Set up local SDK previews | Fern Documentation

Once you configure your SDK, you can use the --preview flag to test generated SDK code locally before publishing. This allows quick iteration on your SDK as you develop:

# Preview all SDKs
fern generate --preview
# Preview SDK for a specific language
fern generate --group <language>-sdk --preview

--preview generates your SDK code into a local .preview/ folder within your fern folder. No changes are published to package managers or GitHub.

--preview preserves any custom code you added to your SDK.

Example usage

Here’s an example of how you can preview a Python SDK.

Generator configuration

sdk-config.yml

schemaVersion: sdk-config/v1
source:
  specs:
    - id: imdb
      type: openapi
      path: ./openapi/openapi.yml
targets:
  - language: python
    package:
      packageName: imdb
    generation:
      naming:
        clientName: imdb
    output:
      delivery: github
      github:
        repository: imdb/imdb-python
      publish:
        registry: pypi
        username: __token__
        password: ${PYPI_TOKEN}

Invoke the Fern CLI

fern generate --target python --preview

Preview your SDK

The resulting folder structure will look like this:

fern

fern.config.json

.preview

fern-python-sdk

...

sdk-config.yml

openapi

openapi.yml