APIs

An API (Application Programming Interface) is a defined contract for how one system requests work or data from another. On the web, that contract is usually expressed over HTTP with JSON payloads.

Common styles

Style	Shape
REST	Resources addressed by URL, HTTP verbs for actions
GraphQL	Single endpoint, client specifies the exact data graph
RPC	Call named procedures (gRPC, JSON-RPC)
Webhooks	Server calls you when an event happens

What a good contract specifies

• Endpoints and the resources they expose.
• Auth — API keys, OAuth tokens, or signed requests.
• Request/response schemas and error formats.
• Rate limits and versioning strategy.

APIs are the connective tissue behind agent tools like Claude Code, which call model and platform APIs under the hood.