What problem does it solve? Codebases accumulate shallow modules whose interfaces are nearly as complex as their implementations, making code hard to test, navigate, and refactor. This Skill provides a shared vocabulary and decision framework for designing deep modules so interfaces stay small while behaviour concentrates behind clean seams. ## Core Features & Use Cases - Deep-module vocabulary: Precise definitions of module, interface, seam, adapter, depth, leverage, and locality, with rejected framings explicitly called out to keep language consistent across a team. - Dependency-aware deepening: Classifies dependencies as in-process, local-substitutable, remote-but-owned, or true external, and prescribes ports-and-adapters or mock strategies for each category. - Design It Twice workflow: Spawns parallel sub-agents that each produce a radically different interface for a candidate module, then compares them on depth, locality, and seam placement with an opinionated recommendation. - Use Case: When refactoring a cluster of shallow service classes, use this Skill to decide where the seam goes, define a port with production and in-memory test adapters, and rewrite tests against the new deep interface. ## Quick Start Ask the assistant to review a module's interface using the codebase-design vocabulary and propose a deeper design with a clear seam and test strategy.
