MCP Weather Server

Instructions

Implementation Reference

• src/index.ts:92-133 (handler)

  The main handler function for the 'get-alerts' tool. Fetches alerts from the National Weather Service API for a given state code, handles errors and empty results, formats the alerts using the formatAlert helper, and returns them as formatted text content.

  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:89-91 (schema)

  Input schema for the 'get-alerts' tool, validating the 'state' parameter as a two-letter string using Zod.

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

• src/index.ts:86-134 (registration)

  Registration of the 'get-alerts' tool on the MCP server, specifying name, description, input schema, and inline 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:48-58 (helper)

  Helper function to format a single weather alert feature into a multi-line string, used within the get-alerts handler.

  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/index.ts:19-35 (helper)

  Generic helper function for making authenticated requests to the National Weather Service API, used by the get-alerts handler.

  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;
    }
  }