Fix: Correct message part ordering in A2A history

src/google/adk/agents/remote_a2a_agent.py

@@ -370,7 +370,7 @@ def _construct_message_parts_from_session(
       if not event.content or not event.content.parts:
         continue
 
-      for part in event.content.parts:
+      for part in reversed(event.content.parts):
 
         converted_part = self._genai_part_converter(part)
         if converted_part:

tests/unittests/agents/test_remote_a2a_agent.py

@@ -695,6 +695,69 @@ async def test_handle_a2a_response_success_with_message(self):
       assert result.custom_metadata is not None
       assert A2A_METADATA_PREFIX + "context_id" in result.custom_metadata
 
+
+    def test_construct_message_parts_from_session_preserves_order(self):
+        """Test that message parts are in correct order with multi-part messages.
+
+        This test verifies the fix for the bug where _present_other_agent_message
+        creates multi-part messages with "For context:" prefix, and ensures the
+        parts are in the correct chronological order (not reversed).
+        """
+        # Create mock events with multiple parts
+        # Event 1: User message
+        user_part = Mock()
+        user_part.text = "User question"
+        user_content = Mock()
+        user_content.parts = [user_part]
+        user_event = Mock()
+        user_event.content = user_content
+        user_event.author = "user"
+
+        # Event 2: Other agent message (will be transformed by
+        # _present_other_agent_message)
+        other_agent_part1 = Mock()
+        other_agent_part1.text = "For context:"
+        other_agent_part2 = Mock()
+        other_agent_part2.text = "[other_agent] said: Response text"
+        other_agent_content = Mock()
+        other_agent_content.parts = [other_agent_part1, other_agent_part2]
+        other_agent_event = Mock()
+        other_agent_event.content = other_agent_content
+        other_agent_event.author = "other_agent"
+
+        self.mock_session.events = [user_event, other_agent_event]
+
+        with patch(
+            "google.adk.agents.remote_a2a_agent._present_other_agent_message"
+        ) as mock_present:
+            # Mock _present_other_agent_message to return the transformed event
+            mock_present.return_value = other_agent_event
+
+            # Mock the converter to track the order of parts
+            converted_parts = []
+
+            def mock_converter(part):
+                mock_a2a_part = Mock()
+                mock_a2a_part.original_text = part.text
+                converted_parts.append(mock_a2a_part)
+                return mock_a2a_part
+
+            self.mock_genai_part_converter.side_effect = mock_converter
+
+            result = self.agent._construct_message_parts_from_session(self.mock_context)
+
+            # Verify the parts are in correct order
+            assert len(result) == 2  # Returns tuple of (parts, context_id)
+            assert len(result[0]) == 3  # 1 user part + 2 other agent parts
+            assert result[1] is None  # context_id
+
+            # Verify order: user part, then "For context:", then agent message
+            assert converted_parts[0].original_text == "User question"
+            assert converted_parts[1].original_text == "For context:"
+            assert (
+                converted_parts[2].original_text == "[other_agent] said: Response text"
+            )
+
   @pytest.mark.asyncio
   async def test_handle_a2a_response_with_task_completed_and_no_update(self):
     """Test successful A2A response handling with non-streeaming task and no update."""