As per a report from XDA-Developers, Android Messages is testing Google Assistant integration so more people will use it. With the general public's dwindling interest in using Allo, Google promised to turn its focus away from Allo and put the energy towards improving the experience on Android Messages, Android's default SMS app. In its current testing state, you can only summon Google within conversations and there is no way to Chat directly with Google Assistant like you can on Allo. XDA was able to get Google Assistant to only respond with recommended restaurants and the...
Source: https://www.gsmarena.com/google_assistant_integration_may_be_coming_to_android_messages-news-32902.php