Conciseness

The body is lean and assumes Claude's competence: it never explains what a lockfile or goroutine is, and lines like 'Instructions and conventions are not concurrency control' are maximally dense. The single-sentence 'Why' motivates the pattern without padding, so every token earns its place; level 4's 'minor instances of over-explanation' would require trimmable material that isn't present.

Actionability

Concrete, executable-in-design guidance: 'Give each actor its own owned file, key, branch, or state directory, and merge only at the read/reporting boundary', named serialization mechanisms ('lockfiles, sequential phases, single-writer actor, or atomic compare-and-swap'), and a worked micro-example ('indexer-state.json + metrics-state.json is not'). It stops short of level 5 because the serialization side has no specific example covering its common cases, leaving a minor gap.

Workflow Clarity

A clear 3-step decision sequence (identify shared state → eliminate the shared write target by default → serialize structurally when one shared write target is a real invariant) with an explicit gate ('Treat "we need a lock" as a design smell to check'). It fits 'clear sequence with most checkpoints present; minor validation gaps' — how to verify the invariant is real is left open — better than level 3, where checkpoints would be missing or implicit.

Progressive Disclosure

No bundle files exist (no references/, scripts/, or assets/), and the skill is under 50 lines with no need for external references. Per the simple-skill guideline, the well-organized structure (title, summary paragraph, 'Why', numbered 'Pattern') earns a 5; there is no inlined content that belongs in separate files and no buried references.

Specificity

Names the domain ('concurrent actors might write to the same file, branch, key, or state object') and exactly two concrete actions ('Eliminate the sharing first', 'serialize structurally'), matching the '1-2 concrete actions' anchor rather than level 4's 'several specific actions'. It is well above level 1-2 vagueness since the actions and targets are concrete, not generic.

Completeness

An explicit trigger clause ('Apply when concurrent actors might write to the same file, branch, key, or state object') answers 'when' with concrete conditions, and 'Eliminate the sharing first; serialize structurally only when one shared writer is a real invariant' clearly answers 'what'. Both are explicitly stated with concrete trigger phrases, matching the level 5 anchor; the 'Use when' cap of 3 does not apply.

Trigger Term Quality

Natural trigger terms are present ('concurrent actors', 'same file, branch, key, or state object', 'write'), covering the shared-state target types comprehensively; a few common synonyms are missing ('race condition', 'merge conflict', 'parallel agents/workers'), which fits 'good keyword coverage; a few natural terms missing' rather than level 5, and is better than level 3's 'missing common variations' since four target types are enumerated.

Distinctiveness Conflict Risk

A clear niche (concurrency control for shared mutable state) with distinct triggers ('concurrent actors might write to the same file, branch, key, or state object') that are unlikely to fire for unrelated skills. It is specific enough to avoid the overlap risk described at levels 3-4.