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 so teams and AI agents name things consistently. - 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 functions, use this Skill to decide where the seam goes, whether a port is justified, 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 vocabulary and suggest deepening opportunities with a testing strategy.
