A message-based identification process can facilitate reliable interoperation between accessories and host devices. During an identification process, the devices can negotiate an operating agreement that specifies particular communications (e.g., messages) that each device is permitted to send to or receive from the other, for example by having one device send a list of specific messages that it intends to send to and/or receive from the other. The other device can review the proposal and either accept or reject it. If a proposal is accepted, the devices can begin interoperating using messages that were included in the agreed-upon proposal while ignoring any received messages that were not included in the agreed-upon proposal.