Building Multi-Protocol Systems

MCP + A2A + ADK: The Full Stack

Production agent systems in 2026 use multiple protocols together. Here's how they fit:

The Three-Protocol Architecture

┌─────────────────────────────────────────────────┐
│              YOUR AGENT (built with ADK)         │
│                                                  │
│  ┌────────────────┐     ┌─────────────────────┐ │
│  │  MCP Clients   │     │   A2A Client         │ │
│  │  (Tools/Data)  │     │   (Agent Delegation) │ │
│  └───────┬────────┘     └──────────┬──────────┘ │
└──────────┼─────────────────────────┼────────────┘
           │                         │
    ┌──────▼──────┐          ┌──────▼──────────┐
    │ MCP Servers │          │ Remote A2A      │
    │ • Database  │          │ Agents          │
    │ • GitHub    │          │ • Travel Agent  │
    │ • Slack     │          │ • Legal Agent   │
    │ • Files     │          │ • Finance Agent │
    └─────────────┘          └─────────────────┘

When to Use Which

Production Implementation

from google.adk import Agent, MCPTool, A2AClient

# MCP tools for direct data access
db_tool = MCPTool(server="postgres-mcp", tool="query_customers")
slack_tool = MCPTool(server="slack-mcp", tool="send_message")

# A2A clients for agent delegation
travel_agent = A2AClient("https://travel.example.com")
legal_agent = A2AClient("https://legal.example.com")

# Build the orchestrator
orchestrator = Agent(
    name="executive_assistant",
    model="gemini-2.5-pro",
    tools=[db_tool, slack_tool],
    a2a_agents=[travel_agent, legal_agent],
    instruction="""You are an executive assistant. 
    Use MCP tools for data access (database, Slack).
    Delegate to the travel agent for booking tasks.
    Delegate to the legal agent for contract review."""
)

Integration Checklist

• ☐ Publish your Agent Card at /.well-known/agent-card.json
• ☐ Register MCP servers for all data/tool access
• ☐ Discover and validate remote A2A agents before production
• ☐ Implement BLOCKED state handling for A2A tasks
• ☐ Set up OpenTelemetry for cross-protocol observability
• ☐ Rate-limit A2A calls to prevent cascade failures
• ☐ Authenticate all inter-agent communication (OAuth 2.0)