The description names the domain (API and interface design) and mentions some specific contexts like REST/GraphQL endpoints, type contracts, and module boundaries, but it doesn't list concrete actions—it says 'guides' design rather than specifying what actions it performs (e.g., 'generates endpoint schemas', 'validates contract compatibility').

Clearly answers both 'what' (guides stable API and interface design) and 'when' with explicit 'Use when...' clauses covering multiple trigger scenarios: designing APIs, module boundaries, REST/GraphQL endpoints, type contracts, and frontend/backend boundaries.

Good coverage of natural terms users would say: 'API', 'REST', 'GraphQL', 'endpoints', 'type contracts', 'module boundaries', 'frontend and backend', 'public interface'. These are terms developers naturally use when working on API design tasks.

While it targets API and interface design specifically, the broad scope of 'any public interface' and 'module boundaries' could overlap with general architecture skills, code organization skills, or specific REST/GraphQL implementation skills. The term 'guides' is also somewhat vague about what differentiates this from general coding advice.

The skill is generally well-structured but includes some unnecessary explanation that Claude would already know (e.g., explaining what HTTP status codes mean, explaining what PATCH vs PUT does). The 'Common Rationalizations' table, while useful for humans, adds bulk that doesn't help Claude execute tasks. The Hyrum's Law section is somewhat verbose. However, the code examples are lean and the tables are efficient.

The skill provides fully executable TypeScript code examples throughout — contract-first interface definitions, validation patterns with Zod-style parsing, REST resource design with concrete URL patterns, discriminated unions, branded types, and pagination response shapes. All examples are copy-paste ready and specific.

The verification checklist at the end provides a clear set of checkpoints, and the principles are well-sequenced (contract first → error semantics → validation → extension → naming). However, there's no explicit step-by-step workflow for designing an API from scratch — the content reads more as a reference guide than a sequenced process. For a design skill this is partially acceptable, but the 'When to Use' triggers don't connect to specific workflow paths.

The content is well-organized with clear sections and headers, but it's a monolithic document with no references to supporting files (except one mention of 'deprecation-and-migration' which has no bundle file). At ~200 lines, some sections like REST API Patterns and TypeScript Interface Patterns could be split into separate reference files. The single reference that exists points to a non-existent file.