diff --git a/backend/test/unit/channel/infrastructure/agent/test_agent_adapter.py b/backend/test/unit/channel/infrastructure/agent/test_agent_adapter.py
index c9b1c6e7..2369724a 100644
--- a/backend/test/unit/channel/infrastructure/agent/test_agent_adapter.py
+++ b/backend/test/unit/channel/infrastructure/agent/test_agent_adapter.py
@@ -1,10 +1,10 @@
 from __future__ import annotations
 
+import sys
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
-import yuxi.services.chat_service  # noqa: F401
 from yuxi.channel.domain.model.message.stream_chat_request import StreamChatRequest
 from yuxi.channel.domain.model.shared.channel_type import ChannelType
 from yuxi.channel.infrastructure.agent.agent_adapter import AgentAdapter
@@ -36,6 +36,17 @@ def stream_request() -> StreamChatRequest:
     )
 
 
+def _make_chat_service_mock(chunks: list[str]):
+    mock_svc = MagicMock()
+
+    async def _stream_gen(*args, **kwargs):
+        for chunk in chunks:
+            yield chunk
+
+    mock_svc.stream_agent_chat = _stream_gen
+    return mock_svc
+
+
 class TestAgentAdapter:
     @pytest.mark.asyncio
     async def test_get_service_user_found(self, adapter: AgentAdapter, fake_session_factory) -> None:
@@ -89,11 +100,11 @@ class TestAgentAdapter:
         result_mock.scalar_one_or_none.return_value = fake_user
         db.execute.return_value = result_mock
 
-        with patch("yuxi.services.chat_service.stream_agent_chat") as mock_stream:
-            mock_stream.return_value = async_iter([
-                '{"response": "hello", "status": "success"}',
-                '{"response": " world", "status": "success"}',
-            ])
+        mock_svc = _make_chat_service_mock([
+            '{"response": "hello", "status": "success"}',
+            '{"response": " world", "status": "success"}',
+        ])
+        with patch.dict(sys.modules, {"yuxi.services.chat_service": mock_svc}):
             result = await adapter.stream_chat(stream_request)
             assert result == "hello world"
 
@@ -104,8 +115,10 @@ class TestAgentAdapter:
         result_mock.scalar_one_or_none.return_value = None
         db.execute.return_value = result_mock
 
-        result = await adapter.stream_chat(stream_request)
-        assert "Error: no service user found" in result
+        mock_svc = _make_chat_service_mock([])
+        with patch.dict(sys.modules, {"yuxi.services.chat_service": mock_svc}):
+            result = await adapter.stream_chat(stream_request)
+            assert "Error: no service user found" in result
 
     @pytest.mark.asyncio
     async def test_stream_chat_error_status(self, adapter: AgentAdapter, stream_request: StreamChatRequest, fake_session_factory) -> None:
@@ -117,11 +130,11 @@ class TestAgentAdapter:
         result_mock.scalar_one_or_none.return_value = fake_user
         db.execute.return_value = result_mock
 
-        with patch("yuxi.services.chat_service.stream_agent_chat") as mock_stream:
-            mock_stream.return_value = async_iter([
-                '{"response": "hello", "status": "success"}',
-                '{"status": "error", "error_message": "something wrong"}',
-            ])
+        mock_svc = _make_chat_service_mock([
+            '{"response": "hello", "status": "success"}',
+            '{"status": "error", "error_message": "something wrong"}',
+        ])
+        with patch.dict(sys.modules, {"yuxi.services.chat_service": mock_svc}):
             result = await adapter.stream_chat(stream_request)
             assert "hello" in result
 
@@ -135,14 +148,9 @@ class TestAgentAdapter:
         result_mock.scalar_one_or_none.return_value = fake_user
         db.execute.return_value = result_mock
 
-        with patch("yuxi.services.chat_service.stream_agent_chat") as mock_stream:
-            mock_stream.return_value = async_iter([
-                '{"status": "error", "error_message": "something wrong"}',
-            ])
+        mock_svc = _make_chat_service_mock([
+            '{"status": "error", "error_message": "something wrong"}',
+        ])
+        with patch.dict(sys.modules, {"yuxi.services.chat_service": mock_svc}):
             result = await adapter.stream_chat(stream_request)
             assert "Error: something wrong" in result
-
-
-async def async_iter(items: list[str]):
-    for item in items:
-        yield item