Conciseness

The body is lean and assumes Claude's competence — it never explains what Zod, Pydantic, status codes, or ETags are, and the one rationale line ("Writing the handler first is how inconsistent APIs happen") earns its place by justifying the contract-first rule. Every token adds project-specific guidance.

Actionability

Mostly executable guidance: a copy-paste-ready error-shape JSON example, an enumerated status-code list, a concrete retry limit ("at most twice, with backoff"), and a specific smoke-test command ("curl -fsS"). Minor gaps keep it below 5 — "Rate-limit public endpoints" names no mechanism and no example shows the validation-error details payload.

Workflow Clarity

A clear sequence is present ("Decide the request, the response, the status codes and the error cases before you write the handler") with an explicit final validation checkpoint ("Smoke-test a new endpoint against the dev server with curl -fsS before you call it done"). Intermediate checkpoints are only implicit, matching the anchor with minor validation gaps rather than the explicit validate-fix-retry loop of the top anchor.

Progressive Disclosure

The skill is under 50 lines with no bundle files and no need for external references, and its four well-organized sections (Contract first, Writes and side effects, External APIs, Limits and caching) make navigation trivial; the one path mentioned (.opencastle/stack/api-config.md) is a project pointer, not a nested reference.

Specificity

Enumerates seven concrete coverage areas ("the contract, input validation, error shapes, status codes, versioning, pagination, retries and caching"), matching the several-specific-actions anchor; not 5 because these are topic nouns rather than concrete actions and body-covered areas like idempotency and authorization are unnamed.

Completeness

Explicitly answers both questions: a full "what" ("Conventions for HTTP endpoints, server actions and calls to external APIs: the contract, input validation...") and a concrete "when" ("Use when adding or changing an endpoint or server action, or wiring an external API"), matching the top anchor exactly.

Trigger Term Quality

Natural user phrasings like "adding or changing an endpoint", "server action", and "wiring an external API" give good keyword coverage; common synonyms such as "route", "REST", or "integration" are missing, so it falls just short of comprehensive.

Distinctiveness Conflict Risk

It carves a clear niche (this project's API conventions) with distinct, specific triggers ("server action", "wiring an external API") that no generic skill would claim, giving minimal conflict risk.