What is Swagger?
A toolset (and historical predecessor of OpenAPI) for designing, documenting, and consuming REST APIs through a machine-readable schema.
Swagger started in 2010 as an internal API description format at Wordnik, became the de facto standard for documenting REST APIs, and was donated to what is now the OpenAPI Initiative in 2016. The "Swagger Specification" was renamed "OpenAPI Specification"; the brand "Swagger" lives on as a family of tools (Swagger UI, Swagger Editor, SwaggerHub) maintained by SmartBear.
Swagger UI is what most developers actually mean when they say "Swagger": a single-page web app that renders an OpenAPI spec into interactive docs, complete with "Try it out" buttons that fire real requests. Drop a swagger-ui.html next to your API, point it at your openapi.json, and you have docs that never go out of date.
In conversation, "swagger" and "openapi" are often used interchangeably, especially for older codebases that still ship swagger.json (older 2.0 spec) rather than openapi.json (3.x). They are not interchangeable formats: 3.x added significant capability (callbacks, links, richer schemas) and most modern tools assume 3.x.
In the wild
- A startup shipping
/api/swaggeras live docs powered by Swagger UI - A team migrating a
swagger.json(2.0) toopenapi.json(3.0) to use newer toolchain features - SwaggerHub used as the design surface where API contracts are reviewed before backend work starts