What is the A2A protocol?

A2A is an open protocol for one agent to hand work to another agent it did not build. MCP wires an agent to tools; A2A wires it to peer agents that keep their own model, prompt and memory. The gap engineers hit first is that A2A specifies discovery and transport, not delegated authority. Nothing in the spec tells a receiving agent who authorized the caller to ask.

How it works

Each server publishes an Agent Card at /.well-known/agent-card.json listing its skills, endpoint and accepted security schemes: API keys, HTTP auth, OAuth 2.0, OpenID Connect and mutual TLS. Clients call it over one of three bindings, JSON-RPC 2.0, gRPC, or HTTP+JSON. Work is modelled as a task with nine states, including working, input_required, auth_required and the terminal completed, failed, canceled and rejected. That state machine is the load-bearing part. A caller never assumes one round trip. It polls or subscribes over Server-Sent Events while the remote agent runs, and the remote side can pause mid-task to ask for more input.

Whether it is worth adopting

Version 1.0 shipped at the protocol's first anniversary in April 2026, with 150+ supporting organizations, SDKs in Python, JavaScript, Java, Go, .NET and Rust, and 22,000+ stars on the core repo. Adoption skews enterprise. Engineers on Hacker News report seeing A2A mostly inside Azure AI Foundry, Amazon Bedrock AgentCore and Google's own stack, while smaller teams wrap remote agents as plain MCP tools and move on. Reach for A2A when the agent on the other end belongs to a different org and has to stay opaque with its own long-running lifecycle. When both agents are yours and share a process, it adds a discovery layer and a state machine on top of a function call already under control.