Bulk update agent skills via CXone REST API

Trying to bulk update agent skill proficiencies. The documentation is sparse on the exact payload structure for the batch endpoint. Sending a POST to /api/v2/users/skills with an array of skill assignments returns a 400 Bad Request. Here is the JSON I am sending:

{
 "users": [
 {
 "userId": "12345",
 "skills": [{ "skillId": "67890", "proficiency": 100 }]
 }
 ]
}

The error message just says “Invalid payload”. What is the correct schema for this call?