What are the best practices for designing API SDKs?

A well-designed API SDK dramatically improves developer experience. Key principles: Idiomatic code — follow language conventions of the target (e.g., snake_case in Python, camelCase in JavaScript, fluent builder patterns in Java). Authentication abstraction — accept API keys or OAuth tokens in the constructor, not per request. Automatic retry with exponential backoff for 429 and 5xx errors. Sensible defaults with full configurability (timeouts, base URL, logging). Strongly-typed request/response models — auto-generated from the OpenAPI spec ensures the SDK stays in sync with the API. Comprehensive error types that wrap HTTP errors into domain-specific exceptions. Pagination helpers that handle cursor traversal transparently. Versioned releases with a changelog. SDK generation tools like Speakeasy or OpenAPI Generator produce multi-language SDKs directly from the OpenAPI spec.