What is the OpenAPI Specification?

The OpenAPI Specification (OAS) is the formal, vendor-neutral standard for describing REST APIs — endpoints, operations, parameters, request/response schemas, authentication methods — in a language-agnostic format, maintained by the OpenAPI Initiative under the Linux Foundation.

It evolved directly from the original Swagger Specification: version 2.0 of the Swagger Specification was donated to the newly formed OpenAPI Initiative and renamed OpenAPI 2.0, with subsequent versions (3.0, 3.1) developed and released independently of the Swagger brand, even though many popular tools built against it still carry the Swagger name.

Because it's an open standard rather than a single vendor's proprietary format, a wide ecosystem of independent tools — documentation renderers, code generators, API gateways, testing frameworks — all understand the same OpenAPI document, which is what makes a single well-written spec reusable across an entire toolchain instead of being locked to one vendor's tooling.