Conciseness

The body is dense and task-focused, but contains minor over-explanation of concepts Claude already knows: the prerequisite 'Familiarity with RESTful API conventions (HTTP methods, status codes, JSON schemas)' and inline comments like '# HTTP 201 Created' and '# HTTP 400 Bad Request'. This matches anchor 4 (efficient with minor trimming opportunities) rather than 5 (every token earns its place).

Actionability

Guidance is fully concrete and copy-paste ready: a complete '#### POST /auth/register' contract template with typed request/response blocks, a paginated list endpoint with a query-parameter table, an executable 'interface ApiError' declaration, and an Output section naming exact interfaces (User, CreateUserRequest, PaginatedResponse<T>). Specific examples cover the common cases, matching anchor 5.

Workflow Clarity

Six clearly sequenced instruction steps plus an Error Handling table pairing causes with solutions provide strong sequencing and remediation guidance. It falls short of anchor 5 because there is no explicit validation step (e.g. verify contract consistency) before sharing the contract in SPAWN REQUEST blocks; this matches anchor 4 (clear sequence, minor validation gaps).

Progressive Disclosure

Four well-signaled, one-level-deep references (${CLAUDE_SKILL_DIR}/references/writing-endpoints.md, typescript-interfaces.md, pagination.md, best-practices.md) are all real files and are restated in a Resources section with descriptions. However, two bundle files (errors.md and the 12.7 KB examples.md) are never referenced from the body, which is a minor organization gap matching anchor 4 rather than 5.

Specificity

The description names the domain and one action-shaped phrase ("needs guidance on designing contracts that coordinate backend and frontend agents") but never states concrete actions such as creating api-contract.md files with typed request/response schemas. It matches anchor 3 (domain plus 1-2 concrete actions, not comprehensive) rather than 4, which requires a list of several specific actions.

Completeness

The 'when' is explicit and strong ("Use when building or modifying API endpoints", "Trigger with phrases like...") but the 'what' is only vaguely implied ("guidance on designing contracts"); the skill's actual output is never stated. This sits between anchor 2 (only 'when' without 'what') and anchor 4 (both clear), and the vague 'what' holds it at 3 rather than 4.

Trigger Term Quality

It quotes a comprehensive set of natural terms — "API contract", "api-contract.md", "shared interface", "TypeScript interfaces", "request response schemas", "endpoint design", "create API", "design endpoint", "API scaffold" — including synonyms and the exact file name, matching the anchor-5 example of comprehensive coverage including synonyms and file names.

Distinctiveness Conflict Risk

Distinct triggers like "api-contract.md" and "request response schemas" carve a clear niche, but broad phrases such as "Use when building or modifying API endpoints" and "create API" could overlap with general API-implementation skills. This matches anchor 4 (mostly distinct, minor overlap risk) rather than 5 (minimal conflict risk).