A2A Protocol Implementation Patterns

Instructions

This skill provides comprehensive patterns for implementing the Agent2Agent (A2A) protocol in Google's Agent Development Kit (ADK). The A2A protocol standardizes communication between AI agents, enabling multi-agent collaboration across different platforms and frameworks.

What is A2A?

The Agent2Agent (A2A) protocol enables AI agents to:

• Discover each other's capabilities through Agent Cards
• Communicate securely using standardized JSON-RPC messages
• Collaborate across different frameworks (CrewAI, LangGraph, ADK)
• Work across deployment platforms (Cloud Run, Agent Engine, GKE)

Key Concept: A2A focuses on agent-to-agent collaboration in natural modalities, complementing MCP (Model Context Protocol) which handles tool/data connections.

Core Patterns

1. Exposing Agents via A2A (Server-Side)

When to use: Make your ADK agent available for other agents to consume

Template: templates/a2a-server.py

Key Components:

• AgentCard at /.well-known/agent.json - Advertises capabilities
• AgentExecutor - Handles incoming requests
• DefaultRequestHandler - Processes JSON-RPC messages
• A2AStarletteApplication - HTTP server implementation

Script: scripts/expose-agent.sh

2. Consuming External Agents (Client-Side)

When to use: Integrate remote A2A agents as sub-agents

Template: templates/a2a-client.py

Key Components:

• A2ACardResolver - Discovers remote agent capabilities
• send_task tool - Sends messages to remote agents
• Session tracking - Maintains context across interactions

Script: scripts/consume-agent.sh

3. Multi-Agent Communication

When to use: Orchestrate multiple specialized agents collaborating on complex tasks

Template: templates/multi-agent-orchestration.py

Pattern:

• Coordinator agent routes tasks
• Specialist agents handle specific domains
• Agent-to-agent messaging via A2A protocol
• Result aggregation and synthesis

Example: examples/purchasing-concierge/

4. Agent Card Configuration

When to use: Define agent capabilities for discovery

Template: templates/agent-card.json

Contents:

• Agent metadata (name, description, version)
• Capabilities and skills
• Supported modalities (text, audio, video)
• Endpoint URLs and protocol version
• Streaming support indicators

Script: scripts/generate-agent-card.sh

Implementation Patterns

Server-Side: Exposing an Agent

# templates/a2a-server.py structure
from adk import Agent
from a2a import AgentExecutor, DefaultRequestHandler, AgentCard

class MyAgentExecutor(AgentExecutor):
    """Handle incoming A2A requests"""
    async def execute(self, request):
        # Process request using your agent
        result = await self.agent.run(request.message)
        return result

# Configure Agent Card
agent_card = AgentCard(
    name="my-agent",
    description="Agent description",
    capabilities=["skill1", "skill2"],
    endpoint="https://my-agent.example.com"
)

# Expose via HTTP
from a2a import A2AStarletteApplication

app = A2AStarletteApplication(
    executor=MyAgentExecutor(),
    card=agent_card
)

Deployment:

# Deploy to Cloud Run
bash scripts/expose-agent.sh --platform cloud-run

# Deploy to Agent Engine
bash scripts/expose-agent.sh --platform agent-engine

# Deploy to GKE
bash scripts/expose-agent.sh --platform gke

Client-Side: Consuming an Agent

# templates/a2a-client.py structure
from adk import Agent
from a2a import A2ACardResolver, send_task

# Discover remote agent
resolver = A2ACardResolver()
agent_card = await resolver.resolve("https://remote-agent.example.com")

# Create tool to communicate with remote agent
send_task_tool = send_task(
    agent_url=agent_card.endpoint,
    session_id="unique-session-id"
)

