SDK, short for software development kit, is a packaged set of libraries, tools, documentation, and code samples that lets developers build software for a specific platform, operating system, or service.
The term covers two common senses. A platform SDK supplies what a developer needs to target an environment — the headers, compilers, debuggers, and emulators behind building for iOS, Android, or a game console. A service SDK is a client library, usually published in several programming languages, that wraps a company's web interface so developers call ordinary functions instead of composing raw HTTP requests, handling authentication, retries, and data types along the way. Both senses share the idea of a kit: several components shipped together so a developer can start building rather than assembling the tooling first.
An SDK is often confused with an 📝Application Programming Interface (API), but the two sit at different layers. An API is the contract — the set of requests a system accepts and the responses it returns. An SDK is a toolkit for working with one or more APIs, and it usually contains them. A single library is one component; an SDK bundles many libraries with documentation and examples. Protocols such as the 📝Model Context Protocol (MCP) now ship official SDKs in several languages, and community-built alternatives compete with them on developer experience.
The SDK story I keep returning to is 📝FastMCP outpacing Anthropic's own MCP SDK purely on developer experience. In a protocol land-grab, the kit builders actually enjoy using is the one that wins the ecosystem.