Conciseness

The body is lean and directive throughout — e.g. 'Interpolate values directly in `markup!`; do not allocate with `format!` first', 'Do not edit generated diagnostic-category registries by hand'. It explains nothing Claude already knows and every token carries a project-specific decision rule. There is no padding to trim, so it fits the top anchor rather than anchor 4's 'minor instances of over-explanation'.

Actionability

For an instruction-only skill the guidance is fully concrete: named APIs ('Use `RuleDiagnostic` for lint and assist rules'), a need-to-mechanism mapping table (note/detail/diff/command/verbose advice), named files ('crates/biome_diagnostics/CONTRIBUTING.md', 'crates/biome_diagnostics/src/advice.rs'), and named skills to load ('lint-rule-development', 'testing-codegen'). Per the rubric's scoring note, absence of code is not penalized when the instruction guidance is this specific.

Workflow Clarity

The implied sequence is coherent — inspect existing diagnostics, choose the API, shape messages/advice, handle categories and severity, then validate — with an explicit Validation section ('Add the narrowest test or fixture that renders the diagnostic', 'Test conditional advice in every branch') and a Review Checklist as the final checkpoint. It is not anchor 4 because both a dedicated validation stage and a completion checklist are present, not merely 'most checkpoints'.

Progressive Disclosure

The body acts as an overview and decision guide that delegates deep detail to clearly signaled one-level references ('Use `crates/biome_diagnostics/CONTRIBUTING.md` as the canonical API and design guide', 'Existing derive examples: search for `#[derive(Diagnostic)]`'), and a References section lists exact paths. Nothing that belongs in a separate file is inlined, and no bundle files exist to restructure. It is not anchor 4 because there are no organization gaps to point to.

Specificity

The description lists multiple concrete capability areas — 'messages, advice, markup, details, code frames, categories, severity, and standalone `Diagnostic` types' — under the concrete actions 'designing or implementing Biome user-facing diagnostic presentation or APIs', giving comprehensive coverage of the domain. It does not fall to 4 because no meaningful capability area of diagnostic presentation is left unnamed.

Completeness

It explicitly answers 'what' ('designing or implementing Biome user-facing diagnostic presentation or APIs, including messages, advice, markup...') and 'when' ('Use this skill when...'), plus an explicit negative trigger ('Do not use for lint matching logic or code-action mutations'). Both are concrete and explicit, matching the top anchor exactly.

Trigger Term Quality

Good keyword coverage with natural terms a Biome contributor would say: 'diagnostic', 'messages', 'advice', 'markup', 'code frames', 'categories', 'severity', 'Diagnostic types'. Not 5 because common user-facing synonyms like 'error', 'warning', or 'error message' are absent, so a few natural phrasings could miss the trigger.

Distinctiveness Conflict Risk

Clear niche (Biome user-facing diagnostic presentation/APIs) with an explicit exclusion clause separating it from adjacent skills ('Do not use for lint matching logic or code-action mutations'), minimizing conflict risk. This is a distinct trigger surface, not merely 'minor overlap risk' as at anchor 4.