Conciseness

The body is almost entirely executable templates, exact file paths, and one line of non-obvious domain knowledge ('subspaceForImpl only reads the slots inline...'). It assumes Claude's competence — no padding, no explanation of what JavaScriptCore or prototypes are — so every token earns its place.

Actionability

Copy-paste-ready C++ templates with exact file paths (DOMClientIsoSubspaces.h, ZigGlobalObject.h, test/internal/source-lints/...) cover the common case well. Minor gaps keep it below level 5: callFoo/constructFoo and Bun::initJSFooClassStructure are referenced but never defined, and the table entry 'jsFooGetter_property' does not match the later definition 'jsFooGetter_prop'.

Workflow Clarity

The section order (Class Structure → Iso Subspaces → ... → Expose to Zig) implies a sequence but there is no explicit step ordering, and validation is only implicit via the mention of 'test/internal/source-lints/iso-subspace-creation.test.ts checks this'. This matches 'steps listed but validation gaps; checkpoints missing or implicit'; level 4 would require an explicit ordered sequence with most checkpoints present.

Progressive Disclosure

A single-file skill (no references/, scripts/, or assets/ bundle exists) with well-organized sections that stay on one coherent procedure. The full class templates are inline rather than split into reference files, which is a minor organization gap consistent with 'good structure; most content is appropriately placed' rather than the fully split level-5 anchor.

Specificity

The description names the domain ('Implements JavaScript classes in C++ using JavaScriptCore') with one concrete action verb; the deliverables (bindings, prototypes, constructors) appear only in the trigger clause, matching 'names domain and 1-2 concrete actions, but not comprehensive'. It does not list several distinct actions (e.g., exposing classes to Zig, registering iso subspaces), which would fit the level-4 anchor.

Completeness

Explicitly answers both parts: 'Implements JavaScript classes in C++ using JavaScriptCore' (what) and 'Use when creating new JS classes with C++ bindings, prototypes, or constructors' (when) with concrete trigger phrases. It is narrower than the level-4 anchor's 'when could be more explicit', since the when-clause names three specific triggering artifacts.

Trigger Term Quality

Includes strong natural terms a user would say: 'JavaScript classes', 'C++ bindings', 'prototypes', 'constructors', 'JavaScriptCore'. Missing a few common variations such as 'JSC bindings', 'native bindings', or 'expose C++ to JS', so it fits 'good keyword coverage; a few natural terms missing' rather than comprehensive level 5.

Distinctiveness Conflict Risk

The JSC/C++-bindings niche is clear and highly specific, but generic words like 'prototypes' and 'constructors' create minor overlap risk with general JavaScript or general C++-binding skills. This fits 'mostly distinct; minor overlap risk with closely related skills' rather than level 5's minimal conflict risk.