Hi, the custom action shouldn’t handle all the messages in talking with agent mode. Rather, you should have a client UI which can switch to a different backend, and the job of the custom action would be to prompt this switch and pause the conversation in Rasa. See this reply for further explanation.
To see an example of how such a handoff can be done, check out our recent addition to the chatroom Rasa widget. There is also an example bot that uses this feature.
edit: Also, the SocketIO addition is included in Rasa Open Source since version 1.10.0, which was released on 28th April.