Conciseness

The style is tight — tables, compact code, no prose padding — but roughly a third of the body re-treads knowledge Claude already has: standard CRUD routes ("GET /api/tasks → List tasks"), pagination and filtering shapes, the 400/401/403/404/409/422/500 status mapping, the camelCase naming table, and standard TypeScript patterns (discriminated unions, branded types). This fits anchor 3 ("mostly efficient but includes some unnecessary explanation or could be tightened") — above anchor 2 because the presentation is not padded and the idempotency, rationalizations, and verification material genuinely earns its tokens.

Actionability

Concrete, near-executable guidance throughout: the boundary-validation Express handler, the atomic idempotency claim with unique-violation handling ("let the unique constraint pick the winner"), the payload-hash guard with an exact 422 response, and specific route/naming tables. Anchor 4 rather than 5 because some snippets are not copy-paste ready — undefined helpers like isUniqueViolation(e) and replayOrReject(key), and ellipses such as "function getTask(id: TaskId): Promise<Task> { ... }".

Workflow Clarity

The numbered Core Principles (1. Contract First through 6. Honouring an Idempotency Key) give a clear design sequence, and the closing 12-item Verification checklist provides explicit validation checkpoints ("The key is claimed in one atomic operation, guarded by a unique constraint"). Anchor 4 rather than 5 because it is reference-style with no feedback loop — no guidance to revisit earlier principles when a checklist item fails — and rather than 3 because the checkpoints are explicit, not implicit or missing.

Progressive Disclosure

Headers and internal structure are good, but this is a 365-line monolithic file where the ~60-line idempotency deep-dive (derivation examples, atomic-claim code, in-flight strategy table, retention guidance) clearly could live in a reference file. The one cross-reference — "See `deprecation-and-migration`" — is neither a link nor a path present in the bundle, matching anchor 3's "references present but not clearly signaled; content that should be separate is inline"; above anchor 2 because section structure is solid, below anchor 4 because references are not clearly signaled or organized.

Specificity

The capability statement is a single generic verb — "Guides stable API and interface design" — while the concrete activities ("creating REST or GraphQL endpoints, defining type contracts between modules, or establishing boundaries between frontend and backend") all live in the when-clause rather than the what. This matches anchor 3 (domain plus 1-2 concrete actions, not comprehensive); it is above anchor 2 because the trigger enumeration names specific activities, but below anchor 4 because the description of capabilities itself does not list several distinct concrete actions.

Completeness

Explicitly answers both parts: what ("Guides stable API and interface design") and when, via two explicit "Use when..." clauses with concrete trigger phrases ("designing APIs, module boundaries, or any public interface"; "creating REST or GraphQL endpoints, defining type contracts..."). This closely mirrors the anchor 5 exemplar structure, and is above anchor 4 because the when is already explicit and specific rather than needing more detail.

Trigger Term Quality

Good natural keyword coverage: "APIs", "REST", "GraphQL", "endpoints", "module boundaries", "type contracts", "frontend and backend" — phrases users would naturally say. A few natural terms are missing (e.g., "API versioning", "schema design", "endpoint design", "breaking changes"), so it fits anchor 4 rather than anchor 5's comprehensive synonym coverage.

Distinctiveness Conflict Risk

The niche is mostly distinct — API/interface design with REST, GraphQL, and type-contract triggers — but "any public interface" and "establishing boundaries between frontend and backend" are broad enough to overlap with general software-architecture or design skills. Fits anchor 4 (mostly distinct, minor overlap risk with closely related skills) rather than anchor 5's minimal-conflict clear niche.