# What Is the A2A (Agent2Agent) Protocol for AI Agents?

> What A2A (Agent2Agent) is, the open protocol that lets one AI agent hand tasks to another: the Agent Card, version 1.0 and its role in UCP and AP2.

URL: https://clione.ai/glossary/a2a/
Lang: en

A2A stands for Agent2Agent, a protocol that lets one [AI agent](/glossary/ai-agent/) hand work to another. If you run into A2A in a payments document, it may mean something else: account-to-account transfers, which have nothing to do with this. What matters to a store is what sits underneath what it sees: **A2A is one of the roads the [agentic commerce](/glossary/agentic-commerce/) protocols are built on**, which is why it appears in the specifications for [UCP (Universal Commerce Protocol)](/glossary/ucp/) and [AP2 (Agent Payments Protocol)](/glossary/ap2/) even though the shopper never sees it.

## How it works

The specification defines it as an open standard for communication between independent AI agent systems, built with different tools, languages or vendors. It has four goals: agents discover what each other can do, agree on how to exchange information (text, files or JSON data), manage tasks together and exchange information securely, **without access to the other's internal state, memory or tools**.[^a2a-spec] Google explained it with two roles: a client agent formulates and communicates the task, and a remote agent carries it out, either to provide information or to take an action.[^google-a2a-launch]

**The calling card.** Every A2A server has to offer an [Agent Card](/glossary/agent-card/): a JSON document with its identity, its capabilities, its skills, the addresses it listens on and what it requires to authenticate. A client finds it in one of three ways: at the well-known address `https://{domain}/.well-known/agent-card.json`, by querying agent catalogs, or because someone configured the address by hand.[^a2a-spec]

**The task.** Work is organized into tasks with states: submitted, working, completed, failed, canceled, rejected, and two waiting states, for when the agent needs more input from the person or an additional authorization. Agents send each other messages made of parts (text, file references or JSON), and a task's result arrives as artifacts, which the specification keeps separate from messages.[^a2a-spec]

**How the answers arrive.** A task can be answered in the same request, live with Server-Sent Events, or later, with notifications the remote agent sends as a [webhook](/glossary/webhook/) to an address the client registered. It is built for long-running tasks, including ones with a person in the loop.[^a2a-spec]

**What it travels over.** Version 1.0 defines three protocol bindings: JSON-RPC 2.0, gRPC and HTTP with JSON. And it supports the usual web security schemes: API keys, HTTP authentication, OAuth 2.0, OpenID Connect and mutual TLS.[^a2a-spec]

## From Google to the Linux Foundation

Google introduced A2A on April 9, 2025, with **more than 50 technology companies** behind it, including Atlassian, PayPal, Salesforce, SAP and Workday, and described it from the start as a complement to Anthropic's [MCP (Model Context Protocol)](/glossary/mcp/).[^google-a2a-launch] On June 23, 2025, at Open Source Summit North America in Denver, it donated A2A to the Linux Foundation so it could be a neutral, community-governed standard.[^google-a2a-anniversary]

Version 1.0 came out in March 2026, **the first stable, production-ready release**, with signed Agent Cards that verify an agent's identity cryptographically.[^google-a2a-anniversary] The specification published today is 1.0.0, and the earlier ones (0.3.0, 0.2.6 and 0.1.0) remain accessible.[^a2a-spec] At its first anniversary, on April 9, 2026, the Linux Foundation counted **more than 150 organizations** backing it and placed it inside services from Microsoft, Google and AWS.[^lf-a2a-150]

## Where it touches commerce

A store does not run into A2A on its own, but inside the shopping protocols. AP2 presents itself as an open extension of A2A: agents collaborate over A2A and use AP2 to secure payments.[^ap2-site] UCP supports four transports, REST, MCP, A2A and one for embedded experiences, and when a merchant declares A2A, the address it publishes is that of its agent's Agent Card.[^ucp-overview] Google sums up the relationship in its first-year review: AP2, UCP and A2UI, a protocol for agent interfaces, were all designed with A2A's extension model.[^google-a2a-anniversary]

## In a store

Here is how the agent of a store that answers catalog questions over A2A might introduce itself. It is a **fictitious example**, written with the fields of the 1.0 specification; a real card also declares its security schemes:[^a2a-spec]

```json
{
  "name": "Northpack Backpacks catalog agent",
  "description": "Answers questions about the catalog: dimensions, materials, stock and shipping times.",
  "supportedInterfaces": [
    {
      "url": "https://northpack.example/a2a/v1",
      "protocolBinding": "JSONRPC",
      "protocolVersion": "1.0"
    }
  ],
  "provider": {
    "organization": "Northpack Backpacks (fictitious example)",
    "url": "https://northpack.example"
  },
  "version": "1.0.0",
  "capabilities": { "streaming": true, "pushNotifications": false },
  "defaultInputModes": ["text/plain"],
  "defaultOutputModes": ["text/plain", "application/json"],
  "skills": [
    {
      "id": "search-products",
      "name": "Search products",
      "description": "Finds backpacks by capacity, use and maximum price.",
      "tags": ["catalog", "search"],
      "examples": ["Waterproof 20-liter backpack for a 15-inch laptop under $90"]
    }
  ]
}
```

A person's [shopping agent](/glossary/shopping-agent/) would read this card at `/.well-known/agent-card.json`, see that there is a search skill, send a message with the request to the declared address and get back a task that moves to "working" and ends with an artifact: the list of backpacks, as text or JSON. At no point does it see how the store's agent queries the catalog.


**Publishing an Agent Card does not put you in any agent storefront.** The specification gives three ways to find one: the well-known address on your domain, an agent catalog or a direct configuration.[^a2a-spec] All three assume the client already knows where to look or trusts whoever maintains the catalog. A2A solves how to talk to an agent you have already found; **it does not decide which agent an assistant recommends to you**, just as having a business card does not bring you customers.


## How to check it

1. **See whether your platform or your payment gateway declares A2A.** In a UCP profile, the `a2a` transport points to the URL of an Agent Card.[^ucp-overview]
2. **Request the card.** On a domain that says it has an A2A agent, `https://{domain}/.well-known/agent-card.json` has to return an AgentCard object.[^a2a-spec]
3. **Read the skills, the input and output modes and the security.** They are what the agent promises to do and what it requires in order to do it.[^a2a-spec]
4. **Check the version** in `protocolVersion`. The current one is 1.0; a guide written for 0.3 or earlier describes a different version of the protocol.[^a2a-spec]
5. **Before connecting a third-party agent, review how it authorizes.** The specification forbids treating the "authorization required" state as an authorization in itself: what gets authorized, and how, is defined by each implementation.[^a2a-spec]

## Related concept: MCP

[MCP](/glossary/mcp/) connects an agent to its tools and data; A2A connects agents to each other. The A2A specification sums it up like this: MCP describes how an agent uses a specific capability, with structured inputs and outputs, and A2A describes how independent agents discover each other, split up tasks and exchange results as peers. An A2A client can hand a task to a remote agent that, to complete it, calls several tools over MCP.[^a2a-spec] The protocol's documentation puts it as an image: **MCP is vertical, it goes deep into one agent; A2A is horizontal, it connects agents**. Its example is a repair shop: the mechanic agent uses MCP to read the car's error codes and A2A to negotiate with a parts supplier's agent.[^a2a-mcp]

To go further, the [agentic commerce reading path](/learn/agentic-commerce/) orders the rest of the protocols, and the entry on [AP2](/glossary/ap2/) explains the extension of A2A that handles payment.