Conciseness

Mostly efficient -- tables, formulas, and checklists are dense with real information -- but it includes several padded sections Claude does not need: six 'Why it works' paragraphs, an 'About the Author' biography, affiliate-tagged 'Further Reading' links, and basic definitions Claude already knows ('DNS resolves domain names; CDN caches static assets', SQL-vs-NoSQL basics). It sits between the 2 anchor (several unnecessary explanations) and the 3 anchor; the consistently tight tabular format keeps it at 3 rather than dropping to 2.

Actionability

Provides concrete, executable guidance: worked formulas ('100M DAU x 5 actions / 86400 = ~5,800 QPS avg, ~30K QPS peak', '500M tweets/day x 300 bytes x 365 days = ~55 TB/year') and specific patterns with parameters ('100 tokens/min per API key; refill at steady rate; reject with 429'). As an instruction-only skill, absent code is not penalized, but some guidance remains at the pattern level ('draw a high-level diagram with APIs, services, data stores') rather than being fully concrete, keeping it below the 5 anchor.

Workflow Clarity

The four-step process is clearly sequenced with time allocations and explicit checkpoints ('Never skip Step 1', 'Get explicit agreement on assumptions before proceeding'), and the Quick Diagnostic table acts as a checklist. It falls short of the 5 anchor because there is no explicit error-recovery or feedback loop (no revise-and-recheck cycle after a design review or estimate fails sanity checks).

Progressive Disclosure

All six referenced files exist, are one level deep (verified: reference files contain only internal anchors, no nested references), and are clearly signaled both per-section ('See: [references/...]') and in a consolidated Reference Files index with one-line descriptions. It misses the 5 anchor because the body duplicates a notable amount of reference content (powers of two, latency numbers, and cache strategies appear both inline and in the reference files), so SKILL.md is longer than a lean overview.

Specificity

Names the domain and several concrete action areas -- 'load balancing, caching, database scaling, and message queues' plus 'back-of-the-envelope estimation' -- but coverage has minor gaps (reliability/operations and monitoring are not named as capabilities). It lists several specific actions rather than only 1-2, placing it above the 3 anchor but short of the comprehensive coverage of the 5 anchor.

Completeness

Explicitly answers 'what' ('Design scalable distributed systems using structured approaches for...') and 'when' with concrete trigger phrases ('Use when the user mentions...' followed by 8 phrases, then 'Also trigger when...'). This matches the anchor for clearly and explicitly answering both what AND when with concrete triggers.

Trigger Term Quality

Eight quoted natural trigger phrases ('system design', 'scale this', 'high availability', 'rate limiter', 'design a URL shortener', 'system design interview', 'capacity planning', 'distributed architecture') plus three additional trigger conditions ('estimating infrastructure requirements', 'choosing between microservices and monoliths', 'designing for millions of concurrent users'). Coverage is comprehensive across synonyms and phrasings a user would naturally say; no common variation is obviously missing.

Distinctiveness Conflict Risk

Occupies a clear niche (distributed system design) with domain-specific triggers, and explicitly disambiguates sibling skills ('For data fundamentals, see ddia-systems. For resilience, see release-it.'), minimizing conflict risk. Uses third-person voice throughout, so no voice penalty applies.