Conciseness

The body is lean and assumes competence — no tutorials on what a physics engine is, just setup code, patterns, and checklists. The 'Common patterns' bullets partially restate what the setup code already shows (Composite.add, wireframes option), a minor trim opportunity that keeps it below the 'every token earns its place' anchor.

Actionability

Code is concrete and copy-paste ready (Engine/Render/Runner creation, bodies, mouse constraint, cleanup with Runner.stop), covering the common cases. The one minor gap is that loading Matter.js itself (script tag or import) is never shown, so the examples are not fully self-contained — the 'concrete code with minor gaps' anchor.

Workflow Clarity

The 4-step workflow is clearly sequenced with sensible conditionality ('only if requested', cleanup for SPA teardown) and a questions section for missing specs. There are no explicit validation/checkpoint steps (e.g., confirming the render loop is running), which keeps it just below the level-5 anchor; the skill is not destructive/batch so no cap applies.

Progressive Disclosure

The skill is a compact single file (~60 lines, mostly code) with no references/, scripts/, or assets/ bundle and no content that warrants splitting out. Sections (Workflow, Minimal setup, Common patterns, Mouse interaction, Cleanup, Questions) are well-organized and easy to navigate — matching the well-organized-sections ideal for a simple skill.

Specificity

Lists several concrete capabilities — 'Engine/World setup, Render/Runner configuration, adding bodies and constraints' — which are specific, though 'scroll/interaction-friendly canvas scenes' is vaguer and coverage has minor gaps (e.g., no mention of collision events or plugins). Fits the 'several specific actions; minor gaps' anchor, not the comprehensive level 5.

Completeness

The 'Use when...' clause explicitly states when to invoke it, and the 'including ...' list describes what it does, so both are present. However, the what is fused into the when-clause and there are no separate user-mention triggers ('or when the user mentions X'), so the 'when' is slightly less explicit than the level-5 anchor.

Trigger Term Quality

Natural terms like 'Matter.js', '2D physics interactions', 'canvas', 'bodies', and 'constraints' match what users would say, but common synonyms such as 'physics engine', 'physics simulation', 'rigid bodies', or 'game physics' are missing. Good coverage with a few natural terms absent — anchor 4, not the synonym-complete level 5.

Distinctiveness Conflict Risk

Naming the specific library 'Matter.js' with its own vocabulary (Engine/World/Render/Runner, bodies, constraints) carves out a clear niche that would rarely trigger for the wrong skill — matching the 'clear niche with distinct triggers' anchor.