Endpoints in Fern Definition | Fern Documentation

Fern Definition isn’t recommended for new customers and Fern isn’t accepting feature requests for this format. It remains supported for existing users.

In Fern, you organize related endpoints into a Service. This grouping improves clarity and makes the generated SDKs more idiomatic.

Service definition

Each service defines:

A base-path: A common prefix for all the endpoints’ HTTP paths
Whether the service requires authentication
Endpoints

user.yml

  service: 
    base-path: /users # This defines the group/namespace for the methods
    auth: false 
    endpoints: {}

To define a service with an empty base path use the empty string: base-path: ""

Section display name

By default, section names in your API Reference come from service file names (e.g., user.yml becomes “User”). To override the display name of a section, use the section property in your docs.yml.

SDK method names

SDK method names are derived directly from the service file name and endpoint key. The file name becomes the namespace and the endpoint key becomes the method name.

For example, given the following definition:

users.yml

service:
  base-path: /users
  auth: false
  endpoints:
    create:
      path: ""
      method: POST
      request: CreateUserRequest

Fern generates a method called client.users.create().

Endpoints

An endpoint includes:

A URL path (Optionally including path parameters)
A Display Name (Optional)
An HTTP Method
Request information (Optional)
- Query-parameters
- Headers
- Request body
Successful (200) response information (Optional)
Error (non-200) responses that this endpoint might return (Optional)

URL path

Each endpoint has a URL path.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    getAllUsers:
      path: /all
      method: GET

The full path for the endpoint is the concatenation of:

The environment URL
The service base-path
The endpoint path

Endpoint display name

The display name will appear as the title of an endpoint. By default, the display name is equal to the ‘Title Case’ of the endpoint name. If you would like to customize the endpoint name, you can set the display name.

In the example below, “Add a new plant to the store” displays as the title of the endpoint page within the API Reference.

user.yml

service:
  base-path: /v3
  auth: false
  endpoints:
    addPlant:
      path: /plant
      display-name: Add a new plant to the store
      method: POST

Path parameters

Supply path parameters for your endpoints to create dynamic URLs.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    getUser:
      path: /{userId} 
      path-parameters: 
        userId: string
      method: GET

Services can also have path-parameters:

project.yml

service: 
  base-path: /projects/{projectId}
  path-parameters: 
    projectId: string 
  auth: false 
  endpoints: 
    ...

A path parameter can’t share a name with another request property (a body property, query parameter, or header), including after camelCase normalization. fern check reports these collisions; set the name property on the path parameter to rename it and resolve the collision:

user.yml

path-parameters:
  plantId:
    type: string
    name: id

Query parameters

Each endpoint can specify query parameters:

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    getAllUsers:
      path: /all
      method: GET
      request:
        # this name is required for idiomatic SDKs
        name: GetAllUsersRequest
        query-parameters:
          limit: optional<integer>

`allow-multiple`

Use allow-multiple to specify that a query parameter is allowed multiple times in the URL, as in ?filter=jane&filter=smith. This will alter the generated SDKs so that consumers can provide multiple values for the query parameter.

user.yml

  ...
  query-parameters:
    filter:
      type: string
      allow-multiple: true

Auth

Each endpoint can override the auth behavior specified in the service.

user.yml

service: 
  base-path: /users 
  auth: false 
  endpoints: 
    getMe: 
      path: "" 
      method: GET 
      # This endpoint will be authed 
      auth: true 
      docs: Return the current user based on Authorization header.

Headers

Each endpoint can specify request headers:

user.yml

service: 
  base-path: /users 
  auth: false 
  endpoints: 
    getAllUsers: 
      path: /all
      method: GET 
      request: 
        # this name is required for idiomatic SDKs name:
        name: GetAllUsersRequest 
        headers: 
          X-Endpoint-Header: string

Services can also specify request headers. These headers will cascade to the service’s endpoints.

user.yml

service: 
  base-path: /users 
  auth: false 
  headers: 
    X-Service-Header: string 
  endpoints: 
    getAllUsers: 
      path: /all 
      method: GET 
      request: 
        # this name is required for idiomatic SDKs 
        name: GetAllUsersRequest 
        headers: 
          X-Endpoint-Header: string

Request body

Endpoints can specify a request body type.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    setUserName:
      path: /{userId}/set-name
      path-parameters:
        userId: string
      method: POST
      request: string

Inlining a request body

If the request body is an object, you can inline the type declaration. This makes the generated SDKs a bit more idiomatic.

user.yml

service: 
  base-path: /users 
  auth: false 
  endpoints: 
    createUser: 
      path: /create 
      method: POST 
      request: 
        # this name is required for idiomatic SDKs 
        name: CreateUserRequest 
        body: 
          properties: 
            userName: string

Success response

Endpoints can specify a response, which is the type of the body that will be returned on a successful (200) call.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    getAllUsers:
      path: /all
      method: GET
      response: list<User>
types:
  User:
    properties:
      userId: string
      name: string

Response status codes

You can also use the status-code field to specify a custom status code for a success response.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    create: :
      path: ""
      method: POST
      request: CreateUserRequest
      response: 
        type: User
        status-code: 201
types:
  User:
    properties:
      userId: string
      name: string

An endpoint has exactly one success response: response takes a single type and a single status-code. There is no array of success responses. To document a second success body shape, model the response type as a discriminated or undiscriminated union, which still renders under one status code. Non-success responses are declared as errors.

