To use an OpenAPI spec, run the command below.

npm install -g fern-api
fern init --openapi <filepath to openapi>

You can also point to a URL

npm install -g fern-api
fern init --openapi <url to OpenAPI>

Fern will generate the following directory with your OpenAPI spec inside of it.

fern/
├─ fern.config.json
└─ api/
  ├─ generators.yml
  └─ openapi/
    └─ openapi.json # <--- your openapi file

Run fern check to validate the OpenAPI spec.

If there are no errors, you can run fern generate.