What is REST API?

A REST API (Representational State Transfer) is an architectural style for web APIs that models a system as a set of resources, each addressable by a URL, and operated on with the standard HTTP methods: GET to read, POST to create, PUT or PATCH to update, DELETE to remove. Responses are typically JSON, status is carried in HTTP status codes, and well-behaved REST APIs are stateless, each request carries everything the server needs, so requests can be load-balanced and cached freely. REST won the public-web API space because it rides directly on HTTP, is trivially inspectable with a browser or curl, and requires no special client library. Its looser conventions are both its strength and weakness: there is no enforced schema by default (though OpenAPI specs add one), and clients can over-fetch or under-fetch data, which is exactly the gap GraphQL and gRPC try to close. REST is profoundly relevant to MCP: a large share of MCP servers are thin adapters that translate an LLM's tool call into one or more REST calls against an underlying SaaS API and shape the response back for the model. For an AI agent, the available endpoints, their auth, and their rate limits are practical operating knowledge worth holding in shared memory across sessions.