SOLID

Overview

Apply SOLID so behavior is easy to change and to test without a pile of mocks.

Companion skills: shape the product with grill → write-spec; prove behavior with tdd + test-design. This skill is only structure and dependency direction.

Core principle: hard to test usually means hard to use — fix the boundaries, don't paper over them with mocks.

Trigger modes (important)

Mode	When	What to do
Gate (primary)	After write-spec (or waived design), before write-plan, if the slice adds modules / ports / IO edges	Full workflow below — announce `Using solid to …`
Rescue	Tests need ≥3 mocks; review/PR coupling; mid-tdd stuck on structure	Full workflow; then return to caller
Smell only (inside grill)	Comparing approaches that imply different boundaries	Do not load this skill as the main skill. Grill keeps the session; option labels may note one boundary smell ("this binds domain to SQL → mock pile later"). Formal solid waits for Gate

Default: if product "what/why" is still open → grill, not solid.