design.md: Skill for Claude Code

Typed Service Contracts (Spec & Handler Pattern)

This skill defines a Vertical Slice Architecture backed by Design by Contract (DbC) principles. It treats application logic as rigorously defined Units of Work where inputs are parsed (not just validated) and errors are treated as values (Result Pattern) rather than exceptions.

When to use this skill

• Building CLIs or Libraries: When you need strict boundaries between user input and system logic.
• Complex Validation: When inputs require transformation (parsing) before being useful (e.g., ensuring a string is a valid file path).
• High-Reliability Requirements: When you cannot afford unhandled runtime exceptions and need exhaustive error handling.
• Testing Focus: When you want to separate data validation tests from business logic tests.

Architecture Components

1. The Spec (spec.ts)

The "Contract" or "Port". It defines the What. It must contain:

• Input Schema: A Zod schema that parses raw input into a valid DTO.
• Output Schema: A Zod schema defining the successful data structure.
• Error Schema: A discriminated union of specific failure modes (not generic errors).
• Result Type: A DiscriminatedUnion of Success | Failure.
• Interface: The capability definition (e.g., interface ConfigureSpec).

2. The Handler (handler.ts)

The "Implementation" or "Adapter". It defines the How. It must:

• Implement the Interface defined in the Spec.
• Be an "Impure" class that handles side effects (File System, API calls).
• NEVER throw exceptions. It must catch internal errors and map them to the Result type.

How to use it

Step 1: Define the Contract (spec.ts)

Follow this template to define the boundaries.

import { z } from 'zod';

// 1. VALIDATION HELPERS (Reusable Refinements)
export const SafePathSchema = z.string()
  .min(1)
  .refine(p => !p.includes('..'), "No traversal allowed");

// 2. INPUT (The Command) - "Parse, don't validate"
export const MyTaskInputSchema = z.object({
  path: SafePathSchema,
  force: z.boolean().default(false),
});
export type MyTaskInput = z.infer<typeof MyTaskInputSchema>;

// 3. ERROR CODES (Exhaustive)
export const MyTaskErrorCode = z.enum([
  'FILE_NOT_FOUND',
  'PERMISSION_DENIED', 
  'UNKNOWN_ERROR'
]);

// 4. RESULT (The Monad)
export const MyTaskSuccess = z.object({
  success: z.literal(true),
  data: z.string(), // The output payload
});

export const MyTaskFailure = z.object({
  success: z.literal(false),
  error: z.object({
    code: MyTaskErrorCode,
    message: z.string(),
    suggestion: z.string().optional(),
    recoverable: z.boolean(),
  })
});

export type MyTaskResult = 
  | z.infer<typeof MyTaskSuccess> 
  | z.infer<typeof MyTaskFailure>;

// 5. INTERFACE (The Capability)
export interface MyTaskSpec {
  execute(input: MyTaskInput): Promise<MyTaskResult>;
}