A computer program product, apparatus, and method for determining extended capability of a channel path in an I/O processing system are provided. The computer program product includes a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method. The method includes receiving a request to provide a channel path description for a channel path, where the channel path includes a channel coupled to a control unit. The method further includes outputting the channel path description for the channel path in response to the request. The channel path description includes a descriptor indicating that the channel path supports a link protocol for commanding an I/O operation, and an extension support indicator specifying whether the channel path supports an extension to the link protocol.