design-an-interfacelisted

# Design an Interface Based on "Design It Twice" from "A Philosophy of Software Design": your first idea is unlikely to be the best. Generate multiple radically different designs, then compare. ## Invocation Position This is an invoked helper skill. It normally runs from `/write-a-prd` when a module boundary is still unresolved, and it can also support refactor or architecture work when interface tradeoffs are the main uncertainty. Use it when the open question is the shape of a module interface: method count, surface area, depth, caller ergonomics, or evolvability. Do not use it when the real uncertainty is the API contract itself — that belongs in `/api-design-review` — or when the work is already clear enough to keep shaping or implementing without a design fork. ## Workflow ### 1. Gather Requirements Before designing, understand: - [ ] What problem does this module solve? - [ ] Who are the callers? (other modules, external users, tests) - [ ] What are the key operations? - [ ] Any constraints? (performance, compatibility, existing patterns) - [ ] What should be hidden inside vs exposed? Ask: "What does this module need to do? Who will use it?" ### 2. Generate Designs (Parallel Sub-Agents) Spawn 3+ sub-agents simultaneously using Task tool. Each must produce a **radically different** approach. ``` Prompt template for each sub-agent: Design an interface for: [module description] Requirements: [gathered requirements] Constraints for this design: [assign a dif