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; 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.

When to Use

• During grill design sections (architecture / components / interfaces)
• Before tdd when introducing new types, packages, or dependency edges
• Reviewing a PR that adds coupling or "needs 5 mocks to unit-test"
• Refactors aimed at testability

When NOT to use: pure glue scripts with no domain logic; drive-by renames; user explicitly asked for a throwaway spike.