Update openhands/memory/conversation_memory.py

- Removed test_process_events_with_user_agent_finish_action as it was not part of the original PR
- Keep only the two original tests: empty and non-empty assistant finish actions
- Both original tests still pass and cover the intended fix

openhands/memory/conversation_memory.py

@@ -293,10 +293,16 @@ class ConversationMemory:
                 action.tool_call_metadata = None
             if role not in ('user', 'system', 'assistant', 'tool'):
                 raise ValueError(f'Invalid role: {role}')
+
+            # Ensure assistant messages have non-empty content (required by some LLM providers like Mistral)
+            if role == 'assistant' and not (action.thought and action.thought.strip()):
+                thought_text = 'Task completed.'
+            else:
+                thought_text = action.thought or ''
             return [
                 Message(
                     role=role,  # type: ignore[arg-type]
-                    content=[TextContent(text=action.thought)],
+                    content=[TextContent(text=thought_text)],
                 )
             ]
         elif isinstance(action, MessageAction):

tests/unit/memory/test_conversation_memory.py

@@ -1583,3 +1583,61 @@ def test_process_ipython_observation_with_vision_disabled(
     assert isinstance(message.content[1], ImageContent)
     # Check that NO explanatory text about filtered images was added when vision is disabled
     assert 'invalid or empty image(s) were filtered' not in message.content[0].text
+
+
+def test_process_events_with_empty_agent_finish_action(conversation_memory):
+    """Test that AgentFinishAction with empty thought gets default content for assistant messages."""
+    # Create an AgentFinishAction with empty thought from assistant
+    empty_finish_action = AgentFinishAction(thought='')
+    empty_finish_action._source = EventSource.AGENT
+
+    initial_user_message = MessageAction(content='Initial user message')
+    initial_user_message._source = EventSource.USER
+
+    messages = conversation_memory.process_events(
+        condensed_history=[empty_finish_action],
+        initial_user_action=initial_user_message,
+        max_message_chars=None,
+        vision_is_active=False,
+    )
+
+    # Should have system message, initial user message, and assistant finish message
+    assert len(messages) == 3
+
+    # Check the assistant finish message
+    assistant_message = messages[2]
+    assert assistant_message.role == 'assistant'
+    assert len(assistant_message.content) == 1
+    assert isinstance(assistant_message.content[0], TextContent)
+    # Should have default content instead of empty string
+    assert assistant_message.content[0].text == 'Task completed.'
+
+
+def test_process_events_with_non_empty_agent_finish_action(conversation_memory):
+    """Test that AgentFinishAction with non-empty thought preserves original content."""
+    # Create an AgentFinishAction with non-empty thought from assistant
+    finish_action = AgentFinishAction(thought='I have completed the task successfully.')
+    finish_action._source = EventSource.AGENT
+
+    initial_user_message = MessageAction(content='Initial user message')
+    initial_user_message._source = EventSource.USER
+
+    messages = conversation_memory.process_events(
+        condensed_history=[finish_action],
+        initial_user_action=initial_user_message,
+        max_message_chars=None,
+        vision_is_active=False,
+    )
+
+    # Should have system message, initial user message, and assistant finish message
+    assert len(messages) == 3
+
+    # Check the assistant finish message
+    assistant_message = messages[2]
+    assert assistant_message.role == 'assistant'
+    assert len(assistant_message.content) == 1
+    assert isinstance(assistant_message.content[0], TextContent)
+    # Should preserve original content
+    assert (
+        assistant_message.content[0].text == 'I have completed the task successfully.'
+    )