test: add unit tests for TodoMiddleware (#1307)

* test: add unit tests for TodoMiddleware

Cover context-loss detection logic:
- _todos_in_messages and _reminder_in_messages helpers
- _format_todos formatting
- Reminder injection when write_todos truncated
- No-op when todos visible or reminder already present
- abefore_model async delegation

* test: fix event loop error in todo middleware async test

Use asyncio.run() instead of get_event_loop().run_until_complete()
to avoid RuntimeError on Python 3.12 where no default event loop
exists in the main thread.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/tests/test_todo_middleware.py

@@ -0,0 +1,156 @@
+"""Tests for TodoMiddleware context-loss detection."""
+
+import asyncio
+from unittest.mock import MagicMock
+
+from langchain_core.messages import AIMessage, HumanMessage
+
+from deerflow.agents.middlewares.todo_middleware import (
+    TodoMiddleware,
+    _format_todos,
+    _reminder_in_messages,
+    _todos_in_messages,
+)
+
+
+def _ai_with_write_todos():
+    return AIMessage(content="", tool_calls=[{"name": "write_todos", "id": "tc_1", "args": {}}])
+
+
+def _reminder_msg():
+    return HumanMessage(name="todo_reminder", content="reminder")
+
+
+def _make_runtime():
+    runtime = MagicMock()
+    runtime.context = {"thread_id": "test-thread"}
+    return runtime
+
+
+def _sample_todos():
+    return [
+        {"status": "completed", "content": "Set up project"},
+        {"status": "in_progress", "content": "Write tests"},
+        {"status": "pending", "content": "Deploy"},
+    ]
+
+
+class TestTodosInMessages:
+    def test_true_when_write_todos_present(self):
+        msgs = [HumanMessage(content="hi"), _ai_with_write_todos()]
+        assert _todos_in_messages(msgs) is True
+
+    def test_false_when_no_write_todos(self):
+        msgs = [
+            HumanMessage(content="hi"),
+            AIMessage(content="hello", tool_calls=[{"name": "bash", "id": "tc_1", "args": {}}]),
+        ]
+        assert _todos_in_messages(msgs) is False
+
+    def test_false_for_empty_list(self):
+        assert _todos_in_messages([]) is False
+
+    def test_false_for_ai_without_tool_calls(self):
+        msgs = [AIMessage(content="hello")]
+        assert _todos_in_messages(msgs) is False
+
+
+class TestReminderInMessages:
+    def test_true_when_reminder_present(self):
+        msgs = [HumanMessage(content="hi"), _reminder_msg()]
+        assert _reminder_in_messages(msgs) is True
+
+    def test_false_when_no_reminder(self):
+        msgs = [HumanMessage(content="hi"), AIMessage(content="hello")]
+        assert _reminder_in_messages(msgs) is False
+
+    def test_false_for_empty_list(self):
+        assert _reminder_in_messages([]) is False
+
+    def test_false_for_human_without_name(self):
+        msgs = [HumanMessage(content="todo_reminder")]
+        assert _reminder_in_messages(msgs) is False
+
+
+class TestFormatTodos:
+    def test_formats_multiple_items(self):
+        todos = _sample_todos()
+        result = _format_todos(todos)
+        assert "- [completed] Set up project" in result
+        assert "- [in_progress] Write tests" in result
+        assert "- [pending] Deploy" in result
+
+    def test_empty_list(self):
+        assert _format_todos([]) == ""
+
+    def test_missing_fields_use_defaults(self):
+        todos = [{"content": "No status"}, {"status": "done"}]
+        result = _format_todos(todos)
+        assert "- [pending] No status" in result
+        assert "- [done] " in result
+
+
+class TestBeforeModel:
+    def test_returns_none_when_no_todos(self):
+        mw = TodoMiddleware()
+        state = {"messages": [HumanMessage(content="hi")], "todos": []}
+        assert mw.before_model(state, _make_runtime()) is None
+
+    def test_returns_none_when_todos_is_none(self):
+        mw = TodoMiddleware()
+        state = {"messages": [HumanMessage(content="hi")], "todos": None}
+        assert mw.before_model(state, _make_runtime()) is None
+
+    def test_returns_none_when_write_todos_still_visible(self):
+        mw = TodoMiddleware()
+        state = {
+            "messages": [_ai_with_write_todos()],
+            "todos": _sample_todos(),
+        }
+        assert mw.before_model(state, _make_runtime()) is None
+
+    def test_returns_none_when_reminder_already_present(self):
+        mw = TodoMiddleware()
+        state = {
+            "messages": [HumanMessage(content="hi"), _reminder_msg()],
+            "todos": _sample_todos(),
+        }
+        assert mw.before_model(state, _make_runtime()) is None
+
+    def test_injects_reminder_when_todos_exist_but_truncated(self):
+        mw = TodoMiddleware()
+        state = {
+            "messages": [HumanMessage(content="hi"), AIMessage(content="sure")],
+            "todos": _sample_todos(),
+        }
+        result = mw.before_model(state, _make_runtime())
+        assert result is not None
+        msgs = result["messages"]
+        assert len(msgs) == 1
+        assert isinstance(msgs[0], HumanMessage)
+        assert msgs[0].name == "todo_reminder"
+
+    def test_reminder_contains_formatted_todos(self):
+        mw = TodoMiddleware()
+        state = {
+            "messages": [HumanMessage(content="hi")],
+            "todos": _sample_todos(),
+        }
+        result = mw.before_model(state, _make_runtime())
+        content = result["messages"][0].content
+        assert "Set up project" in content
+        assert "Write tests" in content
+        assert "Deploy" in content
+        assert "system_reminder" in content
+
+
+class TestAbeforeModel:
+    def test_delegates_to_sync(self):
+        mw = TodoMiddleware()
+        state = {
+            "messages": [HumanMessage(content="hi")],
+            "todos": _sample_todos(),
+        }
+        result = asyncio.run(mw.abefore_model(state, _make_runtime()))
+        assert result is not None
+        assert result["messages"][0].name == "todo_reminder"