Conciseness

The ~58-line body is lean and assumes Claude's competence: no explanation of what Open Library, ISBNs, or JSON are; every section (Workflow, Common commands, Result handling, Boundaries) earns its tokens. Phrases like 'Empty results are honest no-match outcomes, not errors' add non-obvious handling guidance rather than padding, matching the 'every token earns its place' anchor.

Actionability

Three copy-paste-ready executable commands cover all three lookup modes ('--isbn 9780140328721', a real title/author invocation with --limit, and a broad query example), and the result-handling section gives concrete field names to read ('num_found', 'results', 'error', 'error_code'). Exact remaining syntax is delegated to references/scripts.md, which exists and delivers it.

Workflow Clarity

The workflow is a clear sequenced decision procedure: choose the narrowest lookup, return exact records for ISBN vs ranked candidates for search, and two explicit checkpoints — 'If no results are returned, say that Open Library returned no match' and 'If the API request fails, report the structured error and do not invent metadata'. The skill is read-only and low-volume, so the destructive/batch validation cap does not apply, and error-recovery feedback is explicitly present.

Progressive Disclosure

The body is an appropriate overview and keeps exact CLI syntax, output shapes, and error payloads in references/scripts.md, referenced one level deep via a clearly labeled closing link ('For exact script arguments, output formats, and error payloads, see references/scripts.md'). Both referenced bundle paths are real (references/scripts.md and scripts/lookup_book.py), the reference file itself contains no nested references, and security-critical untrusted-data handling is correctly split between the overview and the reference.

Specificity

The description lists multiple concrete, distinct actions — 'retrieve online book metadata, list editions, identify authors and publishers, find publication details, surface subjects, display cover image URLs, or cite Open Library source URLs' — with the lookup keys ('by ISBN, title, author, or broad book query') named explicitly. This matches the anchor for multiple specific concrete actions with comprehensive coverage, not the score-4 anchor that presumes minor gaps.

Completeness

It explicitly answers both questions with a concrete trigger clause: 'Use this skill when the user asks to retrieve online book metadata...' (what and when) and reinforces scope with 'This skill uses Open Library for read-only public book metadata' and an explicit do-not-use list. This mirrors the score-5 anchor example structure exactly.

Trigger Term Quality

Natural user phrasings are comprehensively covered: 'ISBN', 'title', 'author', 'list editions', 'identify authors and publishers', 'find publication details', 'cover image URLs', plus negative triggers like 'ebook downloads, shopping links, review aggregation'. These are the words a user would actually say when they need this skill; it exceeds the score-4 anchor (which expects a few missing natural terms).

Distinctiveness Conflict Risk

The niche is sharply defined (Open Library, read-only public book metadata) and the negative boundary list ('Do not use this skill for ebook downloads, shopping links, review aggregation, broad reading recommendations, metadata editing, or claims about any user's personal collection') actively prevents mis-triggering against recommendation, shopping, or personal-library skills. Minimal conflict risk.