Fern is a toolkit that allows you to input your API Definition and output SDKs and API documentation. Fern is compatible with the OpenAPI specification (formerly Swagger).

The Fern toolkit is available via a command line interface (CLI) and requires Node 18+. To install it, run:

npm install -g fern-api

Initialize Fern with your OpenAPI spec:

fern init --openapi ./path/to/openapi.yml
# or
fern init --openapi https://link.buildwithfern.com/petstore-openapi

Your directory should look like the following:

fern/
├─ fern.config.json
├─ generators.yml # generators you're using
└─ openapi/
  └─ openapi.json # your openapi document

Finally, to invoke the generator, run:

fern generate

🎉 Once the command completes, you'll see your SDK in /generated/sdks/typescript.

Fern can also build and host a documentation website with an auto-generated API reference. Write additional pages in markdown and have them versioned with git. Search, SEO, dark mode, and popular components are provided out-of-the-box. Plus, you can customize the colors, font, logo, and domain name.

Check out docs built with Fern:

• docs.vellum.ai
• docs.superagent.sh
• docs.hume.ai

Get started here.

Generators are process that take your API Definition as input and output artifacts (SDKs, Postman Collections, Server boilerplate, etc.). To add a generator run fern add <generator id>

Generator ID	Latest Version	Changelog	Entrypoint
fernapi/fern-typescript-node-sdk		CHANGELOG.md	cli.ts
fernapi/fern-python-sdk		CHANGELOG.md	cli.py
fernapi/fern-java-sdk		CHANGELOG.md	Cli.java
fernapi/fern-ruby-sdk		CHANGELOG.md	cli.ts
fernapi/fern-go-sdk		CHANGELOG.md	main.go

Fern's server-side generators output boilerplate application code (models and networking logic). This is intended for spec-first or API-first developers, who write their API definition (as an OpenAPI spec or Fern definition) and want to generate backend code. Watch a demo here.

Generator ID	Latest Version	Changelog	Entrypoint
fernapi/fern-typescript-express		CHANGELOG.md	cli.ts
fernapi/fern-fastapi-server		CHANGELOG.md	cli.py
fernapi/fern-java-spring		CHANGELOG.md	Cli.java

Fern's model generators will output schemas or types defined in your OpenAPI spec or Fern Definition.

Generator ID	Latest Version	Changelog	Entrypoint
fernapi/fern-pydantic-model		CHANGELOG.md	cli.py
fernapi/java-model		CHANGELOG.md	Cli.java
fernapi/fern-ruby-model		CHANGELOG.md	cli.ts

Fern's spec generators can output an OpenAPI spec or a Postman collection.

Note: The OpenAPI spec generator is primarly intended for Fern Definition users. This prevents lock-in so that one can always export to OpenAPI.

Generator ID	Latest Version	Changelog	Entrypoint
fernapi/fern-openapi		CHANGELOG.md	cli.ts
fernapi/fern-postman		CHANGELOG.md	cli.ts

Here's a quick look at the most popular CLI commands. View the documentation for all CLI commands.

fern init: adds a new starter API to your repository.

fern check: validate your API definition and Fern configuration.

fern generate: run the generators specified in generators.yml in the cloud.

fern generate --local: run the generators specified in generators.yml in docker locally.

fern add <generator>: include a new generator in your generators.yml. For example, fern add fern-python-sdk.

Fern supports developers and teams that want to be API-first or Spec-first.

Define your API, and use Fern to generate models, networking code and boilerplate application code. The generated code adds type safety to your API implementation - if your backend doesn't implement the API correctly, it won't compile.

Frameworks currently supported:

• Express
• Spring Boot
• FastAPI

For a walkthrough, check out the Fern + Express video.

While we are big fans of OpenAPI, we know it isn't the easiest format to read and write. If you're looking for an alternative, give the Fern Definition a try.

Install the Fern CLI and initialize a Fern Project.

npm install -g fern-api
fern init

This will create the following folder structure in your project:

fern/
├─ fern.config.json # root-level configuration
├─ generators.yml # generators you're using
└─ definition/
  ├─ api.yml  # API-level configuration
  └─ imdb.yml # endpoints, types, and errors

Here's what the imdb.yml starter file looks like:

types:
  MovieId: string

  Movie:
    properties:
      id: MovieId
      title: string
      rating:
        type: double
        docs: The rating scale is one to five stars

  CreateMovieRequest:
    properties:
      title: string
      rating: double

service:
  auth: false
  base-path: /movies
  endpoints:
    createMovie:
      docs: Add a movie to the database
      method: POST
      path: /create-movie
      request: CreateMovieRequest
      response: MovieId

    getMovie:
      method: GET
      path: /{movieId}
      path-parameters:
        movieId: MovieId
      response: Movie
      errors:
        - MovieDoesNotExistError

errors:
  MovieDoesNotExistError:
    status-code: 404
    type: MovieId

Checkout open source projects that are using Fern Definitions:

• Metriport
• Rivet
• Revert

Join our Discord! We are here to answer questions and help you get the most out of Fern.

We welcome community contributions. For guidelines, refer to our CONTRIBUTING.md.