diff --git a/backend/packages/harness/deerflow/agents/middlewares/loop_detection_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/loop_detection_middleware.py
index 5f68a3ce6..8bc4cfd43 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/loop_detection_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/loop_detection_middleware.py
@@ -182,6 +182,23 @@ class LoopDetectionMiddleware(AgentMiddleware[AgentState]):
 
         return None, False
 
+    @staticmethod
+    def _append_text(content: str | list | None, text: str) -> str | list:
+        """Append *text* to AIMessage content, handling str, list, and None.
+
+        When content is a list of content blocks (e.g. Anthropic thinking mode),
+        we append a new ``{"type": "text", ...}`` block instead of concatenating
+        a string to a list, which would raise ``TypeError``.
+        """
+        if content is None:
+            return text
+        if isinstance(content, list):
+            return [*content, {"type": "text", "text": f"\n\n{text}"}]
+        if isinstance(content, str):
+            return content + f"\n\n{text}"
+        # Fallback: coerce unexpected types to str to avoid TypeError
+        return str(content) + f"\n\n{text}"
+
     def _apply(self, state: AgentState, runtime: Runtime) -> dict | None:
         warning, hard_stop = self._track_and_check(state, runtime)
 
@@ -192,7 +209,7 @@ class LoopDetectionMiddleware(AgentMiddleware[AgentState]):
             stripped_msg = last_msg.model_copy(
                 update={
                     "tool_calls": [],
-                    "content": (last_msg.content or "") + f"\n\n{_HARD_STOP_MSG}",
+                    "content": self._append_text(last_msg.content, _HARD_STOP_MSG),
                 }
             )
             return {"messages": [stripped_msg]}
diff --git a/backend/tests/test_loop_detection_middleware.py b/backend/tests/test_loop_detection_middleware.py
index 3bd0c3665..e037b8492 100644
--- a/backend/tests/test_loop_detection_middleware.py
+++ b/backend/tests/test_loop_detection_middleware.py
@@ -1,5 +1,6 @@
 """Tests for LoopDetectionMiddleware."""
 
+import copy
 from unittest.mock import MagicMock
 
 from langchain_core.messages import AIMessage, HumanMessage, SystemMessage
@@ -19,8 +20,13 @@ def _make_runtime(thread_id="test-thread"):
 
 
 def _make_state(tool_calls=None, content=""):
-    """Build a minimal AgentState dict with an AIMessage."""
-    msg = AIMessage(content=content, tool_calls=tool_calls or [])
+    """Build a minimal AgentState dict with an AIMessage.
+
+    Deep-copies *content* when it is mutable (e.g. list) so that
+    successive calls never share the same object reference.
+    """
+    safe_content = copy.deepcopy(content) if isinstance(content, list) else content
+    msg = AIMessage(content=safe_content, tool_calls=tool_calls or [])
     return {"messages": [msg]}
 
 
@@ -229,3 +235,114 @@ class TestLoopDetection:
 
         mw._apply(_make_state(tool_calls=call), runtime)
         assert "default" in mw._history
+
+
+class TestAppendText:
+    """Unit tests for LoopDetectionMiddleware._append_text."""
+
+    def test_none_content_returns_text(self):
+        result = LoopDetectionMiddleware._append_text(None, "hello")
+        assert result == "hello"
+
+    def test_str_content_concatenates(self):
+        result = LoopDetectionMiddleware._append_text("existing", "appended")
+        assert result == "existing\n\nappended"
+
+    def test_empty_str_content_concatenates(self):
+        result = LoopDetectionMiddleware._append_text("", "appended")
+        assert result == "\n\nappended"
+
+    def test_list_content_appends_text_block(self):
+        """List content (e.g. Anthropic thinking mode) should get a new text block."""
+        content = [
+            {"type": "thinking", "text": "Let me think..."},
+            {"type": "text", "text": "Here is my answer"},
+        ]
+        result = LoopDetectionMiddleware._append_text(content, "stop msg")
+        assert isinstance(result, list)
+        assert len(result) == 3
+        assert result[0] == content[0]
+        assert result[1] == content[1]
+        assert result[2] == {"type": "text", "text": "\n\nstop msg"}
+
+    def test_empty_list_content_appends_text_block(self):
+        result = LoopDetectionMiddleware._append_text([], "stop msg")
+        assert isinstance(result, list)
+        assert len(result) == 1
+        assert result[0] == {"type": "text", "text": "\n\nstop msg"}
+
+    def test_unexpected_type_coerced_to_str(self):
+        """Unexpected content types should be coerced to str as a fallback."""
+        result = LoopDetectionMiddleware._append_text(42, "stop msg")
+        assert isinstance(result, str)
+        assert result == "42\n\nstop msg"
+
+    def test_list_content_not_mutated_in_place(self):
+        """_append_text must not modify the original list."""
+        original = [{"type": "text", "text": "hello"}]
+        result = LoopDetectionMiddleware._append_text(original, "appended")
+        assert len(original) == 1  # original unchanged
+        assert len(result) == 2  # new list has the appended block
+
+
+class TestHardStopWithListContent:
+    """Regression tests: hard stop must not crash when AIMessage.content is a list."""
+
+    def test_hard_stop_with_list_content(self):
+        """Hard stop on list content should not raise TypeError (regression)."""
+        mw = LoopDetectionMiddleware(warn_threshold=2, hard_limit=4)
+        runtime = _make_runtime()
+        call = [_bash_call("ls")]
+
+        # Build state with list content (e.g. Anthropic thinking mode)
+        list_content = [
+            {"type": "thinking", "text": "Let me think..."},
+            {"type": "text", "text": "I'll run ls"},
+        ]
+
+        for _ in range(3):
+            mw._apply(_make_state(tool_calls=call, content=list_content), runtime)
+
+        # Fourth call triggers hard stop — must not raise TypeError
+        result = mw._apply(_make_state(tool_calls=call, content=list_content), runtime)
+        assert result is not None
+        msg = result["messages"][0]
+        assert isinstance(msg, AIMessage)
+        assert msg.tool_calls == []
+        # Content should remain a list with the stop message appended
+        assert isinstance(msg.content, list)
+        assert len(msg.content) == 3
+        assert msg.content[2]["type"] == "text"
+        assert _HARD_STOP_MSG in msg.content[2]["text"]
+
+    def test_hard_stop_with_none_content(self):
+        """Hard stop on None content should produce a plain string."""
+        mw = LoopDetectionMiddleware(warn_threshold=2, hard_limit=4)
+        runtime = _make_runtime()
+        call = [_bash_call("ls")]
+
+        for _ in range(3):
+            mw._apply(_make_state(tool_calls=call), runtime)
+
+        # Fourth call with default empty-string content
+        result = mw._apply(_make_state(tool_calls=call), runtime)
+        assert result is not None
+        msg = result["messages"][0]
+        assert isinstance(msg.content, str)
+        assert _HARD_STOP_MSG in msg.content
+
+    def test_hard_stop_with_str_content(self):
+        """Hard stop on str content should concatenate the stop message."""
+        mw = LoopDetectionMiddleware(warn_threshold=2, hard_limit=4)
+        runtime = _make_runtime()
+        call = [_bash_call("ls")]
+
+        for _ in range(3):
+            mw._apply(_make_state(tool_calls=call, content="thinking..."), runtime)
+
+        result = mw._apply(_make_state(tool_calls=call, content="thinking..."), runtime)
+        assert result is not None
+        msg = result["messages"][0]
+        assert isinstance(msg.content, str)
+        assert msg.content.startswith("thinking...")
+        assert _HARD_STOP_MSG in msg.content