SDK Design

Design client libraries (SDKs) with excellent developer experience through intuitive APIs, robust error handling, automatic retries, and consistent patterns across programming languages.

When to Use This Skill

Use when building a client library for a REST API, creating internal service SDKs, implementing retry logic with exponential backoff, handling authentication patterns, creating typed error hierarchies, implementing pagination with async iterators, or designing streaming APIs for real-time data.

Core Architecture Patterns

Client → Resources → Methods

Organize SDK code hierarchically:

Client (config: API key, base URL, retries, timeout)
├─ Resources (users, payments, posts)
│   ├─ create(), retrieve(), update(), delete()
│   └─ list() (with pagination)
└─ Top-Level Methods (convenience)

Resource-Based (Stripe style):

typescript
const client = new APIClient({ apiKey: 'sk_test_...' })
const user = await client.users.create({ email: 'user@example.com' })

Use for APIs <100 methods. Prioritizes developer experience.

Command-Based (AWS SDK v3):

typescript
import { S3Client, PutObjectCommand } from '@aws-sdk/client-s3'
await client.send(new PutObjectCommand({ Bucket: '...' }))

Use for APIs >100 methods. Prioritizes bundle size and tree-shaking.

For detailed architectural guidance, see references/architecture-patterns.md.

Language-Specific Patterns

TypeScript: Async-Only

typescript
const user = await client.users.create({ email: 'user@example.com' })

All methods return Promises. Avoid callbacks.

Python: Dual Sync/Async

python
# Sync
client = APIClient(api_key='sk_test_...')
user = client.users.create(email='user@example.com')

# Async
async_client = AsyncAPIClient(api_key='sk_test_...')
user = await async_client.users.create(email='user@example.com')

Provide both clients. Users choose based on architecture.

Go: Sync with Context

go
client := apiclient.New("api_key")
user, err := client.Users().Create(ctx, req)

Use context.Context for timeout and cancellation.

Authentication

API Key (Most Common)

typescript
const client = new APIClient({ apiKey: process.env.API_KEY })

Store keys in environment variables, never hardcode.

OAuth Token Refresh

typescript
const client = new APIClient({
  clientId: 'id',
  clientSecret: 'secret',
  refreshToken: 'token',
  onTokenRefresh: (newToken) => saveToken(newToken)
})

SDK automatically refreshes tokens before expiry.

Bearer Token Per-Request

typescript
await client.users.list({
  headers: { Authorization: `Bearer ${userToken}` }
})

Use for multi-tenant applications.

See references/authentication.md for OAuth flows, JWT handling, and credential providers.

Retry and Backoff

Exponential Backoff with Jitter

typescript
async function retryWithBackoff<T>(fn: () => Promise<T>, maxRetries: number): Promise<T> {
  let attempt = 0

  while (attempt <= maxRetries) {
    try {
      return await fn()
    } catch (error) {
      attempt++
      if (attempt > maxRetries || !isRetryable(error)) throw error

      const exponential = Math.min(1000 * Math.pow(2, attempt - 1), 10000)
      const jitter = Math.random() * 500
      await sleep(exponential + jitter)
    }
  }
}

function isRetryable(error: any): boolean {
  return (
    error.code === 'ECONNRESET' ||
    error.code === 'ETIMEDOUT' ||
    (error.status >= 500 && error.status < 600) ||
    error.status === 429
  )
}

Retry Decision Matrix:

| Error Type | Retry? | Rationale | |------------|--------|-----------| | 5xx, 429, Network Timeout | ✅ Yes | Transient errors | | 4xx, 401, 403, 404 | ❌ No | Client errors won't fix themselves |

Rate Limit Handling

typescript
if (error.status === 429) {
  const retryAfter = parseInt(error.headers['retry-after'] || '60')
  await sleep(retryAfter * 1000)
}

Respect Retry-After header on 429 responses.

See references/retry-backoff.md for jitter strategies, circuit breakers, and idempotency keys.

