How to handle null user inputs within adaptive cards only

Question

How can I show a user a red color message inside the adaptive card itself, when the user tries to hit the ok button without providing any input in the drop-down present in my adaptive cards. I don't want my Luis agent to handle it I want my UI to handle the above scenario

Answer 1

Adaptive Cards don't currently support client-side validation as per this answer from a MSFT employee. You will have to handle this scenario in your code that processes the user input. If the input is not valid, then you could send a message to the user saying please fill in the required fields X, Y, and Z, followed by the original Adaptive Card.

Edit - Additional sources

See this comment from a month ago about validation coming in Version 1.3, along with this issue which is tracking the current status of the input validation functionality.