Defining Server Tools

One of the core features of MCP is the ability for servers to expose “tools” that clients can execute. This guide explains how to define and register tools in your gomcp server application.

1. Define the Tool Structure (protocol.Tool)

First, you need to define the metadata for your tool using the protocol.Tool struct. This includes:

• Name: A unique string identifier for the tool (e.g., “calculate_sum”, “read_file”).
• Description: (Optional) A human-readable description of what the tool does.
• InputSchema: A protocol.ToolInputSchema defining the expected arguments. This uses a subset of JSON Schema:
  • Type: Should typically be “object”.
  • Properties: A map where keys are argument names and values are protocol.PropertyDetail structs describing the argument’s Type (e.g., “string”, “number”, “boolean”, “array”, “object”), Description, allowed Enum values, or Format.
  • Required: A slice of strings listing the names of mandatory arguments.
• Annotations: (Optional) A protocol.ToolAnnotations struct providing hints about the tool’s behavior (e.g., ReadOnlyHint, DestructiveHint).

Example: An “echo” tool definition

import "github.com/localrivet/gomcp/protocol"

var echoTool = protocol.Tool{
	Name:        "echo",
	Description: "Simple tool that echoes back the input text.",
	InputSchema: protocol.ToolInputSchema{
		Type: "object",
		Properties: map[string]protocol.PropertyDetail{
			"input": {
				Type:        "string",
				Description: "The text to be echoed back by the server.",
			},
			"prefix": {
				Type:        "string",
				Description: "An optional prefix to add to the echoed text.",
				// Note: Not listed in 'Required', so it's optional.
			},
		},
		Required: []string{"input"}, // Only 'input' is mandatory
	},
	Annotations: protocol.ToolAnnotations{
		// Example annotation: This tool doesn't modify state
		ReadOnlyHint: func(b bool) *bool { return &b }(true),
	},
}

2. Implement the Tool Handler Function (server.ToolHandlerFunc)

Next, create a function that implements the actual logic of your tool. This function must match the server.ToolHandlerFunc signature:

type ToolHandlerFunc func(ctx context.Context, arguments map[string]interface{}) (result []protocol.Content, err error)

• It receives a context.Context for cancellation/deadlines.
• It receives the arguments provided by the client in the tools/call request as a map[string]interface{}. You’ll need to perform type assertions to access argument values safely.
• It returns a slice of protocol.Content objects (e.g., protocol.TextContent, protocol.ImageContent) representing the successful result of the tool execution.
• It returns an error if the tool execution fails. This error will be converted into a JSON-RPC error response sent back to the client.

Example: Handler for the “echo” tool

import (
	"context"
	"fmt"
	"github.com/localrivet/gomcp/protocol"
)

func handleEchoTool(ctx context.Context, args map[string]interface{}) ([]protocol.Content, error) {
	// Safely get the required 'input' argument
	inputText, ok := args["input"].(string)
	if !ok || inputText == "" {
		// Return an error if 'input' is missing, empty, or wrong type
		return nil, fmt.Errorf("required argument 'input' (string) is missing or invalid")
	}

	// Safely get the optional 'prefix' argument
	prefixText, _ := args["prefix"].(string) // Ignore error if missing/wrong type, default to ""

	// Construct the result message
	resultText := prefixText + inputText

	// Return the result as TextContent
	result := []protocol.Content{
		protocol.TextContent{
			Type: "text", // Always specify the content type
			Text: resultText,
		},
	}
	return result, nil // Return nil error on success
}

3. Register the Tool with the Server

Finally, use the RegisterTool method on your server.Server instance before running the server. Pass the protocol.Tool definition and the corresponding handler function.

import (
	"log"
	"github.com/localrivet/gomcp/server"
	// ... other imports
)

func main() {
	// ... (Initialize serverInfo, opts, srv as shown in server.md) ...

	// Register the echo tool and its handler
	err := srv.RegisterTool(echoTool, handleEchoTool)
	if err != nil {
		log.Fatalf("Failed to register tool '%s': %v", echoTool.Name, err)
	}
	log.Printf("Registered tool: %s", echoTool.Name)

	// ... (Create transport and run server) ...
}

Now, when a client connects and sends a tools/call request for the “echo” tool with valid arguments, the handleEchoTool function will be executed, and its result will be sent back to the client. If the client sends a tools/list request, the echoTool definition will be included in the response.