# API Design

> RESTful API design principles — resource naming, HTTP verbs, status codes, versioning, cursor pagination, error shape, idempotency

- Skill: `nadav011/api-design` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nadav011/api-design`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nadav011/api-design/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: Nadav011 (https://skillmd.com/u/nadav011)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nadav011/api-design

---

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# API Design

---

## Resource Naming

Resources are nouns, not verbs. URLs identify things; HTTP methods describe what to do with them.

```
# ❌ WRONG — verbs in paths, inconsistent case
GET  /getUsers
POST /createUser
POST /deleteUser?id=123
GET  /user_orders/456

# ✅ CORRECT — plural nouns, kebab-case, hierarchical
GET    /users
POST   /users
GET    /users/:id
PATCH  /users/:id
DELETE /users/:id
GET    /users/:id/orders
GET    /users/:id/orders/:orderId
```

**Rules:**
- **Plural nouns** for collections: `/users`, `/orders`, `/products`
- **kebab-case** for multi-word paths: `/payment-methods`, `/access-tokens`
- **Nest sparingly** — maximum two levels deep. `/users/:id/orders` is fine; `/users/:id/orders/:orderId/line-items/:itemId/discounts` is not
- **No file extensions** in URLs (no `.json`, `.xml`)
- **Lowercase** everywhere

---

## HTTP Verbs and Idempotency

| Method | Meaning | Idempotent | Safe | Body |
|--------|---------|-----------|------|------|
| `GET` | Read resource(s) | Yes | Yes | No |
| `HEAD` | Read metadata only | Yes | Yes | No |
| `POST` | Create resource or non-idempotent action | No | No | Yes |
| `PUT` | Full replacement of resource | Yes | No | Yes |
| `PATCH` | Partial update | No* | No | Yes |
| `DELETE` | Remove resource | Yes | No | No |

\* PATCH is idempotent only if designed that way (e.g. `{ set: { status: "active" } }` vs `{ increment: { count: 1 } }`).

```typescript
// ✅ CORRECT — verb reflects semantics
POST   /users            // creates a new user (not idempotent — two calls = two users)
PUT    /users/:id        // replaces the entire user document
PATCH  /users/:id        // updates specific fields
DELETE /users/:id        // removes user — safe to call twice (second returns 404)

// ❌ WRONG — tunnelling everything through POST
POST /users/update
POST /users/delete
POST /api?action=getUser
```

---

## HTTP Status Codes

Use the most precise code. Never return `200 OK` with `{ success: false }` in the body.

### 2xx — Success

| Code | Name | When to use |
|------|------|-------------|
| `200` | OK | GET/PATCH/PUT succeeded, body contains the resource |
| `201` | Created | POST succeeded, resource was created; include `Location` header |
| `204` | No Content | DELETE / action succeeded, no body to return |
| `202` | Accepted | Request accepted for async processing; poll a status endpoint |

### 4xx — Client Error

| Code | Name | When to use |
|------|------|-------------|
| `400` | Bad Request | Malformed JSON, missing required field |
| `401` | Unauthorized | Not authenticated (no token / token invalid) |
| `403` | Forbidden | Authenticated but not allowed to access this resource |
| `404` | Not Found | Resource doesn't exist |
| `409` | Conflict | Duplicate resource, state conflict (e.g. already cancelled) |
| `422` | Unprocessable Entity | Structurally valid but semantically wrong (validation failure) |
| `429` | Too Many Requests | Rate limited; include `Retry-After` header |

### 5xx — Server Error

| Code | Name | When to use |
|------|------|-------------|
| `500` | Internal Server Error | Unexpected crash; do not leak details |
| `502` | Bad Gateway | Upstream dependency failed |
| `503` | Service Unavailable | Intentional downtime / overload; include `Retry-After` |

---

## Versioning Strategy

**URL prefix versioning** is the most practical approach for public APIs.

```
# ✅ RECOMMENDED — explicit, cacheable, easy to route
GET /v1/users
GET /v2/users

# Alternatives (with trade-offs)
Accept: application/vnd.myapi.v2+json   # Header versioning — harder to test in browser
?version=2                               # Query param — accidentally cached, easy to miss
```

**Rules:**
- Start at `/v1/` from day one, even for internal APIs
- Never remove a version without a documented migration window (minimum 6 months for external consumers)
- Breaking changes = new major version. Additive changes (new fields, new optional params) can ship within the same version
- Document what "breaking" means: removing a field, renaming a field, changing a field type, changing behavior of an existing endpoint

```typescript
// Route structure example
app.use('/v1', v1Router);
app.use('/v2', v2Router);

// Or with Express Router
const v1 = express.Router();
v1.get('/users', getUsersV1);

const v2 = express.Router();
v2.get('/users', getUsersV2); // e.g. cursor pagination instead of offset
```

