quickstart-resources/weather-server-go/main.go

package main

import (
	"cmp"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"strings"

	"github.com/modelcontextprotocol/go-sdk/mcp"
)

const (
	NWSAPIBase = "https://api.weather.gov"
	UserAgent  = "weather-app/1.0"
)

type ForecastInput struct {
	Latitude  float64 `json:"latitude" jsonschema:"Latitude of the location"`
	Longitude float64 `json:"longitude" jsonschema:"Longitude of the location"`
}

type AlertsInput struct {
	State string `json:"state" jsonschema:"Two-letter US state code (e.g. CA, NY)"`
}

type PointsResponse struct {
	Properties struct {
		Forecast string `json:"forecast"`
	} `json:"properties"`
}

type ForecastResponse struct {
	Properties struct {
		Periods []ForecastPeriod `json:"periods"`
	} `json:"properties"`
}

type ForecastPeriod struct {
	Name             string `json:"name"`
	Temperature      int    `json:"temperature"`
	TemperatureUnit  string `json:"temperatureUnit"`
	WindSpeed        string `json:"windSpeed"`
	WindDirection    string `json:"windDirection"`
	DetailedForecast string `json:"detailedForecast"`
}

type AlertsResponse struct {
	Features []AlertFeature `json:"features"`
}

type AlertFeature struct {
	Properties AlertProperties `json:"properties"`
}

type AlertProperties struct {
	Event       string `json:"event"`
	AreaDesc    string `json:"areaDesc"`
	Severity    string `json:"severity"`
	Description string `json:"description"`
	Instruction string `json:"instruction"`
}

func makeNWSRequest[T any](ctx context.Context, url string) (*T, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, fmt.Errorf("failed to create request: %w", err)
	}

	req.Header.Set("User-Agent", UserAgent)
	req.Header.Set("Accept", "application/geo+json")

	client := http.DefaultClient
	resp, err := client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("failed to make request to %s: %w", url, err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		body, _ := io.ReadAll(resp.Body)
		return nil, fmt.Errorf("HTTP error %d: %s", resp.StatusCode, string(body))
	}

	var result T
	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
		return nil, fmt.Errorf("failed to decode response: %w", err)
	}

	return &result, nil
}

func formatAlert(alert AlertFeature) string {
	props := alert.Properties
	event := cmp.Or(props.Event, "Unknown")
	areaDesc := cmp.Or(props.AreaDesc, "Unknown")
	severity := cmp.Or(props.Severity, "Unknown")
	description := cmp.Or(props.Description, "No description available")
	instruction := cmp.Or(props.Instruction, "No specific instructions provided")

	return fmt.Sprintf(`
Event: %s
Area: %s
Severity: %s
Description: %s
Instructions: %s
`, event, areaDesc, severity, description, instruction)
}

func formatPeriod(period ForecastPeriod) string {
	return fmt.Sprintf(`
%s:
Temperature: %d°%s
Wind: %s %s
Forecast: %s
`, period.Name, period.Temperature, period.TemperatureUnit,
		period.WindSpeed, period.WindDirection, period.DetailedForecast)
}

func getForecast(ctx context.Context, req *mcp.CallToolRequest, input ForecastInput) (
	*mcp.CallToolResult, any, error,
) {
	// Get points data
	pointsURL := fmt.Sprintf("%s/points/%f,%f", NWSAPIBase, input.Latitude, input.Longitude)
	pointsData, err := makeNWSRequest[PointsResponse](ctx, pointsURL)
	if err != nil {
		return &mcp.CallToolResult{
			Content: []mcp.Content{
				&mcp.TextContent{Text: "Unable to fetch forecast data for this location."},
			},
		}, nil, nil
	}

	// Get forecast data
	forecastURL := pointsData.Properties.Forecast
	if forecastURL == "" {
		return &mcp.CallToolResult{
			Content: []mcp.Content{
				&mcp.TextContent{Text: "Unable to fetch forecast URL."},
			},
		}, nil, nil
	}

	forecastData, err := makeNWSRequest[ForecastResponse](ctx, forecastURL)
	if err != nil {
		return &mcp.CallToolResult{
			Content: []mcp.Content{
				&mcp.TextContent{Text: "Unable to fetch detailed forecast."},
			},
		}, nil, nil
	}

	// Format the periods
	periods := forecastData.Properties.Periods
	if len(periods) == 0 {
		return &mcp.CallToolResult{
			Content: []mcp.Content{
				&mcp.TextContent{Text: "No forecast periods available."},
			},
		}, nil, nil
	}

	// Show next 5 periods
	var forecasts []string
	for i := range min(5, len(periods)) {
		forecasts = append(forecasts, formatPeriod(periods[i]))
	}

	result := strings.Join(forecasts, "\n---\n")

	return &mcp.CallToolResult{
		Content: []mcp.Content{
			&mcp.TextContent{Text: result},
		},
	}, nil, nil
}

func getAlerts(ctx context.Context, req *mcp.CallToolRequest, input AlertsInput) (
	*mcp.CallToolResult, any, error,
) {
	// Build alerts URL
	stateCode := strings.ToUpper(input.State)
	alertsURL := fmt.Sprintf("%s/alerts/active/area/%s", NWSAPIBase, stateCode)

	alertsData, err := makeNWSRequest[AlertsResponse](ctx, alertsURL)
	if err != nil {
		return &mcp.CallToolResult{
			Content: []mcp.Content{
				&mcp.TextContent{Text: "Unable to fetch alerts or no alerts found."},
			},
		}, nil, nil
	}

	// Check if there are any alerts
	if len(alertsData.Features) == 0 {
		return &mcp.CallToolResult{
			Content: []mcp.Content{
				&mcp.TextContent{Text: "No active alerts for this state."},
			},
		}, nil, nil
	}

	// Format alerts
	var alerts []string
	for _, feature := range alertsData.Features {
		alerts = append(alerts, formatAlert(feature))
	}

	result := strings.Join(alerts, "\n---\n")

	return &mcp.CallToolResult{
		Content: []mcp.Content{
			&mcp.TextContent{Text: result},
		},
	}, nil, nil
}

func main() {
	// Create MCP server
	server := mcp.NewServer(&mcp.Implementation{
		Name:    "weather",
		Version: "1.0.0",
	}, nil)

	// Add get_forecast tool
	mcp.AddTool(server, &mcp.Tool{
		Name:        "get_forecast",
		Description: "Get weather forecast for a location",
	}, getForecast)

	// Add get_alerts tool
	mcp.AddTool(server, &mcp.Tool{
		Name:        "get_alerts",
		Description: "Get weather alerts for a US state",
	}, getAlerts)

	// Run server on stdio transport
	if err := server.Run(context.Background(), &mcp.StdioTransport{}); err != nil {
		log.Fatal(err)
	}
}