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 that hide complexity behind small interfaces. ## Core Features & Use Cases - Deep-module vocabulary: Precise definitions of module, interface, implementation, depth, seam, adapter, leverage, and locality, with rejected framings explicitly called out. - 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. - Design It Twice workflow: Spawns parallel sub-agents to produce radically different interface designs, then compares them on depth, locality, and seam placement. - Use Case: When refactoring a cluster of shallow service classes, use this Skill to decide where the seam goes, which dependencies need adapters, and how to write tests at the new interface while deleting obsolete unit tests. ## Quick Start Ask the AI to review a module's interface using the codebase-design skill and propose a deeper design with a clean seam and test strategy.
