diff --git a/libs/langchain_v1/langchain/agents/middleware/todo.py b/libs/langchain_v1/langchain/agents/middleware/todo.py
index 2cb0b94154fc0..15781d85cfd77 100644
--- a/libs/langchain_v1/langchain/agents/middleware/todo.py
+++ b/libs/langchain_v1/langchain/agents/middleware/todo.py
@@ -136,7 +136,7 @@ def write_todos(
     return Command(
         update={
             "todos": todos,
-            "messages": [ToolMessage(f"Updated todo list to {todos}", tool_call_id=tool_call_id)],
+            "messages": [ToolMessage("Todo list updated.", tool_call_id=tool_call_id)],
         }
     )
 
@@ -149,9 +149,7 @@ def _write_todos(
     return Command(
         update={
             "todos": todos,
-            "messages": [
-                ToolMessage(f"Updated todo list to {todos}", tool_call_id=runtime.tool_call_id)
-            ],
+            "messages": [ToolMessage("Todo list updated.", tool_call_id=runtime.tool_call_id)],
         }
     )
 
diff --git a/libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_todo.py b/libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_todo.py
index 6da56153256e5..0cd2439fc84ab 100644
--- a/libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_todo.py
+++ b/libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_todo.py
@@ -296,43 +296,32 @@ def mock_handler(req: ModelRequest) -> ModelResponse:
 
 
 @pytest.mark.parametrize(
-    ("todos", "expected_message"),
+    "todos",
     [
-        ([], "Updated todo list to []"),
-        (
-            [{"content": "Task 1", "status": "pending"}],
-            "Updated todo list to [{'content': 'Task 1', 'status': 'pending'}]",
-        ),
-        (
-            [
-                {"content": "Task 1", "status": "pending"},
-                {"content": "Task 2", "status": "in_progress"},
-            ],
-            (
-                "Updated todo list to ["
-                "{'content': 'Task 1', 'status': 'pending'}, "
-                "{'content': 'Task 2', 'status': 'in_progress'}]"
-            ),
-        ),
-        (
-            [
-                {"content": "Task 1", "status": "pending"},
-                {"content": "Task 2", "status": "in_progress"},
-                {"content": "Task 3", "status": "completed"},
-            ],
-            (
-                "Updated todo list to ["
-                "{'content': 'Task 1', 'status': 'pending'}, "
-                "{'content': 'Task 2', 'status': 'in_progress'}, "
-                "{'content': 'Task 3', 'status': 'completed'}]"
-            ),
-        ),
+        [],
+        [{"content": "Task 1", "status": "pending"}],
+        [
+            {"content": "Task 1", "status": "pending"},
+            {"content": "Task 2", "status": "in_progress"},
+        ],
+        [
+            {"content": "Task 1", "status": "pending"},
+            {"content": "Task 2", "status": "in_progress"},
+            {"content": "Task 3", "status": "completed"},
+        ],
     ],
 )
 def test_todo_middleware_write_todos_tool_execution(
-    todos: list[dict[str, Any]], expected_message: str
+    todos: list[dict[str, Any]],
 ) -> None:
-    """Test that the write_todos tool executes correctly."""
+    """Test that the write_todos tool executes correctly.
+
+    The tool message is a neutral "Todo list updated." ack; the actual
+    state lives in the `todos` field of the Command update. Dumping the
+    full list inline (including `status: completed` entries) was priming
+    some models to interpret the tool result as "everything done, wrap
+    up briefly" instead of continuing to deliver the substantive answer.
+    """
     tool_call = {
         "args": {"todos": todos},
         "name": "write_todos",
@@ -341,7 +330,7 @@ def test_todo_middleware_write_todos_tool_execution(
     }
     result = write_todos.invoke(tool_call)
     assert result.update["todos"] == todos
-    assert result.update["messages"][0].content == expected_message
+    assert result.update["messages"][0].content == "Todo list updated."
 
 
 @pytest.mark.parametrize(