Conciseness

The body is dense and lean — cheatsheet tables, short code blocks, and directive rules like 'Never mutate. Always create new values.' No section explains concepts Claude already knows (no 'immutability is a principle of FP...' padding), and every line adds guidance. Matches the 5 anchor ('lean and efficient; every token earns its place'); not a 4 because there is no identifiable over-explanation to trim.

Actionability

Provides copy-paste-ready patterns: a per-operation mutable-vs-immutable cheatsheet ('arr.splice(i,1)' → 'arr.filter((_,idx) => idx !== i)' or 'arr.toSpliced(i,1)'), three executable nested-update approaches, and an explicit decision rule ('1-2 levels → manual spread. 3+ levels or many fields → Immer or helper functions'). Matches the 5 anchor; not a 4 because the examples are fully executable and cover the common cases across the listed languages.

Workflow Clarity

This is a single-purpose decision-guidance skill, and the single action ('Never mutate. Always create new values') is unambiguous and reinforced by a numbered pre-decision checklist ('Before Choosing Immutable vs Mutable, Ask Yourself'), an explicit approach-selection rule, and a 'When NOT to Enforce' escape hatch. Per the simple-skill exception the single action being unambiguous supports a 5; the destructive/batch validation cap does not apply since the skill involves no destructive or batch operations.

Progressive Disclosure

A single well-organized file with clear scannable sections (cheatsheet, nested updates, enforcement, performance, NEVER, when-not-to-enforce) and no nested or buried references — there are no bundle files at all. Fits the 4 anchor ('good structure; most content appropriately placed; minor organization gaps'); not a 5 because at ~93 lines everything is inlined, and material like the per-language notes or the Immer guide would naturally live in one-level-deep reference files, and the compact single-file 5 exception is scoped to skills under 50 lines.

Specificity

Lists multiple concrete capability areas — 'immutable update patterns (spread, copy, freeze)', 'nested updates without mutation', 'readonly enforcement', and 'performance trade-offs' — which is comprehensive coverage of the domain, matching the 5 anchor ('multiple specific concrete actions'). It is not a 4 because no meaningful capability area of the skill is missing from the enumeration.

Completeness

Explicitly answers 'what' ('Covers immutable update patterns... nested updates... readonly enforcement, and performance trade-offs') and 'when' via an explicit 'Activate when:' clause with concrete triggers. This matches the 5 anchor verbatim in structure; not a 4 because the 'when' guidance is not merely present but specific and multi-condition.

Trigger Term Quality

Trigger list includes natural user phrasings and synonyms — 'immutability, spread, readonly, frozen, copy-on-write, or structural sharing' plus situational phrases like 'fixing bugs caused by unintended mutation' and named ecosystems (React, Redux). Matches the 5 anchor's 'comprehensive coverage of natural terms including synonyms'; not a 4 because no common synonym or variation is obviously missing (file extensions are not applicable to a language-level skill).

Distinctiveness Conflict Risk

The niche (immutability/immutable updates) is mostly distinct with distinctive terms like 'copy-on-write' and 'structural sharing', but the trigger 'updating objects or arrays' is very broad and could overlap with general state-management or framework skills. Fits the 4 anchor ('mostly distinct; minor overlap risk with closely related skills'); not a 5 because that broad trigger creates real misfire risk, and not a 3 because the core triggers are unmistakably about immutability.