char-index-mcp

Instructions

Implementation Reference

• char_index_mcp/server.py:65-89 (handler)

  Implements the find_nth_substring tool logic using a while loop with str.find() to locate the starting index of the nth occurrence of the substring, handling overlaps by advancing start by 1.

  def find_nth_substring(
      text: Annotated[str, "Text to search in"],
      substring: Annotated[str, "Substring to find"],
      n: Annotated[int, "Which occurrence to find (1-based)"] = 1
  ) -> int:
      """Find starting index of nth occurrence of a substring. Returns -1 if not found."""
      if not substring:
          raise ValueError("substring cannot be empty")
      if n < 1:
          raise ValueError("n must be >= 1")
      
      count = 0
      start = 0
      
      while True:
          index = text.find(substring, start)
          if index == -1:
              return -1
          
          count += 1
          if count == n:
              return index
          
          start = index + 1

• char_index_mcp/server.py:64-64 (registration)

  The @mcp.tool() decorator from FastMCP registers the find_nth_substring function as an MCP tool.

  @mcp.tool()

• char_index_mcp/server.py:65-69 (schema)

  The function signature with Annotated type hints defines the input schema: text (str), substring (str), n (int, default 1) and output int.

  def find_nth_substring(
      text: Annotated[str, "Text to search in"],
      substring: Annotated[str, "Substring to find"],
      n: Annotated[int, "Which occurrence to find (1-based)"] = 1
  ) -> int: