Hello user87148,   Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.   Upon reviewing our logs for your PUT requests to the /contacts endpoint, I found that the JSON body was missing from the requests. I did some testing with the JSON request body that you provided and found that the request was successful with this format. With further testing, I was able to replicate the "update_source is missing, update_source does not have a valid value" error response that you received when completely removing the JSON body from the request.   It seems likely that some part of your workflow may not be adding the JSON body to the PUT request before it is sent.     Please have a look and let us know if you have any other questions!   Regards, ... View more