Sezzle's current v2 API documentation is based on the OpenAPI Specification (OAS), a standard, programming language-agnostic interface for RESTful APIs. You can read more about OpenAPI here.
The Sezzle v1 API is based on the immediate predecessor to OpenAPI, called Swagger 2.0
You can find a useful interactive visual guide to the differences between Swagger 2.0 and OpenAPI 3.x here.
Access the raw OpenAPI specifications
View the Sezzle v2 OpenAPI
View the Sezzle v1 OpenAPI