Conciseness

The body is dense and instructional — typed error classes with exact status codes ("ValidationError (400) ... RateLimitError (429)"), an RFC 7807 field list, and a tight error-cause-solution table — with little explanation of concepts Claude already knows. Minor trimmable padding exists: the Overview paragraph restates the Instructions, and two Resources bullets ("Express error handling best practices", "HTTP status code decision tree for API responses") carry no URLs or specifics. Anchor 4 ("Efficient; minor instances of over-explanation that could be trimmed") fits better than 3 because no section is genuinely padded.

Actionability

Guidance is concrete and executable: exact envelope fields ("type, title, status, detail, instance"), validation error shape ("field, message, code properties"), an example RFC 7807 JSON body, a one-line Express middleware signature ("app.use((err, req, res, next) => {...})"), and a defined output file layout. It is not a 5 because no complete copy-paste code exists in the body itself — implementation details are delegated to references/implementation.md, leaving minor gaps in the main file.

Workflow Clarity

The 9-step Instructions sequence is coherent and correctly ordered (audit → envelope → typed classes → middleware → validation formatting → env config → monitoring → process-level handlers → tests), ending in a verification step ("Write tests verifying that each error type produces the correct HTTP status code"). Anchor 4 fits rather than 5 because the validation step lacks a feedback loop — there is no explicit "if tests fail, fix and re-run" guidance — and the destructive/batch cap does not apply since the skill is additive.

Progressive Disclosure

The body is a clear, well-sectioned overview (~66 lines) with three well-signaled, one-level-deep references that all exist in the bundle: "See references/implementation.md for the full implementation guide", "Refer to references/errors.md for comprehensive error patterns", and "See references/examples.md for additional examples". Each reference is placed at the end of its relevant section, no content that belongs in a separate file is inlined, and none of the reference files nest further references. This matches anchor 5 exactly.

Specificity

The description names the domain and 1-2 concrete actions — "Implement standardized error handling with proper HTTP status codes and error responses" — but does not enumerate what the skill actually does (no mention of RFC 7807, typed error classes, centralized middleware, or error monitoring, which the body covers). It matches anchor 3 ("Names domain and 1-2 concrete actions, but not comprehensive") better than anchor 4 because the actions listed are generic rather than several specific ones.

Completeness

Both what ("Implement standardized error handling with proper HTTP status codes and error responses") and when ("Use when implementing standardized error handling") are present, plus explicit trigger phrases. However, the "Use when" clause is tautological — it restates the what rather than describing user scenarios — so the "when" could be more specific, matching anchor 4 rather than anchor 5's fully concrete trigger guidance.

Trigger Term Quality

Provides natural trigger phrases a user would actually say: "add error handling", "standardize errors", "implement error responses". This is good keyword coverage per anchor 4, though common synonyms like "exception handling", "API errors", or "status codes" are missing, which keeps it below anchor 5's comprehensive synonym coverage.

Distinctiveness Conflict Risk

"Standardized error handling" with HTTP status codes and error responses is a clear niche with distinct triggers, but it has minor overlap risk with closely related skills (general API-development, logging, or debugging skills that might also trigger on "error handling"). Anchor 4 ("Mostly distinct; minor overlap risk with closely related skills") is the best fit.