A method, system, and computer usable program product for source identification for multipart content validation are provided in the illustrative embodiments. An identification information of a source server is obtained by parsing an update request, the update request being configured to request from the source server an update of a part of the multipart content. The identification information is recorded. A change in a part of the multipart content is observed. The change in the part results in an updated multipart content. A debugging directive is constructed, the debugging directive including the identification information of the source server. The debugging directive is inserted relative to the part that has changed thereby associating the source server with the part of the multipart content. The updated multipart content including the debugging directive is transmitted to a validation server executing in a second data processing system.