# Use in your agent
my_agent = Agent(
    tools=[send_task_tool],
    # ... other config
)

# Agent can now invoke remote agent
result = await my_agent.run("Ask the remote agent to do something")

Multi-Agent Orchestration

# templates/multi-agent-orchestration.py structure
from adk import Agent
from a2a import A2ACardResolver, send_task

# Discover specialist agents
resolver = A2ACardResolver()
research_agent = await resolver.resolve("https://research-agent.example.com")
analysis_agent = await resolver.resolve("https://analysis-agent.example.com")
writing_agent = await resolver.resolve("https://writing-agent.example.com")

# Coordinator agent
coordinator = Agent(
    name="coordinator",
    tools=[
        send_task(agent_url=research_agent.endpoint),
        send_task(agent_url=analysis_agent.endpoint),
        send_task(agent_url=writing_agent.endpoint)
    ],
    instructions="""
    You coordinate multiple specialist agents:
    1. Use research agent to gather information
    2. Use analysis agent to process findings
    3. Use writing agent to synthesize results
    """
)

# Execute multi-agent workflow
result = await coordinator.run("Research and write a report on AI agents")

Agent Card Structure

{
  "id": "my-agent",
  "name": "My Agent",
  "description": "Description of agent capabilities",
  "version": "1.0.0",
  "url": "https://my-agent.example.com",
  "capabilities": {
    "skills": [
      {
        "name": "skill1",
        "description": "First skill description"
      },
      {
        "name": "skill2",
        "description": "Second skill description"
      }
    ],
    "modalities": ["text", "image"],
    "streaming": true
  },
  "protocol": {
    "version": "0.3",
    "transport": "grpc"
  }
}

Generation:

bash scripts/generate-agent-card.sh \
  --name "my-agent" \
  --description "Agent description" \
  --skills "skill1,skill2" \
  --modalities "text,image" \
  --url "https://my-agent.example.com"

Protocol Configuration

gRPC Transport (A2A v0.3+)

# templates/grpc-config.py
from a2a import A2AStarletteApplication, GrpcTransport

app = A2AStarletteApplication(
    executor=MyAgentExecutor(),
    transport=GrpcTransport(
        host="0.0.0.0",
        port=50051,
        secure=True,
        cert_file="/path/to/cert.pem",
        key_file="/path/to/key.pem"
    )
)

Security Cards (A2A v0.3+)

# templates/security-card.py
from a2a import SecurityCard, sign_card

# Create security card
security_card = SecurityCard(
    issuer="my-organization",
    audience=["trusted-agent-1", "trusted-agent-2"],
    permissions=["read", "write"]
)

# Sign the card
signed_card = sign_card(
    card=security_card,
    private_key="/path/to/private-key.pem"
)

JSON-RPC Message Format

Request:

{
  "id": "request-uuid",
  "jsonrpc": "2.0",
  "method": "message/send",
  "params": {
    "message": "Task description",
    "session_id": "session-uuid",
    "context": {}
  }
}

Response:

{
  "id": "request-uuid",
  "jsonrpc": "2.0",
  "result": {
    "message": "Agent response",
    "artifacts": [],
    "status": "completed"
  }
}

Scripts

1. Expose Agent via A2A

bash scripts/expose-agent.sh --platform cloud-run --region us-central1

What it does:

• Generates Agent Card at /.well-known/agent.json
• Creates Dockerfile with A2A server
• Deploys to specified platform
• Configures networking and security
• Returns agent endpoint URL

2. Consume Remote Agent

bash scripts/consume-agent.sh --url https://remote-agent.example.com

What it does:

• Resolves Agent Card from remote URL
• Validates capabilities
• Generates client code
• Creates send_task tool wrapper
• Provides integration example

3. Generate Agent Card

bash scripts/generate-agent-card.sh \
  --name "my-agent" \
  --description "Agent description" \
  --skills "research,analysis,writing"

What it does:

• Creates JSON Agent Card
• Validates against A2A schema
• Generates /.well-known/agent.json
• Provides endpoint configuration

4. Validate A2A Configuration

bash scripts/validate-a2a.sh --config agent-card.json

What it does:

• Checks Agent Card schema
• Validates endpoint accessibility
• Tests JSON-RPC message format
• Verifies security configuration