Error Handling

Typed Error Hierarchy

typescript
class APIError extends Error {
  constructor(
    message: string,
    public status: number,
    public code: string,
    public requestId: string
  ) {
    super(message)
    this.name = 'APIError'
  }
}

class RateLimitError extends APIError {
  constructor(message: string, requestId: string, public retryAfter: number) {
    super(message, 429, 'rate_limit_error', requestId)
  }
}

class AuthenticationError extends APIError {
  constructor(message: string, requestId: string) {
    super(message, 401, 'authentication_error', requestId)
  }
}

Error Handling in Practice

typescript
try {
  const user = await client.users.create({ email: 'invalid' })
} catch (error) {
  if (error instanceof RateLimitError) {
    await sleep(error.retryAfter * 1000)
  } else if (error instanceof AuthenticationError) {
    console.error('Invalid API key')
  } else if (error instanceof APIError) {
    console.error(`${error.message} (Request ID: ${error.requestId})`)
  }
}

Include request ID in all errors for debugging.

See references/error-handling.md for user-friendly messages, validation errors, and debugging support.

Pagination

Async Iterators (Recommended)

TypeScript:

typescript
for await (const user of client.users.list({ limit: 100 })) {
  console.log(user.id, user.email)
}

Python:

python
async for user in client.users.list(limit=100):
    print(user.id, user.email)

SDK automatically fetches next page.

Implementation

typescript
class UsersResource {
  async *list(options?: { limit?: number }): AsyncGenerator<User> {
    let cursor: string | undefined = undefined

    while (true) {
      const response = await this.client.request('GET', '/users', {
        query: { limit: String(options?.limit || 100), ...(cursor ? { cursor } : {}) }
      })

      for (const user of response.data) yield user

      if (!response.has_more) break
      cursor = response.next_cursor
    }
  }
}

Manual Pagination

typescript
let cursor: string | undefined = undefined
while (true) {
  const response = await client.users.list({ limit: 100, cursor })
  for (const user of response.data) console.log(user.id)
  if (!response.has_more) break
  cursor = response.next_cursor
}

Provide both automatic and manual options.

See references/pagination.md for cursor vs. offset pagination and Go channel patterns.

Streaming

Server-Sent Events

typescript
async *stream(path: string, body?: any): AsyncGenerator<any> {
  const response = await fetch(url, {
    headers: { 'Accept': 'text/event-stream' },
    body: JSON.stringify(body)
  })

  const reader = response.body!.getReader()
  const decoder = new TextDecoder()

  while (true) {
    const { done, value } = await reader.read()
    if (done) break

    const chunk = decoder.decode(value)
    for (const line of chunk.split('\n')) {
      if (line.startsWith('data: ')) {
        const data = line.slice(6)
        if (data === '[DONE]') return
        yield JSON.parse(data)
      }
    }
  }
}

// Usage
for await (const chunk of client.posts.stream({ prompt: 'Write a story' })) {
  process.stdout.write(chunk.content)
}

Idempotency Keys

Prevent duplicate operations during retries:

typescript
import { randomUUID } from 'crypto'

if (['POST', 'PATCH', 'PUT'].includes(method)) {
  headers['Idempotency-Key'] = options?.idempotencyKey || randomUUID()
}

// Usage
await client.charges.create(
  { amount: 1000 },
  { idempotencyKey: 'charge_unique_123' }
)

Server deduplicates requests by key.

Versioning

Semantic Versioning

• 1.0.0 → 1.1.0: New features (safe)
• 1.1.0 → 2.0.0: Breaking changes (review)
• 1.0.0 → 1.0.1: Bug fixes (safe)

Deprecation Warnings

typescript
function deprecated(message: string, since: string) {
  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    const originalMethod = descriptor.value
    descriptor.value = function (...args: any[]) {
      console.warn(`[DEPRECATED] ${propertyKey} since ${since}. ${message}`)
      return originalMethod.apply(this, args)
    }
    return descriptor
  }
}

@deprecated('Use users.list() instead', 'v2.0.0')
async getAll() { return this.list() }

API Version Pinning

typescript
const client = new APIClient({
  apiKey: 'sk_test_...',
  apiVersion: '2025-01-01'
})

See references/versioning.md for migration strategies.

