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API Design

API design is the practice of defining the contract between a service and its consumers in a way that is consistent, predictable, and resilient to change. A well-designed API reduces integration friction, makes versioning safe, and communicates intent through naming and structure rather than documentation alone. This skill covers the three dominant paradigms - REST, GraphQL, and gRPC - along with OpenAPI specs, pagination strategies, versioning, error formats, and authentication patterns.

When to use this skill

Trigger this skill when the user:

• Asks how to name, structure, or version API endpoints
• Needs to choose between REST, GraphQL, or gRPC for a new service
• Wants to write or review an OpenAPI / Swagger specification
• Asks about HTTP status codes and when to use each
• Needs to implement pagination (offset, cursor, keyset)
• Asks about authentication schemes (API key, OAuth2, JWT)
• Wants a consistent error response format across their API
• Needs to design request/response schemas or query parameters

Do NOT trigger this skill for:

• Internal function/method interfaces inside a single service - use clean-code or clean-architecture skills
• Database schema design unless it is driven by API contract requirements

Key principles

1. Consistency over cleverness - Every endpoint, field name, error shape, and status code should follow the same pattern throughout the API. Consumers should be able to predict behavior for an endpoint they have never used before.

2. Resource-oriented design - Model your API around nouns (resources), not verbs (actions). POST /orders is better than POST /createOrder. The HTTP method carries the verb.

3. Proper HTTP semantics - Use the right method (GET is safe + idempotent, PUT/DELETE are idempotent, POST is neither). Use correct status codes: 201 for creation, 204 for empty success, 400 for client errors, 404 for not found, 409 for conflicts, 429 for rate limiting.

4. Version from day one - Include a version in your URL or header before publishing. v1 in the path costs nothing; removing a breaking change from a production API costs everything.

5. Design for the consumer - Shape responses around what the client needs, not around what the database returns. Clients should not have to join, filter, or transform data after receiving a response.

Core concepts

REST resources

REST treats everything as a resource identified by a URL. Resources are manipulated through a uniform interface: GET, POST, PUT, PATCH, DELETE. Collections live at /resources and individual items at /resources/{id}. Sub-resources express ownership: /users/{id}/orders.

GraphQL schema

GraphQL exposes a single endpoint and lets clients declare exactly which fields they need. The schema is the contract - it defines types, queries, mutations, and subscriptions. Best for: UIs that need flexible data fetching, aggregating multiple back-end services, or reducing over/under-fetching.

gRPC + Protobuf

gRPC uses Protocol Buffers as its IDL and HTTP/2 as transport. It generates strongly-typed client/server stubs. Best for: internal service-to-service communication where performance, type safety, and streaming matter more than browser compatibility.

When to use which

Common tasks

1. Design RESTful resource endpoints

Use lowercase, hyphen-separated plural nouns. Never use verbs in the path.

# Collections
GET    /v1/articles          - list
POST   /v1/articles          - create

# Single resource
GET    /v1/articles/{id}     - read
PUT    /v1/articles/{id}     - full replace
PATCH  /v1/articles/{id}     - partial update
DELETE /v1/articles/{id}     - delete

# Sub-resources
GET    /v1/users/{id}/orders - list orders for a user

# Actions that don't map to CRUD (use verb noun under resource)
POST   /v1/orders/{id}/cancel
POST   /v1/users/{id}/password-reset

2. Write an OpenAPI 3.1 spec

Always use $ref to pull components out of paths for reuse. See references/openapi-patterns.md for the full component library (security schemes, reusable responses, discriminators, webhooks).

openapi: 3.1.0
info:
  title: Articles API
  version: 1.0.0

servers:
  - url: https://api.example.com/v1

paths:
  /articles:
    get:
      operationId: listArticles
      summary: List articles
      tags: [Articles]
      parameters:
        - { name: cursor, in: query, schema: { type: string } }
        - { name: limit,  in: query, schema: { type: integer, default: 20, maximum: 100 } }
      responses:
        '200':
          description: Paginated list of articles
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ArticleListResponse'
        '400': { $ref: '#/components/responses/BadRequest' }

    post:
      operationId: createArticle
      summary: Create an article
      tags: [Articles]
      security:
        - bearerAuth: []
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              required: [title]
              properties:
                title: { type: string, maxLength: 255 }
                body:  { type: string }
      responses:
        '201':
          description: Article created
          content:
            application/json:
              schema: { $ref: '#/components/schemas/Article' }
        '422': { $ref: '#/components/responses/UnprocessableEntity' }

components:
  schemas:
    Article:
      type: object
      required: [id, title, status, createdAt]
      properties:
        id:        { type: string, format: uuid }
        title:     { type: string, maxLength: 255 }
        status:    { type: string, enum: [draft, published, archived] }
        createdAt: { type: string, format: date-time }

    ArticleListResponse:
      type: object
      required: [data, pagination]
      properties:
        data:
          type: array
          items: { $ref: '#/components/schemas/Article' }
        pagination:
          type: object
          properties:
            nextCursor: { type: [string, "null"] }
            hasMore:    { type: boolean }

  responses:
    BadRequest:
      description: Invalid request
      content:
        application/problem+json:
          schema: { $ref: '#/components/schemas/ProblemDetails' }
    UnprocessableEntity:
      description: Validation failed
      content:
        application/problem+json:
          schema: { $ref: '#/components/schemas/ProblemDetails' }

  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      bearerFormat: JWT

3. Implement cursor-based pagination

Cursor pagination is stable under concurrent writes; offset pagination is not.

interface PaginationParams {
  cursor?: string;
  limit?: number;
}

interface PaginatedResult<T> {
  data: T[];
  pagination: {
    nextCursor: string | null;
    hasMore: boolean;
  };
}

async function listArticles(
  params: PaginationParams
): Promise<PaginatedResult<Article>> {
  const limit = Math.min(params.limit ?? 20, 100);

