Agent2Agent Protocol (A2A) v0.3 - Google's Open Standard for Direct Agent-to-Agent Communication, Now at the Linux Foundation

What A2A Is

A2A is a universal agent-to-agent communication protocol - the standard enabling any two AI agents, regardless of builder or framework, to discover each other, authenticate securely, and collaborate. It is the "HTTP for AI agents." A2A is complementary to MCP, not competing: MCP connects an agent to its tools (agent-to-tool); A2A connects agents to other agents (agent-to-agent). Google's documentation explicitly recommends using them together.

Core Architecture

Every A2A agent publishes an Agent Card - a JSON metadata document at /.well-known/agent.json advertising its identity, capabilities, service endpoint, and authentication requirements. Agent Cards enable discovery without a prior relationship. A buying agent queries a registry, receives Agent Cards for matching sellers, and initiates direct authenticated communication without an exchange.

Tasks are the unit of work: structured objects with a defined lifecycle (submitted → working → input-required → completed / failed). Long-running campaigns maintain state across the lifecycle, with agents pushing status updates to registered notification endpoints.

Version History

• April 9 2025 (v0.1): Core spec - Agent Cards, task management, basic auth. 50+ partners.
• May 20 2025 (v0.2 / Google I/O): Stateless interactions, OpenAPI-like auth schemas, official Python SDK. Microsoft Azure and SAP announce support.
• June 2025: Donated to Linux Foundation - vendor-neutral governance.
• July 31 2025 (v0.3): gRPC support, signed Agent Cards (cryptographic identity verification), extended Python SDK, native ADK integration, Agent Engine deployment support.