# Module guide

- A module is a unit with an interface and an implementation; its interface includes invariants, ordering, errors, configuration, and performance obligations as well as types.
- Prefer depth: useful behavior behind a small interface. Assess what callers must learn and where future changes will concentrate.
- A seam is where the interface lives. Name the dependency that varies there and the concrete adapters that justify it; do not add a speculative port for one fixed implementation.
- Test proposed behavior through the interface visible to callers. Identify the existing tests that would stay meaningful after an internal refactor.
- Classify dependencies as in-process, locally substitutable, owned remote, or external before recommending an adapter or verification seam.
- State what each candidate hides, what it exposes, and how an implementer can prove the selected behavior. A diagram or example may clarify a design, but it is not verification evidence.