Add unit tests for chat history cleanup logic

backend/tests/test_agent_chat_tool.py

@@ -0,0 +1,28 @@
+import unittest
+from types import SimpleNamespace
+
+from backend.agent.tools.chat import _drop_superseded_trailing_user_rows
+
+
+class ChatToolTests(unittest.TestCase):
+    def test_drops_unanswered_trailing_user_messages_except_latest(self):
+        rows = [
+            SimpleNamespace(role="user", content="first question"),
+            SimpleNamespace(role="assistant", content="first answer"),
+            SimpleNamespace(role="user", content="interrupted follow-up"),
+            SimpleNamespace(role="user", content="latest prompt"),
+        ]
+
+        result = _drop_superseded_trailing_user_rows(rows)
+
+        self.assertEqual([row.content for row in result], ["first question", "first answer", "latest prompt"])
+
+    def test_keeps_normal_alternating_history(self):
+        rows = [
+            SimpleNamespace(role="user", content="first question"),
+            SimpleNamespace(role="assistant", content="first answer"),
+            SimpleNamespace(role="user", content="latest prompt"),
+        ]
+
+        self.assertIs(_drop_superseded_trailing_user_rows(rows), rows)
+