Respect the conversational UI. The full interaction should take place natively within the app. The goal is to recognize the user's intent and provide the right content with minimum user input. Every question asked should bring the user closer to the answer they want. If you need so much information that you're playing a game of 20 Questions, then switch to a form and deliver the content another way.
Beyond users, bots must also please the messaging apps themselves. Take Facebook Messenger. Executives have confirmed that advertisements within Discover — their hub for finding new bots to engage with — will be the main way Messenger monetizes its 1.3 billion monthly active users. If standing out among the 100,000 other bots on the platform wasn't difficult enough, we can assume Messenger will only feature bots that don't detract people from the platform.
In one particularly striking example of how this rather limited bot has made a major impact, U-Report sent a poll to users in Liberia about whether teachers were coercing students into sex in exchange for better grades. Approximately 86% of the 13,000 Liberian children U-Report polled responded that their teachers were engaged in this despicable practice, which resulted in a collaborative project between UNICEF and Liberia’s Minister of Education to put an end to it.
The classic historic early chatbots are ELIZA (1966) and PARRY (1972). More recent notable programs include A.L.I.C.E., Jabberwacky and D.U.D.E (Agence Nationale de la Recherche and CNRS 2006). While ELIZA and PARRY were used exclusively to simulate typed conversation, many chatbots now include functional features such as games and web searching abilities. In 1984, a book called The Policeman's Beard is Half Constructed was published, allegedly written by the chatbot Racter (though the program as released would not have been capable of doing so).
Chatting with a bot should be like talking to a human that knows everything. If you're using a bot to change an airline reservation, the bot should know if you have an unused credit on your account and whether you typically pick the aisle or window seat. Artificial intelligence will continue to radically shape this front, but a bot should connect with your current systems so a shared contact record can drive personalization.