Templates

Python Templates

• templates/a2a-server.py - Server-side agent implementation
• templates/a2a-client.py - Client-side agent consumption
• templates/multi-agent-orchestration.py - Multi-agent coordination
• templates/grpc-config.py - gRPC transport configuration
• templates/security-card.py - Security card implementation

Go Templates

• templates/go/a2a-server.go - Go server implementation
• templates/go/a2a-client.go - Go client implementation

Configuration Templates

• templates/agent-card.json - Agent Card JSON structure
• templates/deployment-config.yaml - Cloud Run/GKE deployment
• templates/security-policy.json - Security configuration

Examples

Example 1: Research Agent Cluster

Location: examples/research-cluster/

Architecture:

• Coordinator agent orchestrates research workflow
• Search agent gathers information
• Analysis agent processes findings
• Writing agent synthesizes results

Communication: All agents communicate via A2A protocol

Example 2: E-Commerce Assistant

Location: examples/ecommerce-assistant/

Architecture:

• Customer-facing agent handles inquiries
• Inventory agent checks product availability
• Pricing agent calculates costs
• Payment agent processes transactions

Pattern: Hierarchical agent structure with A2A messaging

Example 3: Code Review System

Location: examples/code-review/

Architecture:

• Manager agent coordinates review process
• Style agent checks code formatting
• Security agent scans for vulnerabilities
• Performance agent analyzes efficiency

Integration: Each specialist agent is independent A2A service

Example 4: Data Pipeline

Location: examples/data-pipeline/

Architecture:

• Ingestion agent collects data
• Transformation agent processes data
• Validation agent checks quality
• Storage agent persists results

Workflow: Sequential A2A agent execution

Production Best Practices

1. Agent Discovery

• Host Agent Cards at /.well-known/agent.json
• Use semantic versioning for capabilities
• Document all available skills clearly
• Update cards when capabilities change

2. Error Handling

try:
    result = await send_task(remote_agent, task)
except A2AConnectionError:
    # Handle network failures
    result = fallback_handler(task)
except A2AAuthenticationError:
    # Handle auth failures
    log_security_event()
except A2ATimeoutError:
    # Handle timeouts
    retry_with_backoff()

3. Security

• Sign Agent Cards with private keys
• Validate incoming requests
• Use HTTPS/gRPC with TLS
• Implement authentication and authorization
• Rate limit agent-to-agent calls

4. Monitoring

# Track A2A metrics
metrics.record('a2a.request.count', {'agent': 'remote-agent'})
metrics.record('a2a.latency', latency_ms)
metrics.record('a2a.error.rate', error_count)

5. Testing

# Test agent exposure
bash scripts/validate-a2a.sh --config agent-card.json

# Test agent consumption
bash scripts/test-a2a-client.sh --url https://remote-agent.example.com

# Test multi-agent workflow
bash scripts/test-orchestration.sh --config multi-agent-config.yaml

Deployment Platforms

Cloud Run

# Deploy A2A agent to Cloud Run
gcloud run deploy my-agent \
  --source . \
  --region us-central1 \
  --allow-unauthenticated

Agent Card URL: https://my-agent-[hash]-uc.a.run.app/.well-known/agent.json

Agent Engine

# Deploy to Agent Engine
adk deploy --platform agent-engine --agent my-agent

Features: Managed scaling, built-in monitoring, A2A native

GKE (Google Kubernetes Engine)

# Deploy to GKE
kubectl apply -f templates/deployment-config.yaml

Benefits: Full control, custom scaling, multi-region

Framework Interoperability

A2A agents can use different frameworks:

ADK Agent:

from adk import Agent
agent = Agent(name="adk-agent", ...)

CrewAI Agent:

from crewai import Agent
agent = Agent(role="crew-agent", ...)

LangGraph Agent:

from langgraph import StateGraph
agent = StateGraph(...)

