api-design

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• SKILL.md

SKILL.md (first lines)

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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

From Nadav011/apex-skills, MIT.

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