What is an API?

An API specifies exactly what requests are allowed, what parameters they take, and what response format to expect — acting as a contract between the system offering the capability and anything that wants to consume it. Most modern web APIs are REST or GraphQL over HTTP, exchanging JSON, though older or more specialized systems use SOAP, gRPC, or custom protocols. APIs are the connective tissue of nearly all software integration: without them, every two systems that need to talk would require custom, brittle point-to-point code. With a stable API, a calendar app, a payment processor, and a CRM can all be wired together predictably, because each exposes a documented, versioned surface rather than requiring direct database access. For AI agents specifically, APIs are how an agent reaches beyond its own reasoning into the real world — checking a customer record, sending an email, updating a spreadsheet — turning a text-generating model into something that can actually get work done.