Can't update custom field value using Asana API

[Joe_Trollo]

The issue here is that you’re trying to update the definition of a custom field, not the value of a custom field on a task. The API call you want is something like

PUT /tasks/<task-id>

{
  "data": {
    "custom_fields": {
      "1115652263994341": "10.5"
    }
  }
}