Conciseness

The body is lean — tables, short code blocks, and terse directives like 'Retrieve silently. Do NOT narrate the process.' — and the domain thresholds it teaches (X-ray <2 Å for drug design, pLDDT bands) are specifics Claude cannot reliably be assumed to know, so they earn their tokens. It misses 5 because the 'Domain Reasoning' prose partially restates the Quality Assessment tier table and Resolution Use Cases lines, a small redundancy that could be trimmed.

Actionability

Concrete, named tool calls appear throughout (tu.tools.PDBeValidation_get_quality_scores(pdb_id=pdb_id), fallback chains as a table, error responses per error string), giving mostly executable guidance. It is not 5 because Phase 1 mixes runnable calls with comment-only pseudocode ('# By PDB ID: direct retrieval') and undefined variables (name, uniprot_id) that leave minor gaps for copy-paste use.

Workflow Clarity

A clearly sequenced four-phase workflow (Clarify → Disambiguate → Retrieve → Report) with explicit checkpoints: criteria for when to ask vs skip, an Identity Checklist, fallback chains for tool failures, and an error-handling table with retry guidance ('Retry once, provide alternative link'). This matches the top anchor's feedback loops, error recovery, and checklists; score 4 would understate the presence of these explicit validation and recovery structures.

Progressive Disclosure

The body is well-sectioned (Workflow, phases, Quality Assessment, Error Handling, Tool Reference) with no nested or buried references and no bundle files to navigate. It does not reach 5 because at ~130 lines with no external files, the Tool Reference and quality-tier tables are candidates for a one-level-deep reference file that would keep SKILL.md a pure overview.

Specificity

The description lists multiple concrete actions — 'retrieval from RCSB PDB, PDBe, and AlphaFold with disambiguation, quality assessment (resolution, R-factor, pLDDT), and metadata' plus 'distinguishes high-quality experimental (X-ray under 2 Angstrom) vs predicted vs medium-quality structures' — giving comprehensive, specific coverage rather than generic claims. It does not fall to 4 because coverage of the skill's operations (fetch, compare quality, select) is essentially complete, not merely 'several actions with minor gaps'.

Completeness

It explicitly answers 'what' (retrieval with disambiguation, quality assessment, metadata, distinguishing structure tiers) and 'when' with a concrete trigger clause: 'Use for fetching protein structures, structure-quality comparison, and selecting structures for drug design or modeling.' Both halves are explicit and concrete, matching the top anchor; score 4 would require the 'when' to be less explicit than it is.

Trigger Term Quality

Good natural keyword coverage: 'fetching protein structures', 'structure-quality comparison', 'selecting structures for drug design or modeling', plus database names (RCSB PDB, PDBe, AlphaFold) users would mention. It falls short of 5 because common variations like '3D structure', 'protein model', 'download PDB file', or '.pdb' extension are missing; it is above 3 because the phrases present are ones users would naturally say.

Distinctiveness Conflict Risk

It occupies a clear niche — protein structure retrieval from named structural-biology databases with quality-tier assessment — with trigger phrases (PDB, AlphaFold, resolution, pLDDT, drug design) that are unlikely to fire for unrelated skills. Not 4, since even overlap with a general 'drug design' skill is defused by the structure-retrieval framing.