  // Decode opaque cursor back to an internal value
  const afterId = params.cursor
    ? Buffer.from(params.cursor, 'base64url').toString('utf8')
    : null;

  const rows = await db.article.findMany({
    where: afterId ? { id: { gt: afterId } } : undefined,
    orderBy: { id: 'asc' },
    take: limit + 1, // fetch one extra to detect hasMore
  });

  const hasMore = rows.length > limit;
  const data = hasMore ? rows.slice(0, limit) : rows;
  const lastId = data.at(-1)?.id ?? null;

  return {
    data,
    pagination: {
      nextCursor: hasMore && lastId
        ? Buffer.from(lastId).toString('base64url')
        : null,
      hasMore,
    },
  };
}

4. Implement API versioning

Recommendation: URL path versioning for public APIs (/v1/, /v2/), header versioning for internal/partner APIs. Avoid query param versioning - it leaks into caches and logs.

import { Router } from 'express';

// Option A: URL path (public APIs) - each version is a separate router
const v1 = Router(); v1.get('/articles', v1ArticlesHandler);
const v2 = Router(); v2.get('/articles', v2ArticlesHandler);
app.use('/v1', v1);
app.use('/v2', v2);

// Option B: Header versioning (internal/partner APIs)
// Request header: Api-Version: 2
function versionMiddleware(req: Request, res: Response, next: NextFunction) {
  req.apiVersion = parseInt((req.headers['api-version'] as string) ?? '1', 10);
  next();
}

// Option C: Content negotiation
// Accept: application/vnd.example.v2+json

5. Design error response format (RFC 7807)

Always return machine-readable errors. Use application/problem+json content type.

interface ProblemDetails {
  type: string;      // URI identifying the error class
  title: string;     // Human-readable summary (stable per type)
  status: number;    // HTTP status code
  detail?: string;   // Human-readable explanation for this occurrence
  instance?: string; // URI of the specific request (e.g. trace ID)
  [key: string]: unknown; // Extension fields allowed
}

function problemResponse(
  res: Response,
  status: number,
  type: string,
  title: string,
  detail?: string,
  extensions?: Record<string, unknown>
) {
  res.status(status).type('application/problem+json').json({
    type: `https://api.example.com/errors/${type}`,
    title,
    status,
    detail,
    instance: `/requests/${res.locals.requestId}`,
    ...extensions,
  } satisfies ProblemDetails);
}

// Usage
problemResponse(res, 422, 'validation-error', 'Request validation failed',
  'The field "title" must not exceed 255 characters.',
  { fields: [{ field: 'title', message: 'Too long' }] }
);

6. Design authentication

Three patterns, in order of complexity:

import jwt from 'jsonwebtoken';

// JWT middleware - validates token, rejects with 401 on failure
function authMiddleware(req: Request, res: Response, next: NextFunction) {
  const header = req.headers.authorization ?? '';
  if (!header.startsWith('Bearer ')) {
    return problemResponse(res, 401, 'unauthorized', 'Missing bearer token');
  }
  try {
    req.user = jwt.verify(header.slice(7), process.env.JWT_SECRET!) as JwtPayload;
    next();
  } catch {
    problemResponse(res, 401, 'invalid-token', 'Token is invalid or expired');
  }
}

// Scope guard - rejects with 403 if required scope is absent
function requireScope(scope: string) {
  return (req: Request, res: Response, next: NextFunction) => {
    if (!req.user?.scopes?.includes(scope)) {
      return problemResponse(res, 403, 'forbidden', `Scope "${scope}" required`);
    }
    next();
  };
}

app.delete('/v1/articles/:id', authMiddleware, requireScope('articles:write'), handler);

7. Choose REST vs GraphQL vs gRPC

Decision rule: Start with REST. Move to GraphQL when UI teams are blocked by over/under-fetching. Move to gRPC for high-throughput internal services where latency and type safety are critical.

Error handling reference

Gotchas

1. Offset pagination breaks under concurrent writes - Offset-based pagination (?page=2&limit=20) produces incorrect results when rows are inserted or deleted between pages. Use cursor-based pagination (keyset/seek) for any dataset that changes while clients are paginating through it.

2. Breaking changes in "minor" updates - Removing a field, changing a field's type, or narrowing an enum are breaking changes even if you don't bump the version. Consumers fail at runtime with no warning. Use the expand-contract pattern: add the new field, deprecate the old one, remove it only after all consumers have migrated.

3. 422 vs 400 confusion - 400 Bad Request is for malformed requests (unparseable JSON, wrong content type). 422 Unprocessable Entity is for syntactically valid requests that fail business validation (email already taken, negative quantity). Returning 400 for validation errors prevents consumers from distinguishing parse errors from validation failures.

4. URL versioning leaks into caches and logs - Query parameter versioning (?version=2) gets cached incorrectly by HTTP caches that ignore query strings, and pollutes analytics logs. URL path versioning (/v2/) is cleanest for public APIs; header versioning is better for internal APIs that need per-consumer negotiation.

5. DELETE returning 200 with a body vs 204 - Many clients discard the body on 204 No Content responses. If you need to return data from a delete operation, use 200 OK with a body. If nothing needs to be returned, use 204. Mixing them creates client parsing bugs.

References

• RFC 7807 - Problem Details for HTTP APIs
• OpenAPI 3.1 Specification
• Google API Design Guide
• Microsoft REST API Guidelines
• GraphQL Best Practices
• gRPC Concepts
• Stripe API Reference - exemplary REST API design
• references/openapi-patterns.md - reusable OpenAPI 3.1 component patterns

Companion check

On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install:

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Skip entirely if recommended_skills is empty or all companions are already installed.