Beginning with our Q1 Minor Release on April 7th 2017, activity records retrieved via Marketo’s SOAP or REST APIs will include a new field called “marketoGUID”. The value of this field will match the value of the “id” field, but will be a string type field. Some time after the Q2 2018 release, the “id” field will stop being returned as part of these records, and the value of marketoGUID will become a 128-character string.