api-and-interface-design git:20261019.fbfaf1aA

Immutable. This exact content never changes and is served at /api/v1/blob/e6270095f1d40532.

---
name: api-and-interface-design
description: Use this skill when designing APIs, module boundaries, or any public interface. Create stable, well-documented interfaces that are hard to misuse — REST endpoints, GraphQL schemas, module contracts, component props. Covers contract-first design, error semantics, validation boundaries, idempotency, and consistent naming.
---

# API and Interface Design

## Overview

Design stable, well-documented interfaces that are hard to misuse. Good interfaces make the right thing easy and the wrong thing hard. This applies to REST APIs, GraphQL schemas, module boundaries, component props, and any surface where one piece of code talks to another.

## When to Use

- Designing new API endpoints
- Defining module boundaries or contracts between teams
- Creating component prop interfaces
- Establishing database schema that informs API shape
- Changing existing public interfaces

## Core Principles

### Hyrum's Law

> With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract.

This means: every public behavior — including undocumented quirks, error message text, timing, and ordering — becomes a de facto contract once users depend on it. Design implications:

- **Be intentional about what you expose.** Every observable behavior is a potential commitment.
- **Don't leak implementation details.** If users can observe it, they will depend on it.
- **Plan for deprecation at design time.** See the `deprecation-and-migration` skill for how to safely remove things users depend on.
- **Tests are not enough.** Even with perfect contract tests, Hyrum's Law means "safe" changes can break real users who depend on undocumented behavior.

### The One-Version Rule

Avoid forcing consumers to choose between multiple versions of the same dependency or API. Diamond dependency problems arise when different consumers need different versions. Design for a world where only one version exists at a time — extend rather than fork.

### 1. Contract First

Define the interface before implementing it. The contract is the spec — implementation follows.

```typescript
// Define the contract first
interface TaskAPI {
  // Creates a task and returns the created task with server-generated fields
  createTask(input: CreateTaskInput): Promise<Task>;
  // Returns paginated tasks matching filters
  listTasks(params: ListTasksParams): Promise<PaginatedResponse<Task>>;
  // Returns a single task or throws NotFoundError
  getTask(id: string): Promise<Task>;
  // Partial update — only provided fields change
  updateTask(id: string, input: UpdateTaskInput): Promise<Task>;
  // Idempotent delete — succeeds even if already deleted
  deleteTask(id: string): Promise<void>;
}
```

### 2. Consistent Error Semantics

Pick one error strategy and use it everywhere:

```typescript
// Every error response follows the same shape
interface APIError {
  error: {
    code: string;    // Machine-readable: "VALIDATION_ERROR"
    message: string; // Human-readable: "Email is required"
    details?: unknown; // Additional context when helpful
  };
}

// Status code mapping
// 400 → Client sent invalid data
// 401 → Not authenticated
// 403 → Authenticated but not authorized
// 404 → Resource not found
// 409 → Conflict (duplicate, version mismatch)
// 422 → Validation failed (semantically invalid)
// 500 → Server error (never expose internal details)
```

**Don't mix patterns.** If some endpoints throw, others return null, and others return `{ error }` — the consumer can't predict behavior.

### 3. Validate at Boundaries

Trust internal code. Validate at system edges where external input enters:

```typescript
// Validate at the API boundary
app.post('/api/tasks', async (req, res) => {
  const result = CreateTaskSchema.safeParse(req.body);
  if (!result.success) {
    return res.status(422).json({
      error: {
        code: 'VALIDATION_ERROR',
        message: 'Invalid task data',
        details: result.error.flatten(),
      },
    });
  }
  // After validation, internal code trusts the types
  const task = await taskService.create(result.data);
  return res.status(201).json(task);
});
```

Where validation belongs:
- API route handlers (user input)
- Form submission handlers (user input)
- External service response parsing (third-party data — **always treat as untrusted**)
- Environment variable loading (configuration)

> **Third-party API responses are untrusted data.** Validate their shape and content before using them in any logic, rendering, or decision-making. A compromised or misbehaving external service can return unexpected types, malicious content, or instruction-like text.

