The OpenAPI Specification, previously known as the Swagger Specification, is a specification for machine-readable interface files for describing, producing, consuming, and visualizing RESTful web services. Previously part of the Swagger framework, it became a separate project in 2016, overseen by the OpenAPI Initiative, an open-source collaboration project of the Linux Foundation. Swagger and some other tools can generate code, documentation, and test cases given an interface file.
Swagger development began in early 2010 by Tony Tam, who was working at online dictionary company Wordnik. After a meetup talk with Kin Lane (OpenAPI Steering committee) and Tony Tam on June 12, 2014, Kin Lane approached Owen Rubel who was working on a similar approach and borrowed his ideas for the new Swagger 2.0 spec (which would eventually become OpenAPI).
In November 2015, SmartBear announced that it was creating a new organization called the OpenAPI Initiative under the sponsorship of the Linux Foundation. Other founding member companies included 3scale, Apigee, Capital One, Google, IBM, Intuit, Microsoft, PayPal, and Restlet. SmartBear donated the Swagger specification to the new group. RAML and API Blueprint were also under consideration by the group.
In September 2016, the API World conference presented an API Infrastructure award to SmartBear for its ongoing work on Swagger.
In July 2017, the OpenAPI Initiative released version 3.0.0 of its specification. MuleSoft, the main contributor to the alternative RESTful API Modeling Language (RAML), joined the OAS and open-sourced their API Modeling Framework tool, which can generate OAS documents from RAML input.
In February 2021, the OpenAPI Initiative released version 3.1.0. Major Changes In OpenAPI Specification 3.1.0 include JSON Schema vocabularies alignment, New top-level element for describing Webhooks that are registered and managed out of band, Support for identifying API licenses using the standard SPDX identifier, and PathItems object is now optional to make it simpler to create reusable libraries of components.
|3.1.0||2021-02-15||Release of the OpenAPI Specification 3.1.0|
|3.0.3||2020-02-20||Patch release of the OpenAPI Specification 3.0.3|
|3.0.2||2018-10-08||Patch release of the OpenAPI Specification 3.0.2|
|3.0.1||2017-12-06||Patch release of the OpenAPI Specification 3.0.1|
|3.0.0||2017-07-26||Release of the OpenAPI Specification 3.0.0|
|2.0||2014-09-08||Release of Swagger 2.0|
|1.2||2014-03-14||Initial release of the formal document|
|1.1||2012-08-22||Release of Swagger 1.1|
|1.0||2011-08-10||First release of the Swagger Specification|
Applications implemented based on OpenAPI interface files can automatically generate documentation of methods, parameters and models. This helps keep the documentation, client libraries, and source code in sync.
The OpenAPI Specification is language-agnostic. With OpenAPI's declarative resource specification, clients can understand and consume services without knowledge of server implementation or access to the server code.
Tools that work with OpenAPI
The OpenAPI Initiative maintains a list of implementations for version 3.0 of the specification. SmartBear still brands its OpenAPI tools with the Swagger moniker. The Swagger UI framework allows both developers and non-developers to interact with the API in a sandbox UI that gives insight into how the API responds to parameters and options. Swagger can handle both JSON and XML.
Swagger Codegen contains a template-driven engine to generate documentation, API clients and server stubs in different languages by parsing the OpenAPI definition. In July, 2018, William Cheng, the top contributor to Swagger Codegen, and over 40 other contributors to Swagger Codegen forked the code into a project named OpenAPI Generator under the OpenAPI Tools organization. 
The OpenAPI Initiative sponsors an API Specifications Conference (ASC) annually. The event has its origins in the API Strategy and Practice Conference (APIStrat) which ran for many years and became part of the OpenAPI Initiative in 2016.
- Representational State Transfer
- Overview of RESTful API Description Languages including RAML, WADL, WSDL, and others.
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|url=value (help). Retrieved 23 April 2020.
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- OpenAPI Initiative (OAI) website
- API Specifications Conference (ASC) website
- Swagger website
- OpenAPI Specification on GitHub
- Directory of OpenAPI definitions
- OpenAPI Editor: A rich UI Eclipse OpenAPI (OAS) editor and studio to design, develop and test OAS3/OpenAPI
- OpenAPI for Electronic Data Interchange (EDI)
- OpenAPI editor by Remain API Studio. A rich OpenAPI editor and a full OAS3 specs-compliant development environment
- OpenAPI Editor and Test Studio Usage Wiki