What is an API Definition?

An API Definition is a document that defines the structure of the API. It includes the endpoints, request and response schemas, and authentication requirements.

Fern integrates with several API definition formats:

Why create an API Definition ?

Once you have an API definition, Fern will use it as an input to generate artifacts like SDKs and API reference documentation. Every time you update the API definition, you can regenerate these artifacts and ensure they are always up-to-date.


Client libraries in multiple languages.


A Stripe-like API documentation website.

Postman Collection

A published Postman collection, with example request and responses.

Server Boilerplate

Pydantic models for FastAPI or controllers for your Spring Boot application.