As slim as you want, IOPA is a reference pattern that can run with no IOPA code at all
Always free and open-source, include IOPA in commercial and non-commercial projects alike
The IOPA specs allow applications to be developed independently of the actual platform on which it runs and is particularly well-suited for serverless environments
The pattern and reference implementation is particularly well-suited for Bot APIs such as Facebook Messenger, Amazon Alexa Skills, Microsoft Bot Framework, Google Now, and Whatsapp.