If you support custom fields for Contacts we can use use them when creating or updating a contact. To do so we need an endpoint that returns the customers Custom Fields with their IDs.

GET /fields

{
  "custom_fields": [
        {
            "name": "Unsubscribed",
            "id": "7956yv02anehlhbey3o87yghfo"
        },
        {
            "name": "Responded",
            "id": "76qgerv7956yvey3o87yghfo09"
        },
        {
            "name": "Years at Current Position",
            "id": "87y56yv02anehlhghfoyv02a2e"
        }
    ]
}

We do prefer that all fields including the default fields are included in the response