Weather MCP Server

Instructions

Implementation Reference

• src/index.ts:19-65 (registration)

  Registers the get_alerts tool with the MCP server using server.tool(), including name, description, Zod input schema, and inline async handler function.

  server.tool(
    "get_alerts",
    "Get weather alerts for a state",
    {
      state: z.string().length(2).describe("Two-letter state code (e.g. CA, NY)"),
    },
    async ({ state }) => {
      const stateCode = state.toUpperCase();
      const alertsUrl = `${NWS_API_BASE}/alerts?area=${stateCode}`;
      const alertsData = await makeNWSRequest<AlertsResponse>(alertsUrl);
  
      if (!alertsData) {
        return {
          content: [
            {
              type: "text",
              text: "Failed to retrieve alerts data",
            },
          ],
        };
      }
  
      const features = alertsData.features || [];
      if (features.length === 0) {
        return {
          content: [
            {
              type: "text",
              text: `No active alerts for ${stateCode}`,
            },
          ],
        };
      }
  
      const formattedAlerts = features.map(formatAlert);
      const alertsText = `Active alerts for ${stateCode}:\n\n${formattedAlerts.join("\n")}`;
  
      return {
        content: [
          {
            type: "text",
            text: alertsText,
          },
        ],
      };
    },
  );

• src/index.ts:25-64 (handler)

  The async handler function implementing get_alerts tool logic: uppercases state code, constructs NWS alerts URL, fetches data with makeNWSRequest, handles no-data cases, maps features to formatAlert, joins formatted alerts into text response.

  async ({ state }) => {
    const stateCode = state.toUpperCase();
    const alertsUrl = `${NWS_API_BASE}/alerts?area=${stateCode}`;
    const alertsData = await makeNWSRequest<AlertsResponse>(alertsUrl);
  
    if (!alertsData) {
      return {
        content: [
          {
            type: "text",
            text: "Failed to retrieve alerts data",
          },
        ],
      };
    }
  
    const features = alertsData.features || [];
    if (features.length === 0) {
      return {
        content: [
          {
            type: "text",
            text: `No active alerts for ${stateCode}`,
          },
        ],
      };
    }
  
    const formattedAlerts = features.map(formatAlert);
    const alertsText = `Active alerts for ${stateCode}:\n\n${formattedAlerts.join("\n")}`;
  
    return {
      content: [
        {
          type: "text",
          text: alertsText,
        },
      ],
    };
  },

• src/index.ts:22-24 (schema)

  Zod input schema for get_alerts tool, validating 'state' as exactly 2-character string with description.

  {
    state: z.string().length(2).describe("Two-letter state code (e.g. CA, NY)"),
  },

• src/weather-utils.ts:32-42 (helper)

  Helper function to format a single AlertFeature into a readable multi-line string, used in get_alerts handler.

  export function formatAlert(feature: AlertFeature): string {
    const props = feature.properties;
    return [
      `Event: ${props.event || "Unknown"}`,
      `Area: ${props.areaDesc || "Unknown"}`,
      `Severity: ${props.severity || "Unknown"}`,
      `Status: ${props.status || "Unknown"}`,
      `Headline: ${props.headline || "No headline"}`,
      "---",
    ].join("\n");
  }

• src/weather-utils.ts:3-19 (helper)

  Generic helper function for making authenticated fetch requests to NWS API, returning typed JSON or null on error, used in get_alerts to fetch alerts data.

  export async function makeNWSRequest<T>(url: string): Promise<T | null> {
    const headers = {
      "User-Agent": USER_AGENT,
      Accept: "application/geo+json",
    };
  
    try {
      const response = await fetch(url, { headers });
      if (!response.ok) {
        throw new Error(`HTTP error! status: ${response.status}`);
      }
      return (await response.json()) as T;
    } catch (error) {
      console.error("Error making NWS request:", error);
      return null;
    }
  }