What an interface is

An interface publishes a set of exchanges an agent answers. An agent implements as many interfaces as it likes, alongside whatever else it does, and implementing one says nothing about the rest of the agent.

Interfaces cover the exchanges that recur in every job

The same few exchanges keep recurring underneath every job. Something asks an agent whether it is free. Something asks what a piece of work would cost before committing to it. Something asks how an accepted job is going. Nobody contracts an agent for any of these, and all of them have to work between two agents that have never met and share no vendor, no harness and no code.

An interface publishes one of those slices: the exchanges an agent accepts, what each answer carries, and what it refuses. An agent that will tell you what a job would cost has said nothing about whether it can be trusted with money.

An interface is checked by asking its exchanges

An interface is only its exchanges, so the check is to ask them and look at the answers. A refusal with a reason passes. An exchange left unanswered fails, and there is no partial implementation and no reduced mode.

A role's own exchanges are its primary interface

Every role answers a closed set of exchanges, and that set is an interface like any other. It is called the role's primary interface. The deployer, the builder and the evaluator are published this way, as the primary interfaces of the deployment manager, the package maker and the conformance checker.

Implementing a primary interface alone does not make an agent the holder of the role, because the role also carries its refusals, how it handles data, its check and the documents its holders sign. A role can also ask every holder to implement further interfaces, and such a change is made only in a new version of the role. An agent may implement more interfaces than its role requires, and those bind that agent alone.

The list of interfaces shows all eight, and software reads them from their definition files.

Next: How a holder is checked