The A2A (Agent-to-Agent) protocol has undergone a critical transition from proof-of-concept to production deployment over the past few months. The core specification defines three key mechanisms. Agent Card: each agent publishes a standardized JSON capability declaration card containing the agent's name, description, supported skills list, input/output formats, and authentication method. Task Delegation: one agent can delegate subtasks to another agent, supporting both synchronous and asynchronous modes, with the latter using callback URLs for results. Identity & Auth: combining OAuth 2.0 and DID (Decentralized Identifier) for cross-trust-domain authentication.
In Agent Harness training, the A2A protocol is positioned as a complementary layer to MCP rather than a competitor. MCP solves Agent-to-Tool standardized connections, while A2A solves Agent-to-Agent task collaboration. Their relationship can be analogized to web architecture: MCP is HTTP (resource access protocol), A2A is SMTP (message delivery protocol). Recommended learning path: master MCP first (since tool calling is the agent's foundational capability), then learn A2A (since agent collaboration is advanced multi-agent capability). Production-grade A2A implementations mean cross-platform agent collaboration is no longer a lab concept.