Improve Codebase Architecture

Surface architectural friction and propose deepening opportunities, refactors that turn shallow modules into deep ones. The aim is testability and AI-navigability.

Glossary

Use these terms exactly in every suggestion. Consistent language is the point, don't drift into "component," "service," "API," or "boundary." Full definitions in LANGUAGE.md.

• Module, anything with an interface and an implementation (function, class, package, slice).
• Interface, everything a caller must know to use the module: types, invariants, error modes, ordering, config. Not just the type signature.
• Implementation, the code inside.
• Depth, leverage at the interface: a lot of behaviour behind a small interface. Deep = high leverage. Shallow = interface nearly as complex as the implementation.
• Seam, where an interface lives; a place behaviour can be altered without editing in place. (Use this, not "boundary.")
• Adapter, a concrete thing satisfying an interface at a seam.
• Leverage, what callers get from depth.
• Locality, what maintainers get from depth: change, bugs, knowledge concentrated in one place.

Key principles (see LANGUAGE.md for the full list):

• Deletion test: imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep.
• The interface is the test surface.
• One adapter = hypothetical seam. Two adapters = real seam.

This skill is informed by the project's domain model. The domain language gives names to good seams; ADRs record decisions the skill should not re-litigate.

Process

1. Explore

Read the project's domain glossary and any ADRs in the area you're touching first.

Then use the Agent tool with subagent_type=Explore to walk the codebase. Don't follow rigid heuristics, explore organically and note where you experience friction: