Adding Tools & Schemas

1. Defining Argument Structs

In Go, define your tool arguments using standard Go structs with jsonschema tags:

type QueryDatabaseArgs struct {
    Query    string `json:"query" jsonschema:"The SQL query or search keyword to execute"`
    Limit    int    `json:"limit,omitempty" jsonschema:"Optional maximum number of records to return (default: 20)"`
    Category string `json:"category,omitempty" jsonschema:"Optional domain category filter"`
}

The Go SDK automatically inspects these struct tags to generate OpenAPI/JSON Schema metadata for connecting LLMs.

2. Implementing the Handler

Tool handlers must conform to the standard signature:

func queryDatabaseHandler(ctx context.Context, req *mcp.CallToolRequest, args QueryDatabaseArgs) (*mcp.CallToolResult, *QueryDatabaseResult, error) {
    if strings.TrimSpace(args.Query) == "" {
        return &mcp.CallToolResult{
            Content: []mcp.Content{
                &mcp.TextContent{Text: "Error: query parameter is required."},
            },
            IsError: true,
        }, nil, nil
    }

    result := &QueryDatabaseResult{ ... }

    // Dual-Emit: Markdown for general LLMs + typed JSON struct for programmatic agents
    return &mcp.CallToolResult{
        Content: []mcp.Content{
            &mcp.TextContent{Text: formatMarkdown(result)},
        },
    }, result, nil
}

3. Registering the Tool in mcp.go

Use mcp.AddTool to register your tool with human-readable titles, descriptions, and read-only annotations:

mcp.AddTool(server, &mcp.Tool{
    Name:        "query_database",
    Title:       "Query Database",
    Description: "Executes structured queries against the operational datastore.",
    Annotations: &mcp.ToolAnnotations{ReadOnlyHint: true},
}, queryDatabaseHandler)