I've seen several similar questions and the crux of the answer seems to be that I should add "Id" to the field name and pass the user's id. Here for instance:
How to update "Person or Group" column in SharePoint List using REST API
But when try that I get an error saying the field is not recognized. The field name in this case is Agent.
@status=400 @reason_phrase="Bad Request" @response_body="{\"error\":{\"code\":\"invalidRequest\",\"message\":\"Field 'AgentId' is not recognized\"
All of the other field types update as expected, including single line of text, choice, and date/time, but this person/group field has me banging my head against the wall. Any help would be appreciated. Thanks.