Conciseness

The body is largely lean — decision tables, code, and a checklist instead of prose — and assumes Claude knows Python/pytest. It is not level 5 because of trimmable redundancy: Steps 4 and 6 each spend 3-4 lines re-explaining the same motivation before deferring to the same reference file, and the Anti-patterns table repeats 'Always include Step 6' twice. It is clearly above level 3, which expects unnecessary explanations of things Claude already knows.

Actionability

Concrete test code is given for the duplicate-key, hash-mismatch, and TTL-expiry cases, and the referenced bundle file (verified to exist) supplies full commutativity and concurrent-duplicate tests, covering the common cases. It falls short of the copy-paste-ready level-5 anchor because the snippets use undefined names (process_charge, charge_processor, endpoint/freezer fixtures, IdempotencyConflictError) with no fixture or setup definition, so they are templates rather than runnable tests.

Workflow Clarity

Steps 1-7 are clearly sequenced, opening with a classification decision table and closing with an explicit per-handler recipe checklist (Step 7), which satisfies the checklist part of the level-5 anchor. It is not 5 because there is no feedback-loop guidance — e.g., what to conclude or fix when the concurrent-duplicate test fails or how to interpret a race that only surfaces under load — which the anchor expects alongside the checklist. This is a test-authoring skill, not a destructive/batch operation, so the workflow-clarity cap of 3 does not apply.

Progressive Disclosure

The bundle is used correctly: the core idempotency-key pattern and primary test patterns stay inline in SKILL.md, while the deeper commutativity and race-condition material is split into references/idempotency-patterns.md (verified to exist), linked with well-signaled one-level-deep markdown links from Steps 4 and 6. This matches 'Clear overview with well-signaled one-level-deep references; content appropriately split; easy navigation' exactly.

Specificity

The description lists five concrete capabilities — "idempotency-key handling", "retry-safe semantics (exactly-once vs at-least-once vs at-most-once)", "side-effect commutativity verification", "fingerprint-based dedup", "idempotency-window tuning" — which comprehensively covers the skill's domain. It matches the anchor 'Lists multiple specific concrete actions; comprehensive coverage' rather than the level-4 anchor, which expects minor gaps in coverage; no significant capability from the body is missing.

Completeness

Both halves are explicit: 'what' is the build-an-X capability with the enumerated techniques, and 'when' is the concrete clause 'Use when authoring tests for any system where the same input could be processed twice (SQS Standard at-least-once, RabbitMQ requeue, ...)'. This is the anchor-5 example pattern (what + when with concrete trigger phrases), not the level-4 anchor where 'when' is only generically stated.

Trigger Term Quality

The 'Use when' clause enumerates natural phrases users would actually say: "SQS Standard at-least-once, RabbitMQ requeue, retry-on-error logic, webhook redelivery, browser double-click, mobile-network retry", plus synonyms like idempotency tests and redelivery. This matches 'Comprehensive coverage of natural terms including synonyms'; the leading 'Build-an-X' jargon describes what the skill does rather than a trigger term, so it does not lower this dimension.

Distinctiveness Conflict Risk

The niche is clear (idempotency test authoring) with distinct technology-specific triggers, but the body itself lists closely related sibling skills (sqs-tests, rabbitmq-tests, cron-job-test-author, webhook-delivery-tester), so a request like 'write tests for my SQS consumer' could plausibly match more than one skill. This fits 'Mostly distinct; minor overlap risk with closely related skills' rather than level 5, which requires minimal conflict risk with no adjacent competing skills.