Fix: Correct message part ordering in A2A history

The previous logic iterated through session events in reverse and then
reversed the resulting list of parts. This caused incorrect ordering for
multi-part messages generated by `_present_other_agent_message`.

Specifically, introductory text like "For context:" was appearing after
the content it was meant to introduce.

This change simplifies the implementation by removing both reversals. It now
processes events in chronological order, ensuring all message parts are
appended in the correct sequence.

Signed-off-by: Eran Cohen <[email protected]>

Author: eranco74

src/google/adk/agents/remote_a2a_agent.py

@@ -356,7 +356,7 @@ def _construct_message_parts_from_session(
     """
     message_parts: list[A2APart] = []
     context_id = None
-    for event in reversed(ctx.session.events):
+    for event in ctx.session.events:
       if _is_other_agent_reply(self.name, event):
         event = _present_other_agent_message(event)
       elif event.author == self.name:
@@ -378,7 +378,7 @@ def _construct_message_parts_from_session(
         else:
           logger.warning("Failed to convert part to A2A format: %s", part)
 
-    return message_parts[::-1], context_id
+    return message_parts, context_id
 
   async def _handle_a2a_response(
       self, a2a_response: A2AClientEvent | A2AMessage, ctx: InvocationContext

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."""