starlark-mcp: Skill for Claude Code

Create Extension

Creates a new Starlark-based MCP extension for the starlark-mcp server.

Instructions

When the user requests a new extension:

1. Gather Requirements

   • Ask the user for the extension name (lowercase with underscores)
   • Ask what tools/capabilities the extension should provide
   • Determine if the extension needs:
     • External API access (HTTP calls)
     • Database access (SQLite, PostgreSQL)
     • File system operations
     • System commands (via exec.run())

2. Choose Implementation Approach

   • IMPORTANT: Default to using existing Starlark modules first
   • Use exec.run() for CLI tools (docker, git, kubectl, etc.) - this is almost always the right choice
   • Use http module for REST APIs
   • Use existing sqlite or postgres modules for databases
   • Only create new Rust modules if:
     • The CLI tool doesn't exist or lacks JSON output
     • You need direct library access for performance
     • The existing modules don't provide required functionality
   • Remember: exec.run() is simple, fast to implement, and what users already have installed

3. Create Extension File

   • Create extensions/{name}.star file
   • Follow the standard Starlark MCP extension structure

4. Extension Structure

   # Tool handler functions
   def tool_name(params):
       """Tool description"""
       # Extract parameters
       param = params.get("param_name", "default")
   
       # Validate inputs
       if not param:
           return error_response("param_name is required")
   
       # Implement logic
       result = do_something(param)
   
       # Return MCP response
       return {
           "content": [{"type": "text", "text": result}],
       }
   
   # Helper functions
   def error_response(message):
       """Create an error response"""
       return {
           "content": [{"type": "text", "text": "Error: " + message}],
           "isError": True,
       }
   
   # Extension definition
   def describe_extension():
       """Define the extension"""
       return Extension(
           name = "extension_name",
           version = "1.0.0",
           description = "Extension description",
           tools = [
               Tool(
                   name = "tool_name",
                   description = "Tool description",
                   parameters = [
                       ToolParameter(
                           name = "param_name",
                           param_type = "string",
                           required = True,
                           description = "Parameter description",
                       ),
                   ],
                   handler = tool_name,
               ),
           ],
       )