AI Engineering/Phase 16/Lesson 12

A2A — The Agent-to-Agent Protocol

Google announced A2A in April 2025; by April 2026 the spec is at https://a2a-protocol.org/latest/specification/ and 150+ organizations back it. A2A is the horizontal complement to MCP (Lesson 13): where MCP is vertical (agent ↔ tools), A2A is peer-to-peer (agent ↔ agent). It defines Agent Cards (discovery), tasks with artifacts (text, structured data, video), opaque task lifecycles, and auth. Production systems increasingly pair MCP with A2A. Google Cloud rolled A2A support into Vertex AI Agent Builder during 2025-2026.

Learn + Build 1.3 hr Python (stdlib, `http.server`, `json`)

Prerequisites: Phase 16 · 04 (Primitive Model)

#Problem

Your agent needs to call another agent on another system. How? You can expose an HTTP endpoint, define a bespoke JSON schema, and hope the other side speaks it. Every pair of agents becomes a custom integration.

A2A is the universal wire protocol for that call. Standard discovery, standard task model, standard transport, standard artifacts. Like HTTP+REST but for agents as first-class citizens.

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The text and code of this lesson are based on AI Engineering from Scratch by Rohit Ghumare, MIT licensed.