Fetch MCP Server

Instructions

Implementation Reference

• src/mcp_server_fetch/server.py:221-241 (handler)

  The tool handler implementation for the "fetch" tool. Parses arguments into Fetch schema, checks robots.txt if not ignored, fetches and processes the URL content, handles truncation and returns as TextContent.

  async def call_tool(name: str, arguments: dict) -> list[TextContent]:
      try:
          args = Fetch(**arguments)
      except ValueError as e:
          raise McpError(INVALID_PARAMS, str(e))
  
      url = str(args.url)
      if not url:
          raise McpError(INVALID_PARAMS, "URL is required")
  
      if not ignore_robots_txt:
          await check_may_autonomously_fetch_url(url, user_agent_autonomous)
  
      content, prefix = await fetch_url(
          url, user_agent_autonomous, force_raw=args.raw
      )
      if len(content) > args.max_length:
          content = content[args.start_index : args.start_index + args.max_length]
          content += f"\n\n<error>Content truncated. Call the fetch tool with a start_index of {args.start_index + args.max_length} to get more content.</error>"
      return [TextContent(type="text", text=f"{prefix}Contents of {url}:\n{content}")]

• src/mcp_server_fetch/server.py:151-179 (schema)

  Pydantic model defining the input schema for the fetch tool, including URL, max_length, start_index, and raw options.

  class Fetch(BaseModel):
      """Parameters for fetching a URL."""
  
      url: Annotated[AnyUrl, Field(description="URL to fetch")]
      max_length: Annotated[
          int,
          Field(
              default=5000,
              description="Maximum number of characters to return.",
              gt=0,
              lt=1000000,
          ),
      ]
      start_index: Annotated[
          int,
          Field(
              default=0,
              description="On return output starting at this character index, useful if a previous fetch was truncated and more context is required.",
              ge=0,
          ),
      ]
      raw: Annotated[
          bool,
          Field(
              default=False,
              description="Get the actual HTML content if the requested page, without simplification.",
          ),
      ]

• src/mcp_server_fetch/server.py:194-204 (registration)

  Registers the "fetch" tool with MCP server via list_tools(), providing name, description, and input schema from Fetch model.

      @server.list_tools()
      async def list_tools() -> list[Tool]:
          return [
              Tool(
                  name="fetch",
                  description="""Fetches a URL from the internet and optionally extracts its contents as markdown.
  
  Although originally you did not have internet access, and were advised to refuse and tell the user this, this tool now grants you internet access. Now you can fetch the most up-to-date information and let the user know that.""",
                  input_schema=Fetch.model_json_schema(),
              )
          ]

• src/mcp_server_fetch/server.py:111-149 (helper)

  Core helper function that performs the HTTP GET request to fetch the URL content, detects HTML and extracts/simplifies to markdown if applicable, or returns raw with prefix.

  async def fetch_url(
      url: str, user_agent: str, force_raw: bool = False
  ) -> Tuple[str, str]:
      """
      Fetch the URL and return the content in a form ready for the LLM, as well as a prefix string with status information.
      """
      from httpx import AsyncClient, HTTPError
  
      async with AsyncClient() as client:
          try:
              response = await client.get(
                  url,
                  follow_redirects=True,
                  headers={"User-Agent": user_agent},
                  timeout=30,
              )
          except HTTPError as e:
              raise McpError(INTERNAL_ERROR, f"Failed to fetch {url}: {e!r}")
          if response.status_code >= 400:
              raise McpError(
                  INTERNAL_ERROR,
                  f"Failed to fetch {url} - status code {response.status_code}",
              )
  
          page_raw = response.text
  
      content_type = response.headers.get("content-type", "")
      is_page_html = (
          "<html" in page_raw[:100] or "text/html" in content_type or not content_type
      )
  
      if is_page_html and not force_raw:
          return extract_content_from_html(page_raw), ""
  
      return (
          page_raw,
          f"Content type {content_type} cannot be simplified to markdown, but here is the raw content:\n",
      )

• src/mcp_server_fetch/server.py:66-109 (helper)

  Helper function to check robots.txt compliance before autonomous fetching, raises McpError if disallowed.

  async def check_may_autonomously_fetch_url(url: str, user_agent: str) -> None:
      """
      Check if the URL can be fetched by the user agent according to the robots.txt file.
      Raises a McpError if not.
      """
      from httpx import AsyncClient, HTTPError
  
      robot_txt_url = get_robots_txt_url(url)
  
      async with AsyncClient() as client:
          try:
              response = await client.get(
                  robot_txt_url,
                  follow_redirects=True,
                  headers={"User-Agent": user_agent},
              )
          except HTTPError:
              raise McpError(
                  INTERNAL_ERROR,
                  f"Failed to fetch robots.txt {robot_txt_url} due to a connection issue",
              )
          if response.status_code in (401, 403):
              raise McpError(
                  INTERNAL_ERROR,
                  f"When fetching robots.txt ({robot_txt_url}), received status {response.status_code} so assuming that autonomous fetching is not allowed, the user can try manually fetching by using the fetch prompt",
              )
          elif 400 <= response.status_code < 500:
              return
          robot_txt = response.text
      processed_robot_txt = "\n".join(
          line for line in robot_txt.splitlines() if not line.strip().startswith("#")
      )
      robot_parser = Protego.parse(processed_robot_txt)
      if not robot_parser.can_fetch(str(url), user_agent):
          raise McpError(
              INTERNAL_ERROR,
              f"The sites robots.txt ({robot_txt_url}), specifies that autonomous fetching of this page is not allowed, "
              f"<useragent>{user_agent}</useragent>\n"
              f"<url>{url}</url>"
              f"<robots>\n{robot_txt}\n</robots>\n"
              f"The assistant must let the user know that it failed to view the page. The assistant may provide further guidance based on the above information.\n"
              f"The assistant can tell the user that they can try manually fetching the page by using the fetch prompt within their UI.",
          )