Adjudicate whether users can operate an interface — not whether it looks good. The job is a two-layer engine: principles (Gestalt, the UX laws, Nielsen/Shneiderman heuristics, Norman, cognitive load, accessibility) are the why; UI pattern families are the what; the principles select among the patterns. Every recommendation cites its source — the law, heuristic, or author it traces to.

When this applies

Reach for this skill when the concern is operability: "is this usable", "hard to use", "users get stuck / confused / drop off here", a heuristic / usability evaluation, applying a UX law (Fitts, Hick, Miller/Cowan, Jakob, …), reasoning about affordances / signifiers / feedback / cognitive load, choosing among UI pattern families (navigation, forms, search/filter, data display, feedback, action, disclosure, onboarding), or the usability side of accessibility (WCAG POUR, inclusive design). The core audit mode dispatches its heuristic evaluation here.

Not the visual look (core design/audit), the microcopy itself (content-design), the route through time / IA / funnel (journey), persuasion mechanics (behavioral), or truthful data encoding (data-viz).

Rules

Standing rules for every usability judgement. Kept separate so they don't dissolve into the procedure.

Cite the principle. Every recommendation names its source with originator/year where it has one: "Fitts's law (1954)", "Nielsen #5 error prevention (1994)", "Norman: signifiers over affordances (1988/2013)", "layer-cake scanning (NN/g)". No unsourced opinion. The citeable canon lives in references/usability-principles.md.
Laws are tendencies, not physics. The UX laws hold under conditions; they are cited to ground a recommendation, never as a compliance checklist (framework-as-deliverable is compliance theater). Knowing when a law does not apply is part of citing it honestly — e.g. Miller's 7±2 is about working memory, not visible on-screen menu items.
Fix usability before polish. The aesthetic-usability effect (Kurosu & Kashimura, 1995) means a pretty UI inflates subjective scores and masks real defects in testing. Order matters: resolve operability defects first, then let visual polish (core design) raise the ceiling.
Heuristic evaluation is a complement, not user testing. It finds likely problems (evaluator effect, low inter-rater reliability, unreliable severity), not real-world frequency — pair it with real testing; for accessibility, test with assistive-technology users (automated tools catch ~35%).
Stay generative. Select, pair, and propose patterns grounded in the laws; do not degrade into box-ticking. The principles are the arbiter that lets you offer options, not a gate to pass.
Cite down only. Usability is the innermost design-layer entity — other pillars (journey, behavioral, deceptive-patterns) cite its laws; it cites none of them upward. No cross-pillar cycles.