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The OpenAPI Specification (OAS), previously known as the Swagger Specification, is a specification for a machine-readable interface definition language for describing, producing, consuming and visualizing web services.[1] Originally developed to support the Swagger framework, it became a separate project in 2015, overseen by the OpenAPI Initiative, an open-source collaboration project of the Linux Foundation.[2][3]

An OpenAPI Description (OAD)[4] represents a formal description of an API that tools can use to generate code, documentation, test cases, and more.

History

Logo of the OpenAPI Initiative, the organization that develops the OpenAPI Specification under the Linux Foundation

Swagger development began in early 2010 by Tony Tam, who was working at online dictionary company Wordnik.[5]

In March 2015, SmartBear Software acquired the open-source Swagger API specification from Reverb Technologies, Wordnik’s parent company.[6]

In November 2015, SmartBear announced that it was donating the Swagger specification to 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.[7][8]

On 1 January 2016, the Swagger specification was renamed the OpenAPI Specification (OAS) and was moved to a new GitHub repository.[9]

Consolidation of Formats

Two somewhat similar technologies, MuleSoft‘s RESTful API Modeling Language (RAML) and Apiary’s API Blueprint, had been developed around the same time as what was then still called the Swagger Specification.

The producers of both formats later joined the OpenAPI Initiative: Apiary in 2016[10] and MuleSoft in 2017.[11] Both have added support for the OAS.[12][11]

Version History

In July 2017, the OpenAPI Initiative released version 3.0.0 of its specification.[13]

In February 2021, the OpenAPI Initiative released version 3.1.0.[14] Major changes in OpenAPI Specification 3.1.0 include JSON schema vocabularies alignment, new top-level elements for describing webhooks that are registered and managed out of band, support for identifying API licenses using the standard SPDX identifier, allowance of descriptions alongside the use of schema references and a change to make the PathItems object optional to simplify creation of reusable libraries of components.[15][16][17]

In September 2025, the OpenAPI Initiative released version 3.2.0 of the OAS.[18] Notable features include structured tags, first-class streaming media type support, support for arbitrary HTTP methods, clearer example semantics for serializations, OAuth2 device flow and metadata enhancements, and clarified path templating and routing semantics.[19]

Release dates

Version Date Notes[20]
3.2.0 2025-09-19 Release of the OpenAPI Specification 3.2.0
3.1.2 2025-09-19 Patch release of the OpenAPI Specification 3.1.2
3.1.1 2024-10-24 Patch release of the OpenAPI Specification 3.1.1
3.1.0 2021-02-15 Release of the OpenAPI Specification 3.1.0
3.0.4 2024-10-24 Patch release of the OpenAPI Specification 3.0.4
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

Usage

The OAS describes the format for OpenAPI Descriptions (OADs),[4] which can be used by a variety of applications, libraries, and tools.

Applications can use OADs to automatically generate documentation of methods, parameters and data models. This helps keep the documentation, client libraries and source code in sync.[21]

When an OAD is used to generate source code stubs for servers, the process is called scaffolding.

Relationships to software engineering practices

The paradigm of agreeing on an API contract first and then programming business logic afterwards, in contrast to coding the program first and then writing a retrospective description of its behavior as the contract, is called contract-first development. Since the interface is determined before any code is written, downstream developers can mock the server behavior and start testing right away.[22] In this sense, contract-first development is also a practice of shift-left testing.

Features

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.[1]

Conferences and conference tracks

The OpenAPI Initiative sponsored APIStrat from 2017 to 2019, converting it into the API Specifications Confererence (ASC) from 2020 to 2022.[23] Starting in 2023, the initiative has instead sponsored OpenAPI tracks at multiple conferences throughout the year.[24]

