MCP Server Integration: Turning Any External Service into an Agent Tool

Function Tools are good for custom logic, but to connect existing external services to an Agent—like a MySQL database, Redis cache, or gRPC service—MCP (Model Context Protocol) is a more elegant approach.

What is MCP

MCP is a protocol defining how Agents communicate with external services. Its design philosophy: turn any external service into a Tool an Agent can call, without writing hard-coded integration logic in Agent code.

ADK Go Agent ← MCP Client ← MCP Protocol → MCP Server ← External Service
                                                  ↓
                                           MySQL / Redis / gRPC / HTTP

MCP Core Concepts

Using MCP Tools

ADK Go provides mcp package to connect to MCP Servers:

import "google.golang.org/adk/tool/mcp"

mcpTool, err := mcp.NewMCPTool(ctx,
    mcp.WithServerURL("http://localhost:8081/mcp"),
)

Then register to Agent:

Tools: []tool.Tool{
    mcpTool,  // All Tools exposed by MCP Server
}

Complete Example: Connecting a Weather MCP Server

Say there’s an MCP Server exposing two Tools:

• get_weather: query weather
• get_forecast: query forecast

1. Start MCP Server

MCP Server can be implemented in any language, here using Python as example:

# mcp_server.py
from mcp.server import MCPServer

server = MCPServer("weather-service")

@server.tool("get_weather")
def get_weather(city: str) -> str:
    return f"Weather in {city}: sunny, 22°C"

@server.tool("get_forecast")
def get_forecast(city: str, days: int) -> str:
    return f"{city} {days}-day forecast: sunny, rainy, cloudy"

if __name__ == "__main__":
    server.run()

Start:

python mcp_server.py --port 8081

2. Go Side Integration

mcpTool, err := mcp.NewMCPTool(ctx,
    mcp.WithServerURL("http://localhost:8081/mcp"),
)
if err != nil {
    log.Fatalf("Failed to connect to MCP server: %v", err)
}

agent, _ := llmagent.New(llmagent.Config{
    Name:        "weather_assistant",
    Model:       model,
    Instruction: "你是天气助手，用工具查询天气信息。",
    Tools: []tool.Tool{
        mcpTool,  // Register MCP Tool
    },
})

Agent will automatically recognize the two Tools exposed by MCP Server, can call them directly.

MCP Advantages

vs Hand-written Tools

vs REST API Direct Connect

MCP Security Considerations

Auth

MCP Server can add auth:

mcpTool, err := mcp.NewMCPTool(ctx,
    mcp.WithServerURL("http://localhost:8081/mcp"),
    mcp.WithAuth(mcp.BearerToken("your-token")),
)

Network Isolation

Don’t expose internal MCP Server directly to public net. Use VPC or VPN:

mcp.WithTLS(true),  // Force TLS
mcp.WithServerURL("https://internal-mcp.vpc.local/mcp"),

Common Issues

Q: MCP Server unresponsive A: Check if Server is running: curl http://localhost:8081/mcp/health. Confirm network connectivity.

Q: MCP Tool not recognized by Agent A: Confirm MCP Server correctly exposed Tool definitions. Check Agent startup logs, see if MCP Tool loaded successfully.

Q: Hard to access MCP Server from Mainland China A: If MCP Server deployed on internal network, ensure Agent’s network can access Server. Consider exposing MCP Server to public net with TLS.

Next Steps

MCP done. Next, OpenAPI Tool—dynamically generate Tools from OpenAPI spec, suitable for services with REST API docs but no MCP interface.

← Function Tool Performance | OpenAPI Tool →

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