---

## Pagination: Cursor over Offset

Offset pagination breaks when items are inserted or deleted between pages. Cursor pagination is stable.

```typescript
// ❌ WRONG — offset pagination (breaks on concurrent writes)
GET /users?page=3&per_page=20
// If 5 users are inserted before page 3, the user sees duplicates or misses items

// ✅ CORRECT — cursor pagination (stable, scalable)
GET /users?limit=20
GET /users?limit=20&cursor=eyJ1c2VyX2lkIjoiMTIzIn0
```

```typescript
// Response shape for cursor pagination
{
  "data": [ /* array of items */ ],
  "pagination": {
    "nextCursor": "eyJ1c2VyX2lkIjoiMTQzIn0",  // null when no more pages
    "hasMore": true,
    "limit": 20
  }
}
```

```typescript
// Implementation (TypeScript + PostgreSQL)
interface PaginationParams {
  cursor?: string; // opaque base64-encoded JSON
  limit: number;
}

interface CursorPayload {
  createdAt: string;
  id: string;
}

function decodeCursor(cursor: string): CursorPayload {
  return JSON.parse(Buffer.from(cursor, 'base64url').toString('utf8'));
}

function encodeCursor(item: { createdAt: Date; id: string }): string {
  const payload: CursorPayload = {
    createdAt: item.createdAt.toISOString(),
    id: item.id,
  };
  return Buffer.from(JSON.stringify(payload)).toString('base64url');
}

async function getUsers(params: PaginationParams) {
  const limit = Math.min(params.limit, 100); // cap to prevent abuse
  const cursor = params.cursor ? decodeCursor(params.cursor) : null;

  const users = await db.users.findMany({
    take: limit + 1, // fetch one extra to determine hasMore
    where: cursor
      ? {
          OR: [
            { createdAt: { lt: new Date(cursor.createdAt) } },
            { createdAt: new Date(cursor.createdAt), id: { gt: cursor.id } },
          ],
        }
      : undefined,
    orderBy: [{ createdAt: 'desc' }, { id: 'asc' }],
  });

  const hasMore = users.length > limit;
  const data = hasMore ? users.slice(0, limit) : users;
  const lastItem = data.at(-1);

  return {
    data,
    pagination: {
      nextCursor: hasMore && lastItem ? encodeCursor(lastItem) : null,
      hasMore,
      limit,
    },
  };
}
```

---

## Error Response Shape (RFC 7807)

All errors use the same shape. Clients can write one error handler.

```typescript
// Consistent error envelope
{
  "error": {
    "code": "VALIDATION_FAILED",       // machine-readable, stable string
    "message": "Request validation failed", // human-readable, may change
    "details": [                        // optional — per-field errors
      { "field": "email", "message": "Must be a valid email address" },
      { "field": "age",   "message": "Must be 18 or older" }
    ],
    "traceId": "01HX4ZK9R3BVNMJ2QPFW5T8CDE" // correlate with server logs
  }
}
```

```typescript
// Express error handler
app.use((err: unknown, _req: Request, res: Response, _next: NextFunction) => {
  const appErr = err instanceof AppError ? err : null;
  const statusCode = appErr?.statusCode ?? 500;

  res.status(statusCode).json({
    error: {
      code: appErr?.code ?? 'INTERNAL_ERROR',
      message: statusCode < 500
        ? (appErr?.message ?? 'Bad request')
        : 'An unexpected error occurred',
      ...(appErr instanceof ValidationError && { details: appErr.fields }),
      traceId: getTraceContext()?.traceId,
    },
  });
});
```

**Rules:**
- `code` is a SCREAMING_SNAKE_CASE string that never changes between versions
- `message` is human-readable and may change — clients must not parse it
- Include `traceId` so users can report a specific request to support
- 5xx errors must NOT include stack traces, SQL, or file paths in the response

---

## Idempotency for Mutations

POST requests are not idempotent by default. Allow clients to safely retry.

```typescript
// Client sends Idempotency-Key header
POST /v1/payments
Idempotency-Key: 7f0c8e2a-5d1f-4b9e-a1c3-2e4f6d8b0a9c
Content-Type: application/json

{ "amount": 9900, "currency": "USD", "customerId": "cus_abc" }
```

```typescript
// Server: store result keyed by idempotency key
async function createPayment(
  req: Request,
  res: Response,
): Promise<void> {
  const idempotencyKey = req.headers['idempotency-key'] as string | undefined;

  if (idempotencyKey) {
    // Check if we already processed this key
    const cached = await cache.get(`idem:${idempotencyKey}`);
    if (cached) {
      // Return exact same response as original — same status code, same body
      res.status(cached.status).json(cached.body);
      return;
    }
  }

  const payment = await paymentService.create(req.body);
  const responseBody = { id: payment.id, status: payment.status };

  if (idempotencyKey) {
    // Cache for 24h — long enough to cover network retries
    await cache.set(`idem:${idempotencyKey}`, {
      status: 201,
      body: responseBody,
    }, { ttl: 86400 });
  }

  res.status(201).json(responseBody);
}
```

**Rules:**
- Idempotency keys should be client-generated UUIDs
- Cache the response (status + body), not just the fact that it happened
- TTL of at least 24 hours for the idempotency store
- Return the exact same response on replay — do not re-execute the mutation
- Required for: payment endpoints, order creation, any operation with real-world side effects

---

## Response Envelope Consistency

Pick one shape and use it everywhere. Mixing patterns forces clients to write multiple parsers.

```typescript
// ✅ CORRECT — consistent envelope
// Single resource
{ "data": { "id": "usr_123", "email": "..." } }

// Collection
{ "data": [...], "pagination": { "nextCursor": "...", "hasMore": true, "limit": 20 } }

// Created resource
// HTTP 201 + Location: /v1/users/usr_123
{ "data": { "id": "usr_123", ... } }

// Action with no return value
// HTTP 204 — empty body

// Error
{ "error": { "code": "...", "message": "...", "traceId": "..." } }
```

---

## Checklist

- [ ] All paths use plural nouns and kebab-case
- [ ] No verbs in paths — HTTP method carries the verb
- [ ] Versioning prefix present from day one (`/v1/`)
- [ ] Correct status code for each case (201 for created, 204 for deleted, 422 for validation, etc.)
- [ ] All errors use the same `{ error: { code, message, traceId } }` shape
- [ ] Cursor pagination with opaque cursors (not page/offset)
- [ ] `Idempotency-Key` support on POST endpoints with side effects
- [ ] `Location` header returned on 201 Created responses
- [ ] `Retry-After` header returned on 429 and 503 responses
- [ ] API depth capped at two levels of nesting