Grabbing the OpenAPI description of Sage Pay Pi from here:
https://developer.sage.com/api/payments/api/
until Sage Pay get those files onto a public repository.
This will likely be out-of-date, but they are needed for code generation, and it's thge best we can manage for now.
Code samples are released by Sage Pay under Apache v2.0
OpenAPI Description released under Creative Commons Attribution 4.0
The use of Sage’s API is covered under Sage Developer Terms and Conditions