diff --git a/backend/packages/harness/deerflow/agents/middlewares/dangling_tool_call_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/dangling_tool_call_middleware.py
index 6026d834e..5fcc238fe 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/dangling_tool_call_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/dangling_tool_call_middleware.py
@@ -26,6 +26,11 @@ from langchain_core.messages import ToolMessage
 
 logger = logging.getLogger(__name__)
 
+# Workaround for issue #2894: malformed write_file calls can carry huge Markdown
+# payloads in invalid tool-call args. Keep recovery error details short so the
+# synthetic ToolMessage does not echo large or malformed content back to the model.
+_MAX_RECOVERY_ERROR_DETAIL_LEN = 500
+
 
 class DanglingToolCallMiddleware(AgentMiddleware[AgentState]):
     """Inserts placeholder ToolMessages for dangling tool calls before model invocation.
@@ -98,9 +103,25 @@ class DanglingToolCallMiddleware(AgentMiddleware[AgentState]):
     @staticmethod
     def _synthetic_tool_message_content(tool_call: dict) -> str:
         if tool_call.get("invalid"):
+            name = tool_call.get("name")
             error = tool_call.get("error")
-            if isinstance(error, str) and error:
-                return f"[Tool call could not be executed because its arguments were invalid: {error}]"
+            error_text = error[:_MAX_RECOVERY_ERROR_DETAIL_LEN] if isinstance(error, str) and error else ""
+            # Workaround for issue #2894: malformed write_file calls can carry huge Markdown
+            # payloads in invalid tool-call args. Keep recovery guidance actionable without
+            # echoing large or malformed content back to the model.
+            if name == "write_file":
+                details = f" Parser error: {error_text}" if error_text else ""
+                return (
+                    "[write_file failed before execution: the tool-call arguments were not valid JSON, "
+                    "so no file was written. This often happens when the model tries to write a very "
+                    "large Markdown file in a single tool call, especially when `content` contains "
+                    "unescaped quotes, inline JSON, backslashes, or code fences. Do not retry the same "
+                    "large `write_file` payload for this artifact; provide the report/content directly "
+                    "as normal assistant text in your next response. If a file write is still needed "
+                    f"later, split the file into smaller sections instead of one large payload.{details}]"
+                )
+            if error_text:
+                return f"[Tool call could not be executed because its arguments were invalid: {error_text}]"
             return "[Tool call could not be executed because its arguments were invalid.]"
         return "[Tool call was interrupted and did not return a result.]"
 
diff --git a/backend/tests/test_dangling_tool_call_middleware.py b/backend/tests/test_dangling_tool_call_middleware.py
index 34f1ac035..2ccce9fa9 100644
--- a/backend/tests/test_dangling_tool_call_middleware.py
+++ b/backend/tests/test_dangling_tool_call_middleware.py
@@ -333,8 +333,27 @@ class TestBuildPatchedMessagesPatching:
         assert patched[1].tool_call_id == "write_file:36"
         assert patched[1].name == "write_file"
         assert patched[1].status == "error"
+        assert "write_file failed before execution" in patched[1].content
+        assert "no file was written" in patched[1].content
+        assert "very large Markdown file in a single tool call" in patched[1].content
+        assert "Do not retry the same large `write_file` payload" in patched[1].content
+        assert "split the file into smaller sections" in patched[1].content
+        assert "normal assistant text" in patched[1].content
+        assert "Failed to parse tool arguments" in patched[1].content
+        assert 'bad {"json"}' not in patched[1].content
+
+    def test_non_write_file_invalid_tool_call_uses_generic_recovery_message(self):
+        mw = DanglingToolCallMiddleware()
+        msgs = [_ai_with_invalid_tool_calls([_invalid_tc(name="search", tc_id="search:1")])]
+
+        patched = mw._build_patched_messages(msgs)
+
+        assert patched is not None
+        assert patched[1].tool_call_id == "search:1"
+        assert patched[1].name == "search"
         assert "arguments were invalid" in patched[1].content
         assert "Failed to parse tool arguments" in patched[1].content
+        assert "write_file failed before execution" not in patched[1].content
 
     def test_valid_and_invalid_tool_calls_are_both_patched(self):
         mw = DanglingToolCallMiddleware()