mcp-nixos

Instructions

Implementation Reference

• mcp_nixos/server.py:1111-1141 (handler)

  Main handler function for 'darwin_options_by_prefix' tool. Registers via @mcp.tool() decorator, parses nix-darwin HTML documentation using parse_html_options helper, filters by prefix, sorts, and formats results as plain text list.
  @mcp.tool() async def darwin_options_by_prefix(option_prefix: str) -> str: """Get nix-darwin options matching a specific prefix. Useful for browsing options under a category or finding exact option names. Args: option_prefix: The prefix to match (e.g., 'system.defaults' or 'services') Returns: Plain text list of options with the given prefix, including descriptions """ try: options = parse_html_options(DARWIN_URL, "", option_prefix) if not options: return f"No nix-darwin options found with prefix '{option_prefix}'" results = [] results.append(f"nix-darwin options with prefix '{option_prefix}' ({len(options)} found):\n") for opt in sorted(options, key=lambda x: x["name"]): results.append(f"• {opt['name']}") if opt["description"]: results.append(f" {opt['description']}") results.append("") return "\n".join(results).strip() except Exception as e: return error(str(e))
• mcp_nixos/server.py:247-336 (helper)

  Key helper function parse_html_options that fetches and parses HTML documentation from nix-darwin (or home-manager) sites using BeautifulSoup, extracts option names, descriptions, types from <dt>/<dd> pairs, applies prefix/query filters. Called by darwin_options_by_prefix with DARWIN_URL and empty query.
  def parse_html_options(url: str, query: str = "", prefix: str = "", limit: int = 100) -> list[dict[str, str]]: """Parse options from HTML documentation.""" try: resp = requests.get(url, timeout=30) # Increase timeout for large docs resp.raise_for_status() # Use resp.content to let BeautifulSoup handle encoding detection # This prevents encoding errors like "unknown encoding: windows-1252" soup = BeautifulSoup(resp.content, "html.parser") options = [] # Get all dt elements dts = soup.find_all("dt") for dt in dts: # Get option name name = "" if "home-manager" in url: # Home Manager uses anchor IDs like "opt-programs.git.enable" anchor = dt.find("a", id=True) if anchor: anchor_id = anchor.get("id", "") # Remove "opt-" prefix and convert underscores if anchor_id.startswith("opt-"): name = anchor_id[4:] # Remove "opt-" prefix # Convert _name_ placeholders back to <name> name = name.replace("_name_", "<name>") else: # Fallback to text content name_elem = dt.find(string=True, recursive=False) if name_elem: name = name_elem.strip() else: name = dt.get_text(strip=True) else: # Darwin and fallback - use text content name = dt.get_text(strip=True) # Skip if it doesn't look like an option (must contain a dot) # But allow single-word options in some cases if "." not in name and len(name.split()) > 1: continue # Filter by query or prefix if query and query.lower() not in name.lower(): continue if prefix and not (name.startswith(prefix + ".") or name == prefix): continue # Find the corresponding dd element dd = dt.find_next_sibling("dd") if dd: # Extract description (first p tag or direct text) desc_elem = dd.find("p") if desc_elem: description = desc_elem.get_text(strip=True) else: # Get first text node, handle None case text = dd.get_text(strip=True) description = text.split("\n")[0] if text else "" # Extract type info - look for various patterns type_info = "" # Pattern 1: <span class="term">Type: ...</span> type_elem = dd.find("span", class_="term") if type_elem and "Type:" in type_elem.get_text(): type_info = type_elem.get_text(strip=True).replace("Type:", "").strip() # Pattern 2: Look for "Type:" in text elif "Type:" in dd.get_text(): text = dd.get_text() type_start = text.find("Type:") + 5 type_end = text.find("\n", type_start) if type_end == -1: type_end = len(text) type_info = text[type_start:type_end].strip() options.append( { "name": name, "description": description[:200] if len(description) > 200 else description, "type": type_info, } ) if len(options) >= limit: break return options except Exception as exc: raise DocumentParseError(f"Failed to fetch docs: {str(exc)}") from exc
• mcp_nixos/server.py:1111-1111 (registration)

  FastMCP decorator @mcp.tool() that registers the darwin_options_by_prefix function as an MCP tool.
  @mcp.tool()
• mcp_nixos/server.py:1112-1119 (schema)

  Input schema defined by function signature (option_prefix: str) and docstring Args section.
  async def darwin_options_by_prefix(option_prefix: str) -> str: """Get nix-darwin options matching a specific prefix. Useful for browsing options under a category or finding exact option names. Args: option_prefix: The prefix to match (e.g., 'system.defaults' or 'services')