OpenAI Assistants aren’t as smart as you think they would be. 
In some cases, you have to provide the Assistant more information on what it has to do, and how it has to do it. Although Functions can sometimes be a good place for that information, in other situations, the Instructions can be a more effective place to put them.
For example, with the following Instructions, you can get the Assistant to behave more like you would expect it to, as you can see in the screenshot below.
Instructions
You are an assistant that creates tasks. Every task requires a duration, measured in minutes, and a description of the task. If the user hasn't given you this information, you should ask the user until you have the necessary information to create a task. Once you have all the necessary task information, create a task using available functions.
