This shouldn’t be this hard. The SharePoint team really needs to work on the programming model. This web API is bad bad.

For “reasons” I found myself needing to update a SharePoint list item to set the value of a Person field. It turns out that this is insanely annoying because what the ListItem API expects is the resolved User Id (it’s some rando number that’s assigned to the user when they access the site for the first time.) I’m working in a PowerAutomate flow and unfortunately that’s not a value that comes along with a user’s object. That would be too helpful.

If you try to use the Claims string to set the PersonFieldStringId field in a MERGE, it just doesn’t work. You get some terrible error message like Bad Gateway or something equally useless. Also, if you try to set through the navigation property like PersonField: { "Claims": ... }, that will succeed, but not actually work.

After being very frustrated and increasingly annoyed, I finally found someone who said to use the validateUpdateListItem method on the item. It worked. Apparently, this method “Validates and sets the values of the specified collection of fields for the list item.”

In case you find yourself on this site, wondering how this can all be so unnecessarily complicated, here’s the request you want to make.

URI: https://tenant.sharepoint.com/sites/MySite/_api/web/lists/GetByTitle('My List Name')/items(1234)/validateUpdateListItem
Method: POST
Headers: Content-Type=application/json;odata=verbose
Body:
{
  "formValues": [
    {
      "FieldName": "Your_x0020_Field_x0020_Name",
      "FieldValue": "[{'Key':'i:0#.f|membership|user@domain.com'}]"
    }
  ]
}