Conciseness

The body is prose-free and table/code-dense (no padded explanations), but a substantial share restates concepts Claude already knows — the HTTP method idempotency/safety table, the 200/201/204/400/401/403/404 status-code reference, and Bearer-token auth basics. This lands on 'mostly efficient but includes some unnecessary explanation or could be tightened'; it is not the level-2 case because there is no padded prose, and not level 4 because the re-taught basics are systematic rather than minor.

Actionability

Everything is concrete and executable: exact URL patterns with GOOD/BAD contrasts, SQL implementations for both pagination styles, runnable TypeScript/Python/Go handlers, exact rate-limit headers, and a pre-ship checklist. This matches 'fully executable; copy-paste ready code or commands; specific examples cover the common cases'.

Workflow Clarity

As a patterns/reference skill it still sequences decisions well: a numbered versioning strategy with a deprecation timeline, a 'When to Activate' section, and a final pre-ship checklist acting as a checkpoint. It stops short of anchor 5 because there are no explicit validate-then-fix feedback loops (e.g., 'if the checklist fails, fix and re-check'), and it sits above anchor 3 because checkpoints (the checklist, the deprecation steps) are explicit.

Progressive Disclosure

No bundle files exist (references/, scripts/, assets/ are absent) and the entire ~520-line reference lives inline in SKILL.md; content that clearly belongs in separate files — three language-specific implementation examples and full reference tables — is inlined with no external references. Section headers are well-organized, matching 'some structure but could be better organized; content that should be separate is inline' rather than level 2's 'minimal structure', since navigation via headers is genuinely easy.

Specificity

Names multiple concrete capability areas — 'resource naming, status codes, pagination, filtering, error responses, versioning, and rate limiting' — giving comprehensive coverage of the REST design domain with no filler. This matches the anchor 'lists multiple specific concrete actions; comprehensive coverage'; the level below (4) would require minor coverage gaps, and none are evident.

Completeness

It explicitly answers both: what ('REST API design patterns including resource naming, status codes, pagination, filtering, error responses, versioning, and rate limiting') and when ('Use when designing or reviewing REST endpoints, resource names, status codes, pagination, or versioning'), with concrete trigger phrases. This matches the anchor-5 example's structure; anchor 4 would require the 'when' to be less explicit, which it is not.

Trigger Term Quality

The 'Use when' clause carries natural terms users would say — 'designing or reviewing REST endpoints, resource names, status codes, pagination, or versioning'. This is good coverage but a few natural phrasings (e.g., 'error handling', 'rate limiting', 'API design') are present in the what-clause yet omitted from the triggers, so it sits between the good (4) and comprehensive (5) anchors, closer to 4.

Distinctiveness Conflict Risk

It occupies a clear niche (REST API design conventions) with distinct triggers like 'REST endpoints', 'resource names', and 'versioning', minimizing risk of firing for unrelated skills. The only adjacent space ('reviewing endpoints') mildly overlaps generic code-review skills, but not enough to drop below the 'clear niche' anchor.