Fetch MCP Server

Instructions

Implementation Reference

• src/Fetcher.ts:46-65 (handler)

  Core handler function that fetches HTML content using _fetch, applies length limits, and formats the response for MCP.

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

• src/index.ts:31-55 (registration)

  Tool registration in ListToolsRequest handler, defining name, description, and input schema.

    name: "fetch_html",
    description: "Fetch a website and return its unmodified contents as HTML",
    inputSchema: {
      type: "object",
      properties: {
        url: {
          type: "string",
          description: "URL of the website 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:144-147 (handler)

  Dispatch logic in CallToolRequest handler that invokes Fetcher.html for 'fetch_html' tool calls.

  if (request.params.name === "fetch_html") {
    const fetchResult = await Fetcher.html(validatedArgs);
    return fetchResult;
  }

• src/types.ts:5-10 (schema)

  Zod schema used to validate input arguments for fetch_html and other 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/Fetcher.ts:15-44 (helper)

  Private helper method for performing the actual fetch request with security checks and error handling.

  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`);
      }
    }
  }