What is OpenAPI?

An OpenAPI file, also known as an OpenAPI document or OpenAPI spec, enables a developer to describe an API. This spec also describes how to use this API, including available endpoints, operation parameters, authentication methods and other information. These specifications are written in either YAML or JSON, and JSON schema is used to describe the data formats within an API.

OpenAPI serves as both documentation and as a contract (in spirit, describing what an API should do—it is not legally binding) between API consumers and producers. It essentially serves as “one source of truth” that provides the instructions in a standardized format.

This standardization simplifies API consumption and integration. It enables humans and computers alike to understand the function and structure of a given API without access to source code or needing to understand the inner workings of the API. It also lets developers work with an API, regardless of what language it was written in.

OpenAPI automates interactive, up-to-date documentation, eliminating some repetitive documentation work and helping to ensure that documentation remains current. OpenAPI also enables code generation for client SDKs and the generation of other boilerplate code automatically from the spec, reducing human error and further minimizing developer workload.

Tools that use OpenAPI specs, including SwaggerUI, can render an API spec in an interactive interface. This interface not only helps visualize the spec, but it also enables real API calls, directly from the browser or web service, for testing and validation purposes.

By standardizing how REST APIs are described, OpenAPI helps improve interoperability and tooling, and support operations throughout the API lifecycle. A strong, well-maintained spec can be a foundational tool for implementing a comprehensive API strategy, supporting integration, collaboration, error prevention and stronger API management.

The complete specification can be found on GitHub.

As a de facto industry standard, OpenAPI provides a secure, automated and widely understood document for API development.

OpenAPI was originally created by developer Tony Tam and first released in 2011 under the name Swagger. At the time, API specifications were often static documents that required tedious updating and were frequently out of date. The Swagger specification included several innovations in API development, including a “try it out” button to test API calls live and in real-time.

Swagger made documentation a focus of its existence. It enabled developers to include annotations, similar to a sticky note, to their source code. Swagger could read these annotations and update the documentation automatically, ensuring that the documentation stays up to date without tedious and repetitive update work. Swagger also introduced a format for describing APIs in machine-readable JSON, which made it language-agnostic: regardless of the language of a backend, Swagger would output a universal JSON file.

Swagger was acquired by SmartBear Software in 2015, was renamed OpenAPI soon after, and donated to the new OpenAPI Initiative (OAI), under the umbrella of the Linux Foundation. The current version is OpenAPI 3.1.

The OpenAPI spec is a standardized, human- and machine-readable document that defines API structure and syntax. All OpenAPI documents include certain components and conform to the same basic structure, but some specs contain more information than others. At a minimum, a spec must includeopenAPI andinfo fields, and at least onepath , component or webhook field.¹

openAPI: Notes the OpenAPI version that the document uses, such as “3.1.1”

info: Includes general API metadata, such as the API title and version number, a description of the API, terms of service, and contact information. 

servers: A list of base URLs where the API can be accessed (which might include both staging and production environments). This list includes variables for environment-specific hosts.

paths: Describes the paths and associated HTTP methods (or operations—for example, GET, PUT, POST) and the parameters, request bodies and responses for each path item.

components: Lists reusable objects such as data schemas, parameters, responses, headers and security definitions. These objects can be referenced throughout the document. Components are referenced, simply enough, with $ref. This strategy keeps an API design as streamlined as possible. 

security: States the security schemes and authentication mechanisms that the API uses, such as OAuth or API keys. It also enables the application of security schemes globally, or by operation.

tags: High-level list of tags that are used to organize operations. Using tags can help improve document clarity (for example, “Users” or “Orders”).

external documentation: Links to guides, onboarding docs and other external documentation for the API

webhooks: Describes inbound calls that the API receives. 

OpenAPI provides a single source of truth for API developers and consumers, and provides features that add value and efficiency to API projects.

OpenAPI was initially created with a heavy focus on documentation for REST APIs, and that focus remains at its core today. Documentation is standardized, interactive, updated in real-time and provides a contract with a single source of truth.

Various tools exist to read OpenAPI docs and export code. One of those tools is Swagger Codegen, which reads documents written according to the OpenAPI spec. Swagger Codegen can then produce API client code software development kits (SDKs), server-side code (stubs) and readable webpages with up-to-date documentation that are human-readable.

One of OpenAPI’s most innovative features remains the “try it out” button, which enables real-time testing and validation right from a browser. Swagger UI, a free open-source tool, enables a visual interface in which a developer can send actual requests to help ensure that they respond as desired. This tool makes it easier to instantly verify that a request is functioning.

OpenAPI is commonly used with integration platforms as a service, or iPaaS.

iPaaS platforms use OpenAPI specifications to understand and connect different applications in an IT ecosystem. When applications expose OpenAPI specifications that describe their APIs, iPaaS platforms can use this information to automatically discover endpoints, authentication methods and data structures. This strategy makes it faster to build integrations, such as connecting an ecommerce platform with accounting software.

OpenAPI also enables frontend and backend developers to work in parallel. Parallel work is preferable to, as is sometimes the case, the frontend team being forced to wait until the backend team has their database up and running. With an OpenAPI contract, the frontend team can create a mock API server that behaves like a real one, enabling and optimizing testing before the backend development is complete. 

API governance—the standards, policies and practices that direct how an organization develops and uses APIs—is vital to help ensure that APIs operate seamlessly, consistently and without unneeded repetition. Because OpenAPI acts as a contract among developers, governance and security can be baked in from the start.

Take API gateways, which handle tasks like traffic routing, monitoring and rate limiting, for example. API gateways can typically consume OpenAPI specs and enforce any traffic limits or other security measures laid out in the spec.

The same applies for security rules. An OpenAPI spec lets developers declare security requirements (such as the use of such as OAuth 2.0 and API keys), and these requirements can be enforced downstream (by a gateway, perhaps.)  Outlining security features in an API contract helps ensure that developers don’t institute unsupported security schemes.

OpenAPI can strengthen API management, the scalable process of creating, publishing and managing API connections, in several ways. For example, because OpenAPI specs are machine-readable and standardized, catalog and portal software can automatically index them. This standardization makes APIs easier to find and understand across an organization. This discoverability helps prevent teams from unknowingly building duplicative APIs.

OpenAPI also supports consistent management and governance by making organizational standards explicit and enforceable. Teams can define requirements, such as versioning conventions, error response formats or naming patterns, directly in or alongside the spec. Because these expectations are documented in a standardized format, they can be automatically validated against new APIs as they are added. This validation reduces reliance on manual review and helps ensure that APIs remain consistent as an organization’s portfolio grows.