All communicate via A2A protocol - Framework is transparent to clients

Integration with Gemini API

# templates/gemini-integration.py
from adk import Agent
from vertexai.preview.generative_models import GenerativeModel

# ADK agent using Gemini
agent = Agent(
    name="gemini-agent",
    model=GenerativeModel("gemini-2.0-flash-exp"),
    tools=[send_task_tool]
)

# Expose via A2A
from a2a import A2AStarletteApplication
app = A2AStarletteApplication(
    executor=GeminiAgentExecutor(agent)
)

A2A + MCP Integration

# templates/a2a-mcp-integration.py
from adk import Agent
from a2a import send_task
from mcp import use_mcp_server

# Agent with both A2A (agents) and MCP (tools)
agent = Agent(
    name="hybrid-agent",
    # A2A: Communicate with other agents
    tools=[
        send_task(agent_url="https://research-agent.example.com")
    ],
    # MCP: Connect to data sources
    mcps=[
        use_mcp_server("filesystem"),
        use_mcp_server("database")
    ]
)

Use Case: Agent uses MCP for data access, A2A for agent collaboration

Requirements

Environment Variables:

• GOOGLE_CLOUD_PROJECT - GCP project ID (for Gemini/Vertex AI)
• GOOGLE_APPLICATION_CREDENTIALS - Service account key path
• A2A_AGENT_URL - Your agent's public URL (for card generation)

Dependencies:

pip install google-adk[a2a]
pip install google-cloud-aiplatform
pip install grpcio  # For gRPC transport

Infrastructure:

• GCP project with Vertex AI API enabled
• Cloud Run or Agent Engine (for deployment)
• Domain with HTTPS (for production Agent Cards)

Security: API Key Handling

CRITICAL: When generating any configuration files or code:

• NEVER hardcode actual API keys or secrets

• NEVER include real credentials in examples

• NEVER commit sensitive values to git

• ALWAYS use placeholders: your_service_key_here

• ALWAYS create .env.example with placeholders only

• ALWAYS add .env* to .gitignore (except .env.example)

• ALWAYS read from environment variables in code

• ALWAYS document where to obtain keys

Placeholder format: {service}_{env}_your_key_here

Example:

# .env.example (safe to commit)
GOOGLE_CLOUD_PROJECT=your_project_id_here
GOOGLE_APPLICATION_CREDENTIALS=/path/to/your_service_account_key.json
A2A_AGENT_URL=https://your_agent_url_here

# .env (NEVER commit)
GOOGLE_CLOUD_PROJECT=actual-project-id
GOOGLE_APPLICATION_CREDENTIALS=/actual/path/to/key.json
A2A_AGENT_URL=https://my-agent-xyz.run.app

Troubleshooting

Agent Card Not Found:

• Verify /.well-known/agent.json is accessible
• Check CORS configuration
• Validate JSON schema

Connection Refused:

• Confirm agent is running
• Check firewall rules
• Verify endpoint URL

Authentication Failed:

• Validate security card signature
• Check permissions
• Review audience list

Message Format Error:

• Verify JSON-RPC 2.0 format
• Check message structure
• Validate parameter types

Resources

Official Documentation:

• A2A Protocol: https://a2a-protocol.org/
• ADK A2A Guide: https://google.github.io/adk-docs/a2a/
• Google Cloud Blog: https://cloud.google.com/blog/products/ai-machine-learning/agent2agent-protocol-is-getting-an-upgrade

Code Examples:

• Python A2A Samples: https://github.com/a2aproject/a2a-samples/tree/main/samples/python/agents
• Purchasing Concierge Codelab: https://codelabs.developers.google.com/intro-a2a-purchasing-concierge

Community:

• A2A GitHub: https://github.com/a2aproject
• ADK Python: https://github.com/google/adk-python

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Plugin: google-adk Version: 1.0.0 Protocol Version: A2A v0.3+ Language Support: Python (stable), Go (stable) Deployment Platforms: Cloud Run, Agent Engine, GKE