See also

References

  1. ^ a b “OpenAPI Documentation: Getting Started”. Learn OpenAPI. The OpenAPI Initiative. Retrieved 17 September 2024.
  2. ^ “New Collaborative Project to Extend Swagger Specification for Building Connected Applications and Services”. Archived from the original on 31 October 2023.
  3. ^ “OpenAPI Initiative Charter”. OpenAPI Initiative. Archived from the original on 26 January 2025. Retrieved 12 November 2019.
  4. ^ a b “OpenAPI Documentation: Glossary”. Learn OpenAPI. The OpenAPI Initiative. 2023. Retrieved 17 September 2024.
  5. ^ “Swagger creator joins SmartBear”. 28 September 2015. Retrieved 6 August 2019.
  6. ^ “SmartBear Assumes Sponsorship of Swagger API Open Source Project”. SmartBear. Retrieved 25 March 2015.
  7. ^ “FAQ”. OpenAPI Initiative. Retrieved 12 November 2019.
  8. ^ “New Collaborative Project to Extend Swagger Specification for Building Connected Applications and Services”. linuxfoundation.org. Archived from the original on 27 April 2016. Retrieved 22 April 2016.
  9. ^ OpenAPI Initiative. “OpenAPI Specification”. GitHub. Retrieved 12 November 2019.
  10. ^ Lensmar, Ole (23 February 2016). “OAI Update – new members, OpenAPI Spec 3.0 progress, and more!”. The OpenAPI Initiative. Retrieved 13 October 2024.
  11. ^ a b Avram, Abel (6 May 2017). “The HTTP API space is Consolidating around OAS”. InfoQ. Retrieved 14 May 2017.
  12. ^ Nesetril, Jakub (18 January 2016). “We’ve got Swagger”. Oracle Apiary. Retrieved 13 October 2024.
  13. ^ “The OAI Announces the OpenAPI Specification 3.0.0”. OpenAPIs. 26 July 2017. Retrieved 19 April 2018.
  14. ^ “OpenAPI Specification 3.1.0 Available Now”. Linux.com. 26 April 2021. Retrieved 26 April 2021.
  15. ^ Charboneau, Tyler (7 April 2021). “What’s New in OpenAPI 3.1.0?”. Nordic APIs. Retrieved 7 April 2021.
  16. ^ “OpenAPI Specification 3.1.0 Released”. OpenAPI Initiative. 18 February 2021. Retrieved 18 February 2021.
  17. ^ Sturgeon, Phil (16 February 2021). “Migrating from OpenAPI 3.0 to 3.1.0”. OpenAPI Initiative. Retrieved 16 February 2021.
  18. ^ “Announcing OpenAPI v3.2”. The OpenAPI Initiative Blog. 23 September 2025. Retrieved 15 June 2026.
  19. ^ “What’s New in OpenAPI Specification v3.2.0?”. Nordic APIs. 27 January 2026. Retrieved 15 June 2026.
  20. ^ “OpenAPI Specification Version 3.2.0”. OpenAPI Initiative Publications. Retrieved 15 June 2026.
  21. ^ “OpenAPI Documentation: Introduction”. Learn OpenAPI. The OpenAPI Initiative. 2023. Retrieved 17 September 2024.
  22. ^ Preibisch, Sascha (2018). API Development: A Practical Guide for Business Implementation Success. [Berkeley, CA]: Apress. ISBN 978-1-4842-4140-0. OCLC 1076234393. Having the Swagger (or for that matter, any other machine-readable) document available, team members can start working on their part of the project at the same time.
  23. ^ “Introducing ASC, the API Specifications Conference”. OpenAPI Initiative Blog. 8 May 2019. Retrieved 15 June 2026.
  24. ^ “Call for Proposals: OAI Track at API Days LondonvDate: September 13-14, 2023 Location: 155 Bishopsgate, London, UK”. OpenAPI Initiative Blog. Retrieved 15 June 2026.

Bibliography

  • Haupt, F.; Karastoyanova, D.; Leymann, F.; Schroth, B. (2014). A Model-Driven Approach for REST Compliant Services. ICWS 2014. 2014 IEEE International Conference on Web Services. pp. 129–136. doi:10.1109/ICWS.2014.30. ISBN 978-1-4799-5054-6.
  • Pautasso, Cesare (2021). Beautiful APIs. LeanPub. p. 100.