Overlays | Fern Documentation

Overlays let you customize your OpenAPI specification without modifying the original file. This is useful when:

Your API specification is auto-generated from server code
You need different configurations for SDKs versus documentation
You want to add Fern configurations like pagination or SDK method names

Overlays follow the OpenAPI Overlay Specification.

Overlays are the recommended approach for customizing OpenAPI specifications. Compared to overrides, overlays offer additional capabilities like bulk modifications using JSONPath wildcards and the ability to modify elements within arrays.

Fern still supports overrides for backward compatibility.

Configure overlays

To use overlays, create an overlays file in the same folder as your spec and reference it in generators.yml:

generators.yml

1 api:
2   specs:
3     - openapi: openapi.json
4       overlays: overlays.yml

Define the overlays file

Each action in an overlay targets elements using JSONPath and applies an update or remove operation:

openapi-overlays.yml

1 overlay: 1.0.0 # Required: Overlay Specification version
2 info:
3   title: Customize Plant Store API # Required: Human-readable description of the overlay's purpose
4   version: 1.0.0 # Required: Version identifier for tracking changes to this overlay
5 actions: # Required: Ordered list of changes to apply
6   - target: $.info # JSONPath expression selecting the element to modify
7     update: # Properties to merge with the targeted element
8       x-fern-sdk-group-name: plants
9   - target: $.paths['/plants/{plantId}'].get.parameters[?(@.name == 'plantId')]
10     update:
11       x-fern-parameter-name: id

Fern requires parentheses around JSONPath filter expressions. Use [?(@.name == 'plantId')] instead of [?@.name == 'plantId'].

Modifying OpenAPI properties

Use update to change standard OpenAPI properties like descriptions, summaries, or other fields:

openapi-overlays.yml

1 overlay: 1.0.0
2 info:
3   title: Improve API documentation
4   version: 1.0.0
5 actions:
6   - target: $.paths['/plants'].get
7     update:
8       summary: List all available plants
9       description: Returns a paginated list of plants in the store inventory.

Customizing with Fern extensions

Use update to add Fern extensions:

openapi-overlays.yml

1 overlay: 1.0.0
2 info:
3   title: Add Fern SDK customizations
4   version: 1.0.0
5 actions:
6   # Add SDK group and method names
7   - target: $.paths['/plants'].get
8     update:
9       x-fern-sdk-group-name: plants
10       x-fern-sdk-method-name: list
11   - target: $.paths['/plants'].post
12     update:
13       x-fern-sdk-group-name: plants
14       x-fern-sdk-method-name: create
15   # Rename a parameter
16   - target: $.paths['/plants/{plantId}'].get.parameters[?(@.name == 'includeDetails')]
17     update:
18       x-fern-parameter-name: withDetails

Removing elements

Use remove: true to delete elements from your specification:

openapi-overlays.yml

1 overlay: 1.0.0
2 info:
3   title: Remove internal endpoints
4   version: 1.0.0
5 actions:
6   - target: $.paths['/internal/debug']
7     description: Remove debug endpoint from public SDK
8     remove: true

Separate overlays for SDKs and docs

Use different overlays files for SDK generation versus documentation by creating separate folders with their own generators.yml:

$ fern/
>   ├─ fern.config.json
>   ├─ openapi.yml
>   ├─ docs/
>   │   ├─ generators.yml
>   │   └─ docs-overlays.yml
>   └─ sdks/
>       ├─ generators.yml
>       └─ sdk-overlays.yml

sdks/generators.yml

1 api:
2   specs:
3     - openapi: ../openapi.yml
4       overlays: sdk-overlays.yml

Overlays for different environments

Configure different overlays for production versus internal APIs:

generators.yml

1 groups:
2   production:
3     specs:
4       - openapi: openapi.yml
5         overlays: production-overlays.yml
6     generators:
7       ...
8   internal:
9     specs:
10       - openapi: openapi.yml
11         overlays: internal-overlays.yml
12     generators:
13       ...