fix: Correct message part ordering in A2A history

Merges test case from https://github.com/google/adk-python/pull/3262

Fixes: #3260

Co-authored-by: Ayush Agrawal <ayushagra@google.com>
PiperOrigin-RevId: 826119626

src/google/adk/agents/remote_a2a_agent.py

@@ -356,29 +356,33 @@ class RemoteA2aAgent(BaseAgent):
     """
     message_parts: list[A2APart] = []
     context_id = None
+
+    events_to_process = []
     for event in reversed(ctx.session.events):
-      if _is_other_agent_reply(self.name, event):
-        event = _present_other_agent_message(event)
-      elif event.author == self.name:
+      if event.author == self.name:
         # stop on content generated by current a2a agent given it should already
         # be in remote session
         if event.custom_metadata:
           metadata = event.custom_metadata
           context_id = metadata.get(A2A_METADATA_PREFIX + "context_id")
         break
+      events_to_process.append(event)
+
+    for event in reversed(events_to_process):
+      if _is_other_agent_reply(self.name, event):
+        event = _present_other_agent_message(event)
 
       if not event.content or not event.content.parts:
         continue
 
       for part in event.content.parts:
-
         converted_part = self._genai_part_converter(part)
         if converted_part:
           message_parts.append(converted_part)
         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

@@ -735,6 +735,71 @@ class TestRemoteA2aAgentMessageHandling:
       assert A2A_METADATA_PREFIX + "task_id" in result.custom_metadata
       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_submitted_and_no_update(self):
     """Test successful A2A response handling with streaming task and no update."""