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Most Membership.io endpoints return JSON:API-style documents. Use this request header: 
Content-Type: application/vnd.api+json
Accept: application/vnd.api+json
application/json is also accepted by the backend for compatibility, but application/vnd.api+json is the canonical media type.

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Resource response

A single resource response has data.id, data.type, and data.attributes. 
{
  "data": {
    "id": "0197...",
    "type": "hubs",
    "attributes": {
      "title": "Member Academy",
      "slug": "member-academy",
      "created_at": "2026-06-08T12:00:00Z"
    }
  }
}

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List response

A list response returns data as an array. Paginated endpoints also include links and metadata. 
{
  "data": [],
  "links": {
    "self": "/api/v1/teams/team_123/contacts?page[size]=20",
    "next": "/api/v1/teams/team_123/contacts?page[size]=20&page[after]=cursor"
  },
  "meta": {
    "has_more": true
  }
}

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Write request

Many newer write endpoints use a JSON:API envelope: 
{
  "data": {
    "type": "contacts",
    "attributes": {
      "email": "member@example.com",
      "first_name": "Sam"
    }
  }
}
Some alpha endpoints still accept flat request bodies or return flat token bodies. Check the generated OpenAPI page for the exact endpoint before building a client.

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Errors

Errors return an errors array. Each error has an HTTP status string, title, detail, optional code, and optional source pointer. 
{
  "errors": [
    {
      "status": "422",
      "code": "validation_error",
      "title": "Validation Error",
      "detail": "Field required",
      "source": {
        "pointer": "/data/attributes/email"
      },
      "meta": {
        "request_id": "req_..."
      }
    }
  ]
}