API Design Skill

When designing or reviewing an API (REST, RPC, or library), follow these principles:

1. Understand the Consumer

• Who calls this API? (Frontend, other services, CLI, third-party developers)
• What are the most common operations?
• What error conditions do consumers need to handle?
• What's the expected request volume and latency budget?

2. Naming Conventions

• Use consistent, predictable names across all endpoints/methods.
• Nouns for resources, verbs for actions: GET /users, POST /users/:id/activate.
• For library APIs: verb-first for actions (createUser, deleteSession), noun-first for accessors (getUserById).
• Avoid abbreviations unless universally understood (id, url, api).
• Be specific: getActiveUserCount() not getCount().

3. Input Design

• Accept the minimum required input. Optional fields should have sensible defaults.
• Use typed schemas (Zod, JSON Schema) for validation at the boundary.
• Reject invalid input early with clear error messages.
• For REST: use path params for identity (/users/:id), query params for filtering (?status=active), body for creation/mutation.
• For libraries: prefer options objects over long parameter lists.

4. Output Design

• Return consistent shapes. Every endpoint should return the same envelope structure.
• Include enough context for the consumer to act without a follow-up call.
• Paginate list endpoints. Always include total, limit, offset or cursor.
• Use ISO 8601 for dates, consistent casing (camelCase or snake_case, not both).

5. Error Handling

• Use standard HTTP status codes (REST) or typed error codes (RPC/library).
• Every error response must include: error code, human-readable message, and request ID.
• Distinguish client errors (4xx / validation) from server errors (5xx / internal).
• Never expose internal details (stack traces, SQL, file paths) in production errors.
• Document every error code the consumer might receive.

6. Versioning & Evolution

• Version the API from day one (/v1/, header-based, or semver for libraries).
• Additive changes (new fields, new endpoints) are non-breaking.
• Removing or renaming fields is breaking — deprecate first, remove in next major version.
• Document breaking changes in a changelog.

7. Documentation

For each endpoint or method, document:

1. Purpose (one sentence).
2. Input parameters with types and constraints.
3. Output shape with example.
4. Error codes and when they occur.
5. Authentication/authorization requirements.
6. Rate limits if applicable.