MCP Avantage

Instructions

Implementation Reference

• src/index.ts:484-496 (registration)

  Registers the "crypto_exchangeRates" tool with MCP server, specifying name, description, input schema, and handler that calls the generic executeAvantageTool which invokes the Avantage library's crypto.exchangeRates method.

  server.addTool({
    name: "crypto_exchangeRates",
    description:
      "Fetches the realtime exchange rate for a cryptocurrency pair (e.g., BTC to USD).",
    parameters: schemas.CryptoExchangeRateParamsSchema,
    execute: (
      args,
      context // Let type be inferred
    ) =>
      executeAvantageTool("crypto_exchangeRates", args, context, (av, params) =>
        av.crypto.exchangeRates(params.from_currency, params.to_currency)
      ),
  });

• src/schemas.ts:99-102 (schema)

  Defines the Zod input validation schema for the crypto_exchangeRates tool, requiring from_currency and to_currency parameters.

  export const CryptoExchangeRateParamsSchema = z.object({
      from_currency: z.string().describe('The cryptocurrency symbol (e.g., "BTC").'),
      to_currency: z.string().describe('The physical currency symbol (e.g., "USD").'),
  }).describe('Parameters for fetching cryptocurrency exchange rates.')

• src/index.ts:38-115 (helper)

  Shared helper function that manages AVantage client instances, executes the library method passed as avantageMethod (here, av.crypto.exchangeRates), handles authentication, errors, and returns JSON stringified data.

  async function executeAvantageTool<TArgs, TResult>(
    toolName: string,
    args: TArgs,
    context: Context<Record<string, unknown> | undefined>, // Use the imported Context type directly
    avantageMethod: (
      av: AVantage,
      args: TArgs
    ) => Promise<{ error?: boolean; reason?: string; data?: TResult }>
  ): Promise<string> {
    logger.info(`Executing '${toolName}' tool for request ID: ${context}`);
    logger.debug(`Args for ${toolName}: ${JSON.stringify(args)}`);
  
    // --- Authentication & Resource Management ---
    // Access extraArgs safely - it might be null or undefined
    const extraArgsApiKey = context.extraArgs?.apiKey as string | undefined;
    const apiKey = extraArgsApiKey || config.apiKey;
  
    if (!apiKey) {
      logger.error(`'${toolName}' failed: Alpha Vantage API key missing.`);
      throw new UserError(apiKeyErrorMessage);
    }
    logger.debug(
      `Using AV API key (source: ${extraArgsApiKey ? "extraArgs" : "environment"}) for ${toolName}`
    );
  
    try {
      // Get or create AVantage instance managed by ResourceManager
      const av = await resourceManager.getResource<AVantage>(
        apiKey, // Cache key is the resolved API key
        "avantage_client", // Type identifier for logging
        async (key) => {
          // Factory Function
          logger.info(
            `Creating new AVantage instance for key ending ...${key.slice(-4)}`
          );
          // AVantage library reads AV_PREMIUM from process.env internally
          return new AVantage(key);
        },
        async (avInstance) => {
          // Cleanup Function (no-op needed for AVantage)
          logger.debug(`Destroying AVantage instance (no-op)`);
        }
      );
  
      // --- Library Call ---
      const result = await avantageMethod(av, args);
  
      // --- Response Handling ---
      if (result.error) {
        logger.warn(
          `'${toolName}' failed. Reason from avantage: ${result.reason}`
        );
        throw new UserError(result.reason || `Tool '${toolName}' failed.`);
      }
  
      if (result.data === undefined || result.data === null) {
        logger.warn(`'${toolName}' completed successfully but returned no data.`);
        return "null"; // Return string "null" for empty data
      }
  
      logger.info(`'${toolName}' completed successfully.`);
      // Stringify the data part of the response
      return JSON.stringify(result.data);
    } catch (error: any) {
      logger.error(
        `Error during execution of '${toolName}': ${error.message}`,
        error
      );
      // If it's already a UserError, rethrow it
      if (error instanceof UserError) {
        throw error;
      }
      // Otherwise, wrap it in a UserError
      throw new UserError(
        `An unexpected error occurred while executing tool '${toolName}': ${error.message}`
      );
    }
  }