fix: Avoid infinite loop with rolling condensers and history truncati…

…on (#6795)

Co-authored-by: openhands <[email protected]>
Co-authored-by: Calvin Smith <[email protected]>

openhands/memory/condenser/condenser.py

@@ -161,6 +161,12 @@ def __init__(self) -> None:
 
     @override
     def condensed_history(self, state: State) -> list[Event]:
+        # The history should grow monotonically -- if it doesn't, something has
+        # truncated the history and we need to reset our tracking.
+        if len(state.history) < self._last_history_length:
+            self._condensation = []
+            self._last_history_length = 0
+
         new_events = state.history[self._last_history_length :]
 
         with self.metadata_batch(state):

tests/unit/test_condenser.py

@@ -19,6 +19,7 @@
 from openhands.events.observation.observation import Observation
 from openhands.llm import LLM
 from openhands.memory.condenser import Condenser
+from openhands.memory.condenser.condenser import RollingCondenser
 from openhands.memory.condenser.impl import (
     AmortizedForgettingCondenser,
     ImportantEventSelection,
@@ -452,6 +453,49 @@ def test_llm_attention_condenser_invalid_config():
     pytest.raises(ValueError, LLMAttentionCondenser.from_config, config)
 
 
+def test_rolling_condenser_handles_truncation(mock_state: State):
+    """Test that RollingCondenser correctly handles history truncation."""
+
+    class TestRollingCondenser(RollingCondenser):
+        """Test implementation of RollingCondenser that just returns all events."""
+
+        def condense(self, events: list[Event]) -> list[Event]:
+            return events
+
+    condenser = TestRollingCondenser()
+
+    # Initial history with 3 events
+    events = [
+        create_test_event('Event 1', id=1),
+        create_test_event('Event 2', id=2),
+        create_test_event('Event 3', id=3),
+    ]
+    mock_state.history = events
+
+    # First condensation - should return all events
+    results = condenser.condensed_history(mock_state)
+    assert len(results) == 3
+    assert [e._id for e in results] == [1, 2, 3]
+
+    # Simulate truncation - history is now shorter, and the condensation should
+    # just include the truncated history
+    mock_state.history = mock_state.history[-1:]
+
+    results = condenser.condensed_history(mock_state)
+    assert len(results) == 1
+    assert results[0]._id == 3
+
+    # Adding more events and condensing should "rebase" us from the truncated history
+    mock_state.history += [
+        create_test_event('Event 4', id=4),
+        create_test_event('Event 5', id=5),
+    ]
+
+    results = condenser.condensed_history(mock_state)
+    assert len(results) == 3
+    assert [e._id for e in results] == [3, 4, 5]
+
+
 def test_llm_attention_condenser_keeps_first_events(mock_llm, mock_state):
     """Test that the LLMAttentionCondenser keeps the right number of initial events when forgetting."""
     max_size = 4