Software Development Kit definition
An SDK (Software Development Kit) is a packaged set of tools that helps developers build for a specific platform or service. It usually includes client libraries, documentation, code samples and sometimes compilers, emulators or debuggers. The iOS SDK lets developers build iPhone apps, while the Stripe SDK wraps Stripe's API so payments take a few lines of code.
What is inside an SDK?
Contents vary by platform, but most SDKs bundle some combination of the items below. Platform SDKs, such as the Android SDK, are large toolchains for building whole apps; service SDKs, such as a payment or analytics SDK, are often just a library plus documentation.
The quality of an SDK shows in the details: clear error messages, sensible defaults, typed responses and changelogs that explain breaking changes. A good SDK makes the right thing easy, for example by tokenizing card data on the device so raw card numbers never reach your servers.
- Client libraries that wrap an API in idiomatic functions for Swift, Kotlin, JavaScript, Python or other languages
- Documentation, API references and quick-start guides
- Code samples and complete sample apps
- Build tools, compilers, emulators or simulators for platform SDKs
- Debugging and testing utilities, sometimes with mock servers
- Prebuilt UI components, such as checkout or login screens
SDK vs API
An API is the contract: the endpoints or functions a service exposes. An SDK is a toolkit that makes that contract easier to use. You can call the Stripe API directly with raw HTTP requests, or use the Stripe SDK, which handles authentication, retries, pagination, request signing and typed responses for you. Every SDK talks to at least one API, but not every API has an SDK.
SDKs save time and reduce mistakes, especially around security-sensitive steps such as tokenizing card data or refreshing OAuth tokens. The cost is an extra dependency to keep updated and slightly less control over exactly what is sent over the network.
Examples of SDKs
Companies also publish SDKs so partners can integrate faster. A logistics platform might ship JavaScript and Python SDKs so retailers can create shipments in a few lines, generating them from an OpenAPI specification so every language stays in step with the API. Well-known examples include:
- Platform SDKs: the iOS SDK in Xcode, the Android SDK and the Flutter SDK for cross-platform apps
- Payment SDKs: Stripe, Razorpay and PayPal mobile and web SDKs
- Cloud SDKs: AWS SDKs, the Azure SDK and Google Cloud client libraries
- Analytics and messaging SDKs: Firebase, Mixpanel, Segment and OneSignal
- AI SDKs: client libraries from OpenAI, Anthropic and Google for calling language models
How to evaluate an SDK before adding it
Every SDK you add ships inside your product, so check it like code you wrote. Look at maintenance activity and release frequency, open security issues, license terms, binary size on mobile, the data it collects (which affects App Store privacy labels and privacy law) and how many transitive dependencies it pulls in. Prefer official SDKs from the service owner, and pin versions so updates are deliberate.
Mobile teams should be especially careful with analytics and advertising SDKs, which can collect personal data before a user consents. Nexzem reviews third-party SDKs during mobile app development projects and documents what each one collects so the privacy policy stays accurate.