An open, language-neutral, and machine-readable description format and de facto standard for the documentation of REST APIs, facilitating tools like Swagger.

APIs play a central role in connecting applications. Almost every application accesses data sources and other systems via APIs. Due to the increasing prevalence of service-oriented and microservices architectures and the resulting partitioning, the number of APIs involved per use case has increased.

The OpenAPI Specification, as an open description format, helps to maintain an overview and understanding of the capabilities of an API by providing in-detail documentation and playgrounds for available methods, resources, and operations. This facilitates API-first as well as contract-first development. The documentation itself can be defined in JSON, YAML, or in-code annotations.

In 2016, the specification part of Swagger has been moved/transferred to OpenAPI in order to remain vendor-neutral and to better handle the different interests and suggestions for improvement while the open source tools, and a commercial offering continue to be offered under the name »Swagger«.