Configuration Best Practices

typescript
interface ClientConfig {
  apiKey: string
  baseURL?: string
  maxRetries?: number
  timeout?: number
  apiVersion?: string
  onTokenRefresh?: (token: string) => void
}

class APIClient {
  constructor(config: ClientConfig) {
    this.apiKey = config.apiKey
    this.baseURL = config.baseURL || 'https://api.example.com'
    this.maxRetries = config.maxRetries ?? 3
    this.timeout = config.timeout ?? 30000
  }
}

Provide sensible defaults, require only apiKey.

Quick Reference Tables

Authentication Patterns

| Pattern | Use Case | |---------|----------| | API Key | Service-to-service | | OAuth Refresh | User-based auth | | Bearer Per-Request | Multi-tenant |

Retry Strategies

| Strategy | Use Case | |----------|----------| | Exponential Backoff | Default retry | | Rate Limit | 429 responses | | Max Retries | Avoid infinite loops (3-5) |

Pagination Options

| Pattern | Language | Use Case | |---------|----------|----------| | Async Iterator | TypeScript, Python | Automatic pagination | | Generator | Python | Sync pagination | | Channels | Go | Concurrent iteration | | Manual | All | Explicit control |

Reference Documentation

Architecture:

• references/architecture-patterns.md - Resource vs. command organization

Core Patterns:

• references/authentication.md - OAuth, token refresh, credential providers
• references/retry-backoff.md - Exponential backoff, jitter, circuit breakers
• references/error-handling.md - Error hierarchies, debugging support
• references/pagination.md - Cursor vs. offset, async iterators
• references/versioning.md - SemVer, deprecation strategies
• references/testing-sdks.md - Unit testing, mocking, integration tests

Code Examples

TypeScript:

• examples/typescript/basic-client.ts - Simple async SDK
• examples/typescript/advanced-client.ts - Retry, errors, streaming
• examples/typescript/resource-based.ts - Stripe-style organization

Python:

• examples/python/sync-client.py - Synchronous client
• examples/python/async-client.py - Async client with asyncio
• examples/python/dual-client.py - Both sync and async

Go:

• examples/go/basic-client.go - Simple Go client
• examples/go/context-client.go - Context patterns
• examples/go/channel-pagination.go - Channel-based pagination

Best-in-Class SDK Examples

Study these production SDKs:

TypeScript/JavaScript:

• AWS SDK v3 (@aws-sdk/client-*): Modular, tree-shakeable, middleware
• Stripe Node (stripe): Resource-based, typed errors, excellent DX
• OpenAI Node (openai): Streaming, async iterators, modern TypeScript

Python:

• Boto3 (boto3): Resource vs. client patterns, paginators
• Stripe Python (stripe): Dual sync/async, context managers

Go:

• AWS SDK Go v2 (github.com/aws/aws-sdk-go-v2): Context, middleware

Common Pitfalls

Avoid these mistakes:

1. No Retry Logic - All SDKs need automatic retries for transient errors
2. Poor Error Messages - Include request ID, status code, error type
3. No Pagination - Implement automatic pagination with async iterators
4. Hardcoded Credentials - Use environment variables or config files
5. Missing Idempotency - Add idempotency keys to prevent duplicate operations
6. Ignoring Rate Limits - Respect Retry-After header on 429 responses
7. Breaking Changes - Use SemVer, deprecate before removing

Integration with Other Skills

• api-design-principles: API design complements SDK design (error codes → error classes)
• building-clis: CLIs wrap SDKs for command-line access
• testing-strategies: Test SDKs with mocked HTTP, retry scenarios

Next Steps

Review language-specific examples for implementation details. Study references for deep dives on specific patterns. Examine best-in-class SDKs (Stripe, AWS, OpenAI) for inspiration.