Architecture

Core Components

A2A over MQTT introduces a broker-centric transport model while keeping agent semantics unchanged:

Agents

Agents publish a retained Agent Card to a discovery topic. The card describes the agent and may include optional trust metadata.

Clients

Clients subscribe to discovery topics to find agents and publish requests to request topics. Replies are returned on corresponding reply topics.

MQTT Broker

The broker routes discovery, request/reply, and event messages. It may optionally enforce trust policies for Agent Cards or attach broker-managed status via MQTT User Properties when forwarding discovery messages, but those choices are implementation-specific and not required for core conformance.