fix(checkpointer): return InMemorySaver instead of None when not configured (#1016) (#1019)

* fix(checkpointer): return InMemorySaver instead of None when not configured (#1016)

* fix(checkpointer): also fix get_checkpointer() to return InMemorySaver

Make all three checkpointer functions consistent:
- make_checkpointer() (async) → InMemorySaver
- checkpointer_context() (sync) → InMemorySaver
- get_checkpointer() (sync singleton) → InMemorySaver

This ensures DeerFlowClient always has a valid checkpointer.

* fix: address CI failure and Copilot review feedback

- Fix import order in test_checkpointer_none_fix.py (I001 ruff error)
- Fix type annotation: _checkpointer should be Checkpointer | None
- Update docstring: change "None if not configured" to "InMemorySaver if not configured"
- Ensure app config is loaded before checking checkpointer config to prevent incorrect InMemorySaver fallback

---------

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

backend/tests/test_checkpointer.py

@@ -71,8 +71,13 @@ class TestCheckpointerConfig:
 
 
 class TestGetCheckpointer:
-    def test_returns_none_when_not_configured(self):
-        assert get_checkpointer() is None
+    def test_returns_in_memory_saver_when_not_configured(self):
+        """get_checkpointer should return InMemorySaver when not configured."""
+        from langgraph.checkpoint.memory import InMemorySaver
+
+        cp = get_checkpointer()
+        assert cp is not None
+        assert isinstance(cp, InMemorySaver)
 
     def test_memory_returns_in_memory_saver(self):
         load_checkpointer_config_from_dict({"type": "memory"})

backend/tests/test_checkpointer_none_fix.py

@@ -0,0 +1,54 @@
+"""Test for issue #1016: checkpointer should not return None."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+from langgraph.checkpoint.memory import InMemorySaver
+
+
+class TestCheckpointerNoneFix:
+    """Tests that checkpointer context managers return InMemorySaver instead of None."""
+
+    @pytest.mark.anyio
+    async def test_async_make_checkpointer_returns_in_memory_saver_when_not_configured(self):
+        """make_checkpointer should return InMemorySaver when config.checkpointer is None."""
+        from src.agents.checkpointer.async_provider import make_checkpointer
+
+        # Mock get_app_config to return a config with checkpointer=None
+        mock_config = MagicMock()
+        mock_config.checkpointer = None
+
+        with patch("src.agents.checkpointer.async_provider.get_app_config", return_value=mock_config):
+            async with make_checkpointer() as checkpointer:
+                # Should return InMemorySaver, not None
+                assert checkpointer is not None
+                assert isinstance(checkpointer, InMemorySaver)
+
+                # Should be able to call alist() without AttributeError
+                # This is what LangGraph does and what was failing in issue #1016
+                result = []
+                async for item in checkpointer.alist(config={"configurable": {"thread_id": "test"}}):
+                    result.append(item)
+
+                # Empty list is expected for a fresh checkpointer
+                assert result == []
+
+    def test_sync_checkpointer_context_returns_in_memory_saver_when_not_configured(self):
+        """checkpointer_context should return InMemorySaver when config.checkpointer is None."""
+        from src.agents.checkpointer.provider import checkpointer_context
+
+        # Mock get_app_config to return a config with checkpointer=None
+        mock_config = MagicMock()
+        mock_config.checkpointer = None
+
+        with patch("src.agents.checkpointer.provider.get_app_config", return_value=mock_config):
+            with checkpointer_context() as checkpointer:
+                # Should return InMemorySaver, not None
+                assert checkpointer is not None
+                assert isinstance(checkpointer, InMemorySaver)
+
+                # Should be able to call list() without AttributeError
+                result = list(checkpointer.list(config={"configurable": {"thread_id": "test"}}))
+
+                # Empty list is expected for a fresh checkpointer
+                assert result == []