SDK stands for software development kit. The API is the interface a company exposes; the SDK is a helper package that makes that interface easier to use in a specific programming language. A payment company’s raw REST API makes you build each request by hand. Its SDK wraps that into simpler code, closer to “create customer” or “refund payment.”
Imagine assembling office furniture. You could buy raw lumber, measure everything, and drill every hole yourself. Or the manufacturer gives you pre-cut pieces, labeled screws, and a tool that fits the bolts. You’re still building the furniture; the kit saves you from starting with raw materials.
Why you care
SDKs install as a dependency, so your project relies on someone else’s package, which is why versioning, docs, and supply chain safety matter. Agents write code fast, and if an SDK exists, they’ll often use it instead of hand-building every call. That works when the SDK encodes the product’s patterns, and it goes wrong when the agent uses an old version or invents a method name. Tell the agent which SDK version and docs to follow; don’t just say “connect to Stripe” and hope.