
     expect(s1?.content).toBe('part1part2');
   });
+
+  it('18. stream_end arriving after onDisconnect is idempotent (no crash, no side effect)', () => {
+    const { result } = renderUseChat();
+    act(() => {
+      result.current.handleServerMessage({ type: 'stream_delta', id: 'e1', delta: 'partial' });
+    });
+    expect(result.current.messages[0].content).toBe('partial');
+
+    // 断连：finishStreaming 重置 streamingRef
+    act(() => result.current.finishStreaming());
+
+    // 重连后 stream_end 到达（可能在断连前已发出，重连后才收到）
+    expect(() => {
+      act(() => result.current.handleServerMessage({ type: 'stream_end', id: 'e1' }));
+    }).not.toThrow();
+    // stream_end 不应改变已 flush 的内容
+    const e1 = result.current.messages.find((m) => m.id === 'e1');
+    expect(e1?.content).toBe('partial');
+
+    // 紧接着 assistant_response 到达，应正常按 id 替换（断连后的正常恢复路径）
+    act(() =>
+      result.current.handleServerMessage({
+        type: 'assistant_response',
+        id: 'e1',
+        content: 'final',
+        role: 'assistant',
+      }),
+    );
+    expect(result.current.messages.find((m) => m.id === 'e1')?.content).toBe('final');
+  });
+
+  it('19. multiple disconnect-reconnect cycles: finishStreaming is idempotent across cycles', () => {
+    const { result } = renderUseChat();
+
+    // 周期 1
+    act(() => result.current.handleServerMessage({ type: 'stream_delta', id: 'c1', delta: 'a' }));
+    act(() => result.current.finishStreaming());
+    expect(result.current.messages.find((m) => m.id === 'c1')?.content).toBe('a');
+
+    // 周期 2：新流式消息
+    act(() => result.current.handleServerMessage({ type: 'stream_delta', id: 'c2', delta: 'b' }));
+    act(() => result.current.finishStreaming());
+    expect(result.current.messages.find((m) => m.id === 'c2')?.content).toBe('b');
+    // c1 不受影响
+    expect(result.current.messages.find((m) => m.id === 'c1')?.content).toBe('a');
+
+    // 周期 3：连续两次 finishStreaming（无 delta 间隔），验证幂等
+    act(() => result.current.finishStreaming());
+    act(() => result.current.finishStreaming());
+    // 不崩溃，消息状态稳定
+    expect(result.current.messages).toHaveLength(2);
+  });
 });