====== A2A Protocol ======

**Agent-to-Agent (A2A)** is an open-source communication protocol developed by **Google**, launched at Google Cloud Next '25 in April 2025. A2A enables secure, interoperable communication between AI agents from different vendors, frameworks, and cloud environments. It complements Anthropic's [[mcp|Model Context Protocol (MCP)]] by addressing inter-agent networking rather than tool/data integration.

Over 50 companies have adopted A2A, including PayPal, Salesforce, and Atlassian.

===== Architecture =====

A2A uses a **client-server model** built on HTTP, Server-Sent Events (SSE), and JSON-RPC — standard web protocols that integrate with existing IT infrastructure. Key architectural layers include:

  * **Application Layer** — User interfaces (Gradio, Google ADK) that interface with a host agent for orchestration
  * **A2A Protocol Layer** — Standardized messaging, discovery, and task management
  * **Integration Layer** — MCP for tool/context access, semantic indexing for data retrieval
  * **Orchestration Layer** — Multi-agent workflow coordination

Agents act as both **A2A Clients** (initiating tasks) and **A2A Servers** (processing incoming tasks), enabling bidirectional communication.

===== How It Works =====

A2A follows a four-step communication flow:

**1. Agent Discovery**: Clients fetch an **Agent Card** — a metadata JSON file hosted at ''/.well-known/agent.json'' — that describes the agent's capabilities, endpoints, and preferences. This enables automatic discovery and matching of suitable agents.

**2. Task Initiation**: The client submits a **Task Object** (structured JSON with objectives, inputs, and parameters) to the remote agent's server endpoint.

**3. Messaging and Status Updates**: Agents exchange **messages** containing "parts" (text, images, or UI negotiations). The protocol supports push notifications, streaming for real-time progress, and status tracking (pending, working, completed).

**4. Result Delivery**: The server returns **artifacts** in multi-format outputs. Long-running tasks are supported with human-in-the-loop feedback capabilities.

===== Relationship to MCP =====

A2A and MCP are complementary protocols that serve different purposes in the agent ecosystem:

^ Aspect ^ A2A (Google) ^ MCP (Anthropic) ^
| Primary Focus | Inter-agent communication | Single-agent tool/context integration |
| Scope | Multi-agent networking across vendors | Plugins for data/tools within one agent |
| Analogy | Networking layer | Plugin system |
| Use Case | Large-scale collaborative workflows | Enhancing individual agent capabilities |
| Launched | April 2025 | November 2024 |
| Adoption | 50+ companies | Widespread across LLM providers |

In practice, they are used together: A2A handles inter-agent communication while MCP handles each agent's connection to enterprise tools and data sources.

===== Key Features =====

  * **Standards-Based**: Built on HTTP, SSE, and JSON-RPC for easy adoption
  * **Secure by Default**: Enterprise-grade authentication (OAuth, OpenAPI parity)
  * **Long-Running Tasks**: Real-time updates and notifications for tasks spanning hours or days
  * **Modality Agnostic**: Supports text, audio, video streaming, and UI negotiation (iframes, forms)
  * **Framework Agnostic**: Works across any agent framework or cloud platform
  * **Scalable**: Plug-and-play cross-platform interoperability

===== Agent Card Example =====

<code python>
# Fetching an Agent Card for discovery
import httpx

async def discover_agent(base_url: str) -> dict:
    async with httpx.AsyncClient() as client:
        response = await client.get(f'{base_url}/.well-known/agent.json')
        card = response.json()
        # Card contains: name, description, capabilities, endpoints
        print(f'Agent: {card["name"]}')
        print(f'Capabilities: {card["capabilities"]}')
        return card

# Sending a task to a discovered agent
async def send_task(agent_url: str, task: dict) -> dict:
    payload = {
        "jsonrpc": "2.0",
        "method": "tasks/send",
        "params": task,
        "id": "1"
    }
    async with httpx.AsyncClient() as client:
        response = await client.post(agent_url, json=payload)
        return response.json()
</code>

===== References =====

  * [[https://developers.googleblog.com/en/a2a-a-new-era-of-agent-interoperability/|Google Developers Blog — A2A: A New Era of Agent Interoperability]]
  * [[https://github.com/google/a2a|A2A Protocol GitHub Repository]]
  * [[https://cloud.google.com/blog/topics/developers-practitioners/building-connected-agents-with-mcp-and-a2a|Google Cloud — Building Connected Agents with MCP and A2A]]
  * [[https://developers.googleblog.com/developers-guide-to-ai-agent-protocols/|Google Developers Guide to AI Agent Protocols]]

===== See Also =====

  * [[mcp|Model Context Protocol (MCP)]] — Anthropic's tool integration protocol
  * [[multi_agent_systems]] — Multi-agent system architectures
  * [[claude_agent_sdk]] — Anthropic Claude Agent SDK
  * [[openai_agents_sdk]] — OpenAI Agents SDK