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 f96373e..0ae892a 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/loop_detection_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/loop_detection_middleware.py
@@ -21,7 +21,7 @@ from typing import override
 
 from langchain.agents import AgentState
 from langchain.agents.middleware import AgentMiddleware
-from langchain_core.messages import SystemMessage
+from langchain_core.messages import HumanMessage
 from langgraph.runtime import Runtime
 
 logger = logging.getLogger(__name__)
@@ -203,8 +203,13 @@ class LoopDetectionMiddleware(AgentMiddleware[AgentState]):
             return {"messages": [stripped_msg]}
 
         if warning:
-            # Inject a system message warning the model
-            return {"messages": [SystemMessage(content=warning)]}
+            # Inject as HumanMessage instead of SystemMessage to avoid
+            # Anthropic's "multiple non-consecutive system messages" error.
+            # Anthropic models require system messages only at the start of
+            # the conversation; injecting one mid-conversation crashes
+            # langchain_anthropic's _format_messages(). HumanMessage works
+            # with all providers. See #1299.
+            return {"messages": [HumanMessage(content=warning)]}
 
         return None
 
diff --git a/backend/tests/test_loop_detection_middleware.py b/backend/tests/test_loop_detection_middleware.py
index b9aa2da..3bd0c36 100644
--- a/backend/tests/test_loop_detection_middleware.py
+++ b/backend/tests/test_loop_detection_middleware.py
@@ -2,7 +2,7 @@
 
 from unittest.mock import MagicMock
 
-from langchain_core.messages import AIMessage, SystemMessage
+from langchain_core.messages import AIMessage, HumanMessage, SystemMessage
 
 from deerflow.agents.middlewares.loop_detection_middleware import (
     _HARD_STOP_MSG,
@@ -81,7 +81,7 @@ class TestLoopDetection:
         assert result is not None
         msgs = result["messages"]
         assert len(msgs) == 1
-        assert isinstance(msgs[0], SystemMessage)
+        assert isinstance(msgs[0], HumanMessage)
         assert "LOOP DETECTED" in msgs[0].content
 
     def test_warn_only_injected_once(self):