A2A is a protocol for AI agents to discover one another, exchange tasks and collaborate across system and organisational boundaries. Where earlier integration patterns assumed a human or a fixed program on at least one end, A2A assumes neither: two autonomous systems negotiate work between themselves.
That removes the control point most enterprise security was built around. Authorisation models assume a principal — a person who is accountable for the action. When an agent delegates to another agent, which delegates again, the chain from the original human authorisation to the eventual action gets long, and each hop is a place where scope can widen without anyone deciding that it should.
How it differs from MCP. MCP governs an agent calling a tool: the tool is passive and does what it is told. A2A governs an agent calling another agent, which has its own goals, its own tool access and its own credentials. The trust question is harder because the other side is not a function — it is an actor.
How it differs from a service mesh. A mesh authenticates services and encrypts traffic between them. It does not ask whether the task one agent handed another was within the authority of the human who started the chain. That question is semantic, and it has to be answered where the traffic is, at runtime.
Both agents must be authorised, and the data exchange must comply with policy. APERION's AIDA protocol binds agents to human principals with exact action scopes and limits.
Put this in the path of your own agents.
Policy enforced inline between your agents and every model and tool they reach, with a record bound to the human who owns it.
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