A good SDK builds on the fundamentals of good software engineering but SDKs have additional requirements to consider.

When developing software as a team a level of familiarity is reached between the team members. Concepts, approaches, technologies, and terminology are shaped by the company and the goals are typically aligned.

Several years ago I worked on a payroll package developing a core engine that required an API to let third parties write calculations, validations and security gates that would execute as part of its regular operation.

We were a small team and I had many conversations with another developer tasked with building a payroll using the API I would provide. Some methods here, classes there, the odd helper function and I had an API and then we had a mini payroll running.