What is an API Specification? What is OAS(OpenAPI Specification)?

As you’ve learned in the previous articles—covering methods, paths, parameters, request bodies, and responses—the next step is to understand the concept of an API specification (spec) and one of the most widely used standards: the OpenAPI Specification (OAS).

1. What Is an API Specification?

An API specification is a document (or machine‑readable file) that describes exactly how an API works. It acts as a contract between the API provider and consumers, explaining:

What endpoints exist (e.g., /pets/{id})

What methods you can use (GET, POST, etc)

What parameters or request bodies are needed

What response structure (schema) you’ll receive

What authentication is required

In other words: when you see the spec, you know how to call the API correctly and interpret the response safely.

2. Why Use an API Spec?

It standardizes how APIs are described so every developer can quickly understand. (openapis.org)

It enables tooling and automation: you can generate interactive docs, client libraries, mock servers, and tests from the spec. (swagger.io)

It improves collaboration: front-end, back-end, QA teams all work from the same “contract”. (apideck.com)

3. Spec Lifecycle

An API spec is not static—it evolves along with the API. Its lifecycle usually includes:

Design phase

The spec is written before the API is implemented.

Acts as a blueprint for developers, front-end teams, and QA.

You can design an API spec in Apidog in a visual interface intuitively.

Implementation phase

Backend developers implement endpoints according to the spec.

The spec may be updated if changes occur.

In Apidog, developers can generate server stubs from API spec directly.

Testing & Documentation phase

QA teams and tools use the spec to generate tests and mock servers.

Developers and consumers can explore the API using tools like Apidog.

Maintenance phase

Any API change (new endpoint, updated request/response) triggers an update to the spec.

In Apidog, the spec remains the single source of truth for both humans and machines.

Understanding the lifecycle helps you trust the spec and know that it should match the actual API behavior.

4. OpenAPI Specification (OAS)

The OpenAPI Specification (OAS) is a widely adopted standard for writing machine-readable API specs.

Initially introduced as "Swagger" in 2010, the project aimed to simplify API documentation and design. In 2016, the Swagger specification was renamed to OpenAPI Specification and donated to the Linux Foundation. It is now stewarded by the OpenAPI Initiative, which includes industry leaders like Google, Microsoft, and IBM.

Why Use OAS?

Standardization: OAS provides a unified and consistent format (in JSON or YAML) for documenting APIs. This makes it easier for developers, creators, and consumers to understand, integrate with, and build upon APIs across different platforms and programming languages.

Automation: Tools like Apidog can generate interactive and human-friendly documentation directly from OAS files. Other tools use OAS to generate code stubs, server skeletons, and API tests.

Improved Collaboration: A clearly defined OAS document acts as an API contract, streamlining communication and alignment between team members—including backend, frontend, QA, and product stakeholders.

Better Testing: Since OAS files define the expected behavior and structure of APIs, automated testing tools can validate correct implementations before deployment.

A minimal OAS document includes sections such as:

Explanation:

openapi — spec version

info — API metadata (title, description, version)

servers — base URL(s) for requests

paths — endpoints and methods

components — reusable schemas, parameters, and objects

4. How to Use an API Spec

Most APIs provide downloadable spec files in JSON or YAML format. These files are often linked in the documentation.

Example: In the Petstore API, the documentation page has a “Export” button in the bottom.

After obtaining the API spec file, you can click the “+” button on the main interface of Apidog and select “Import.”

Choose the “OpenAPI/Swagger” type, and then drag and drop the spec file you just downloaded. Click “Finish,” and you will be able to visually interact with the API spec in Apidog for debugging, testing, and mocking your API.

5. Key Takeaways

An API specification is your map to understanding and using APIs correctly. The OpenAPI Specification provides a standardized, machine-readable format that enables:

Interactive documentation

Automated testing

Client code generation

Reliable communication between teams

By understanding the purpose, lifecycle, and structure of API specs, you can confidently explore any API, build requests, interpret responses, and leverage tooling to work more efficiently.

You've now learned about the blueprint that holds everything together—the API Specification. This concludes our deep dive into the fundamental building blocks of APIs.

Let's recap everything we've learned in this chapter in the Chapter Summary.

What is an API Specification? What is OAS(OpenAPI Specification)?

1. What Is an API Specification?#

2. Why Use an API Spec?#

3. Spec Lifecycle#

4. OpenAPI Specification (OAS)#

Why Use OAS?#

4. How to Use an API Spec#