Well, not really.

The protocol is pretty simple. Make a request and get back payment headers.

Sign the data, with the proper amounts. The signed transaction is sent to a facilitator. The facilitator validates the transaction, then broadcasts it onto the block chain. The payload is now sent to the agent or user.

The failure would be if an agent is unable to call a tool to perform it in the first place. The failure wouldn't be in the payment protocol.