<div>This paper investigates the application of a formal framework for <em>observation</em> in agent-based systems to the specification of the individual agent's interactive behaviour.
An abstract architecture is defined for agents that is based on the idea of viewing them as <em>observable sources</em> &mdash; of knowledge, services, capabilities.
In this model, only the agent's portion that is directly involved in managing the interactions with the agent's environment is explicitly represented, abstracting away from agent's internal details and focussing on their observable effect.</div>

<div>The applicability of the formal framework as a specification tool is put to test in two ways.
On the one hand, this paper shows how different interactive behaviours can be modelled, such as reactive and proactive message sending, as well as publish/subscribing capabilities.
On the other hand, a specification language is introduced, developed on top of the formal framework, which has the potential to be effectively exploited as a tool in the engineering of multiagent systems.</div>