Pagination

Fern supports offset, cursor, URI, and path-based pagination schemes. To set up auto-pagination in generated SDKs:

Annotate the desired paginated endpoint with the pagination field
Specify the pagination scheme (offset, cursor, next_uri, or next_path)
Specify where your results are located using dot-access notation.

Offset pagination options

Include step in most offset pagination configurations to ensure the offset increments by the page size. Use has-next-page when your API returns a boolean indicator for additional pages.

service:
  base-path: /users
  auth: false
  endpoints:
    list:
      path: ""
      method: GET
      pagination:
        offset: $request.page
        step: $request.page_size # Recommended
        results: $response.data
        has-next-page: $response.has_more
      request:
        name: ListUsersRequest
        query-parameters:
          page: optional<integer>
          page_size: optional<integer>
      response: ListUsersResponse
types:
  ListUsersResponse:
    properties:
      data: list<User>
      has_more: boolean

The pagination field supports the following properties:

Property	Description
`offset`	Path to the offset parameter in the request (e.g., `$request.page`)
`cursor`	Path to the cursor parameter in the request (e.g., `$request.cursor`)
`next_cursor`	Path to the next cursor value in the response (required for cursor pagination)
`next_uri`	Path to the next page’s URL in the response (e.g., `$response.next_page_url`)
`next_path`	Path to the relative path for the next page in the response (e.g., `$response.next_page_path`)
`results`	Path to the results array in the response (e.g., `$response.data`)
`step`	Path to the page size parameter, ensures offset increments correctly
`has-next-page`	Path to a boolean indicator for additional pages

Idempotent endpoints

Mark an endpoint as idempotent to allow SDK users to specify idempotency headers for safe request retries. You must also configure idempotency headers in your api.yml to define which headers are available.

service.yml

service:
  base-path: /transactions
  auth: true
  endpoints:
    send:
      path: ""
      method: POST
      idempotent: true
      request: SendTransactionRequest
      response: Transaction

Error responses

Endpoints can specify error responses, which detail the non-200 responses that the endpoint might return.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    getUser:
      path: /{userId}
      path-parameters:
        userId: string
      method: GET
      response: User
      errors:
        - UserNotFoundError
types:
  User:
    properties:
      userId: string
      name: string
errors:
  UserNotFoundError:
    status-code: 404

You can learn more about how to define errors on the Errors page.

Specifying examples

When you declare an example, you can also specify some examples of how that endpoint might be used. These are used by the compiler to enhance the generated outputs. Examples will show up as comments in your SDKs, API documentation, and Postman collection.

You may add examples for endpoints, types, and errors.

user.yml

service:
  base-path: /users
  auth: false
  endpoints:
    getUser:
      path: /{userId}
      path-parameters:
        userId: string
      method: GET
      response: User
      errors:
        - UserNotFoundError
      examples:
        - path-parameters:
            userId: alice-user-id
          response:
            body:
              userId: alice-user-id
              name: Alice
types:
  User:
    properties:
      userId: string
      name: string
errors:
  UserNotFoundError:
    status-code: 404

If you’re adding an example to an endpoint and the type already has an example, you can reference it using $.

service:
  auth: true
  base-path: /address
  endpoints:
    create:
      method: POST
      path: ""
      request: CreateAddress
      response: Address
      examples:
        - request: $CreateAddress.WhiteHouse
          response:
            body: $Address.WhiteHouseWithID
  CreateAddress:
    properties:
      street1: string
      street2: optional<string>
      city: string
      state: string
      postalCode: string
      country: string
      isResidential: boolean
    examples:
      - name: WhiteHouse
        value:
          street1: 1600 Pennsylvania Avenue NW
          city: Washington DC
          state: Washington DC
          postalCode: "20500"
          country: US
          isResidential: true
  Address:
    extends: CreateAddress
    properties:
      id:
        type: uuid
        docs: The unique identifier for the address.
    examples:
      - name: WhiteHouseWithID
        value:
          id: 65ce514c-41e3-11ee-be56-0242ac120002
          street1: 1600 Pennsylvania Avenue NW
          city: Washington DC
          state: Washington DC
          postalCode: "20500"
          country: US
          isResidential: true

Examples contain all the information about the endpoint call, including the request body, path parameters, query parameters, headers, and response body.

user.yml

examples: 
  - path-parameters: 
      userId: some-user-id 
    query-parameters:
      limit: 50 
    headers: 
      X-My-Header: some-value 
    response: 
      body: 
        response-field: hello

Failed examples

You can also specify examples of failed endpoints calls. Add the error property to a response example to designate which failure you’re demonstrating.

user.yml

examples:
  - path-parameters:
      userId: missing-user-id
    response:
      error: UserNotFoundError
errors:
  UserNotFoundError:
    status-code: 404

If the error has a body, then you must include the body in the example.

user.yml

examples:
  - path-parameters:
      userId: missing-user-id
    response:
      error: UserNotFoundError
      body: "User with id `missing-user-id` was not found"
errors:
  UserNotFoundError:
    status-code: 404
    type: string

Referencing examples from types

To avoid duplication, you can reference examples from types using $.

user.yml

service:
  base-path: /users
  auth: true
  endpoints:
    getUser:
      method: GET
      path: /{userId}
      path-parameters:
        userId: UserId
      examples:
        - path-parameters:
            userId: $UserId.Example1
types:
  UserId:
  type: integer
  examples: 
    - name: Example1
      value: user-id-123