What problem does it solve? Design documents often contain only demo-level API sketches that lack complete contracts, error handling, and realistic examples, leaving developers without an implementable specification. This Skill upgrades those sketches into full API design documents with interface contracts, key code designs, and unified error code definitions. ## Core Features & Use Cases - Interface Contract Design: Defines complete contracts for each endpoint including method, path, authentication, permissions, request parameters, and success/error responses. - Key Code Design: Designs only behavior-critical code such as parameter validation, state transitions, core algorithms, and permission checks, skipping boilerplate. - Unified Error Code System: Produces a consistent error response format and categorized error code list across all endpoints. - Use Case: After finishing a technical design document with design-craft, run this Skill to produce an INDEX.md plus per-module API documents (e.g., users.md, orders.md) ready to hand to the development team. ## Quick Start Ask the AI to design the API for the current feature based on the existing requirement and design documents, then confirm each stage as it progresses.