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 change. 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 so teams and AI agents name things consistently. - Deepening guidance: Classifies dependencies (in-process, local-substitutable, remote-owned, true external) and prescribes ports-and-adapters strategies with replace-don't-layer testing. - 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 become injected ports, and how to write tests at the new interface. ## Quick Start Ask the AI to review a module in your codebase and propose a deeper interface using the codebase-design vocabulary.