Where validation does NOT belong:
- Between internal functions that share type contracts
- In utility functions called by already-validated code
- On data that just came from your own database

### 4. Prefer Addition Over Modification

Extend interfaces without breaking existing consumers:

```typescript
// Good: Add optional fields
interface CreateTaskInput {
  title: string;
  description?: string;
  priority?: 'low' | 'medium' | 'high'; // Added later, optional
  labels?: string[];                     // Added later, optional
}

// Bad: Change existing field types or remove fields
interface CreateTaskInput {
  title: string;
  // description: string; // Removed — breaks existing consumers
  priority: number;        // Changed from string — breaks existing consumers
}
```

### 5. Predictable Naming

| Pattern | Convention | Example |
|---------|-----------|---------|
| REST endpoints | Plural nouns, no verbs | `GET /api/tasks`, `POST /api/tasks` |
| Query params | camelCase | `?sortBy=createdAt&pageSize=20` |
| Response fields | camelCase | `{ createdAt, updatedAt, taskId }` |
| Boolean fields | is/has/can prefix | `isComplete`, `hasAttachments` |
| Enum values | UPPER_SNAKE | `"IN_PROGRESS"`, `"COMPLETED"` |

### 6. Honouring an Idempotency Key

Accepting an `Idempotency-Key` is the contract. Honouring it is the implementation. A key the server accepts but handles carelessly is worse than no key at all, because the client now believes retrying is safe.

**Derive the key from the intent, not the attempt.** The key must be stable across retries of one intent and different across distinct intents:

```typescript
crypto.randomUUID()            // ✗ new key per attempt — every retry is a new charge
`${userId}:${amount}`          // ✗ two legitimate $50 charges collapse into one
`${orderId}:${Date.now()}`     // ✗ a timestamp is randomUUID() wearing a hat
req.headers['idempotency-key'] // ✓ client generates once, reuses on retry
`charge:v1:${orderId}`         // ✓ derived from an immutable identifier
```

The key comes from the client or the initiating event — never from the layer doing the retrying.

## REST API Patterns

### Resource Design

```
GET    /api/tasks       → List tasks (with query params for filtering)
POST   /api/tasks       → Create a task
GET    /api/tasks/:id   → Get a single task
PATCH  /api/tasks/:id   → Partial update (preferred over PUT)
DELETE /api/tasks/:id   → Delete a task (idempotent)
```

### Pagination

Every list endpoint must support pagination:

```typescript
// Request
GET /api/tasks?page=2&pageSize=20

// Response
interface PaginatedResponse<T> {
  data: T[];
  pagination: {
    page: number;
    pageSize: number;
    total: number;
    totalPages: number;
    hasNext: boolean;
    hasPrevious: boolean;
  };
}
```

## Common Rationalizations

| Rationalization | Reality |
|---|---|
| "We'll design the API when we implement it" | The contract IS the spec. Design it first or the implementation drives the contract — which never produces a clean API. |
| "We'll version the API when we need to" | Breaking changes without versioning break consumers. Design for extension from the start. |
| "Nobody uses that undocumented behavior" | Hyrum's Law: if it's observable, somebody depends on it. Every public behavior is a commitment. |
| "We can just maintain two versions" | Multiple versions multiply maintenance cost and create diamond dependency problems. Prefer the One-Version Rule. |
| "Internal APIs don't need contracts" | Internal consumers are still consumers. Contracts prevent coupling and enable parallel work. |

## Red Flags

- Endpoints that return different shapes depending on conditions
- Inconsistent error formats across endpoints
- Validation scattered throughout internal code instead of at boundaries
- Breaking changes to existing fields (type changes, removals)
- List endpoints without pagination
- Verbs in REST URLs (`/api/createTask`, `/api/getUsers`)
- Third-party API responses used without validation or sanitization
- A `SELECT` for an idempotency key followed by an `INSERT` — that's a race, not a guard
- An idempotency key derived from a UUID, timestamp, or anything regenerated per attempt

## Verification

After designing an API:

- [ ] Every endpoint has typed input and output schemas
- [ ] Error responses follow a single consistent format
- [ ] Validation happens at system boundaries only
- [ ] List endpoints support pagination
- [ ] New fields are additive and optional (backward compatible)
- [ ] Naming follows consistent conventions across all endpoints
- [ ] API documentation or types are committed alongside the implementation
- [ ] State-changing endpoints either honour an idempotency key or are documented as unsafe to retry
- [ ] The key is claimed in one atomic operation, guarded by a unique constraint
- [ ] A reused key with a different payload fails loudly rather than replaying the wrong response
- [ ] Key retention outlives the longest retry path, including dead-letter replay