AI Engineering/Phase 13/Lesson 19

A2A — Agent-to-Agent Protocol

MCP is agent-to-tool. A2A (Agent2Agent) is agent-to-agent — an open protocol for letting opaque agents built on different frameworks collaborate. Released by Google in April 2025, donated to the Linux Foundation in June 2025, reaching v1.0 in April 2026 with 150+ supporters including AWS, Cisco, Microsoft, Salesforce, SAP, and ServiceNow. It absorbed IBM's ACP and added the AP2 payments extension. This lesson walks the Agent Card, Task lifecycle, and the two transport bindings.

Build 1.3 hr Python (stdlib, Agent Card + Task harness)

Prerequisites: Phase 13 · 06 (MCP fundamentals), Phase 13 · 08 (MCP client)

#Learning Objectives

Distinguish agent-to-tool (MCP) from agent-to-agent (A2A) use cases.
Publish an Agent Card at /.well-known/agent.json with skills and endpoint metadata.
Walk the Task lifecycle (submitted → working → input-required → completed / failed / canceled / rejected).
Use Messages with Parts (text, file, data) and Artifacts as outputs.

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