Fetch MCP Server

Instructions

Implementation Reference

• src/Fetcher.ts:67-89 (handler)

  Core handler logic for 'fetch_json': fetches URL, parses JSON, stringifies it, applies length limits, returns formatted content or error.

  static async json(requestPayload: RequestPayload) {
    try {
      const response = await this._fetch(requestPayload);
      const json = await response.json();
      let jsonString = JSON.stringify(json);
      
      // Apply length limits
      jsonString = this.applyLengthLimits(
        jsonString,
        requestPayload.max_length ?? 5000,
        requestPayload.start_index ?? 0
      );
      
      return {
        content: [{ type: "text", text: jsonString }],
        isError: false,
      };
    } catch (error) {
      return {
        content: [{ type: "text", text: (error as Error).message }],
        isError: true,
      };
    }

• src/types.ts:5-10 (schema)

  Zod schema for validating input parameters to 'fetch_json' and other fetch tools.

  export const RequestPayloadSchema = z.object({
    url: z.string().url(),
    headers: z.record(z.string()).optional(),
    max_length: z.number().int().min(0).optional().default(downloadLimit),
    start_index: z.number().int().min(0).optional().default(0),
  });

• src/index.ts:109-134 (registration)

  Tool registration in ListTools handler, defining name, description, and input schema for 'fetch_json'.

  {
    name: "fetch_json",
    description: "Fetch a JSON file from a URL",
    inputSchema: {
      type: "object",
      properties: {
        url: {
          type: "string",
          description: "URL of the JSON to fetch",
        },
        headers: {
          type: "object",
          description: "Optional headers to include in the request",
        },
        max_length: {
          type: "number",
          description: `Maximum number of characters to return (default: ${downloadLimit})`,
        },
        start_index: {
          type: "number",
          description: "Start content from this character index (default: 0)",
        },
      },
      required: ["url"],
    },
  },

• src/index.ts:148-150 (handler)

  Dispatch logic in CallToolRequest handler that routes 'fetch_json' calls to Fetcher.json.

  if (request.params.name === "fetch_json") {
    const fetchResult = await Fetcher.json(validatedArgs);
    return fetchResult;

• src/Fetcher.ts:15-44 (helper)

  Shared helper method for performing the HTTP fetch with security checks and error handling, used by 'fetch_json'.

  private static async _fetch({
    url,
    headers,
  }: RequestPayload): Promise<Response> {
    try {
      if (is_ip_private(url)) {
        throw new Error(
          `Fetcher blocked an attempt to fetch a private IP ${url}. This is to prevent a security vulnerability where a local MCP could fetch privileged local IPs and exfiltrate data.`,
        );
      }
      const response = await fetch(url, {
        headers: {
          "User-Agent":
            "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36",
          ...headers,
        },
      });
  
      if (!response.ok) {
        throw new Error(`HTTP error: ${response.status}`);
      }
      return response;
    } catch (e: unknown) {
      if (e instanceof Error) {
        throw new Error(`Failed to fetch ${url}: ${e.message}`);
      } else {
        throw new Error(`Failed to fetch ${url}: Unknown error`);
      }
    }
  }