fix(harness): normalize structured content for titles (#1155)

* fix(harness): normalize structured content for titles

Flatten structured LangChain message content before prompting the title model so list/block payloads don't leak Python reprs into generated thread titles.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

backend/tests/test_title_middleware_core_logic.py

@@ -92,6 +92,36 @@ class TestTitleMiddlewareCoreLogic:
         assert "'" not in title
         assert len(title) == 12
 
+    def test_generate_title_normalizes_structured_message_and_response_content(self, monkeypatch):
+        _set_test_title_config(max_chars=20)
+        middleware = TitleMiddleware()
+        fake_model = MagicMock()
+        fake_model.ainvoke = AsyncMock(
+            return_value=MagicMock(content=[{"type": "text", "text": '"结构总结"'}]),
+        )
+        monkeypatch.setattr(
+            "deerflow.agents.middlewares.title_middleware.create_chat_model",
+            lambda **kwargs: fake_model,
+        )
+
+        state = {
+            "messages": [
+                HumanMessage(content=[{"type": "text", "text": "请帮我总结这段代码"}]),
+                AIMessage(content=[{"type": "text", "text": "好的，先看结构"}]),
+            ]
+        }
+
+        title = asyncio.run(middleware._generate_title(state))
+
+        prompt = fake_model.ainvoke.await_args.args[0]
+        assert "请帮我总结这段代码" in prompt
+        assert "好的，先看结构" in prompt
+        # Ensure structured message dict/JSON reprs are not leaking into the prompt.
+        assert "{'type':" not in prompt
+        assert "'type':" not in prompt
+        assert '"type":' not in prompt
+        assert title == "结构总结"
+
     def test_generate_title_fallback_when_model_fails(self, monkeypatch):
         _set_test_title_config(max_chars=20)
         middleware = TitleMiddleware()