GetResponse API Custom Fields

We're deep into automating our lead nurturing workflows and a critical piece involves programmatically updating specific custom fields for existing GetResponse contacts via their API. This synchronization is crucial for our CRM to accurately reflect and drive GetResponse segmentation for targeted campaigns.

However, we're hitting an unexpected roadblock: a consistent 400 Bad Request or 422 Unprocessable Entity when attempting these updates. The issue seems to stem from the payload structure for custom-fields data, especially when trying to update multiple fields or fields with specific data types. The GetResponse documentation on this particular API integration nuance feels a bit ambiguous for complex update scenarios.

We've been trying a PUT request to /contacts/{contactId} with a payload structured like this, but it's consistently failing:

PUT /v3/contacts/{contactId}
Authorization: api-key {your_api_key}
Content-Type: application/json

{
  "customFields": [
    {
      "customFieldId": "abc123def456",
      "value": ["new_value_for_field_1"]
    },
    {
      "customFieldId": "ghi789jkl012",
      "value": ["another_value"]
    }
  ]
}

And the typical error response we're seeing is similar to:

{
  "message": "Invalid custom field value format",
  "code": 1007
}

Can anyone share the definitive correct JSON payload structure for updating multiple custom fields for a single contact via the GetResponse API? Are there specific requirements for different custom field types (e.g., text, number, dropdown) when sending the value array? Any common pitfalls for this type of API integration that we might be overlooking?

Thanks in advance for any help!