What is a RESTful API's uniform interface constraint?

The uniform interface constraint is the central distinguishing feature of REST. It has four sub-constraints. Resource identification: resources are identified by stable URIs (e.g., /users/42) — the URI identifies the concept, not a specific representation. Manipulation through representations: clients interact with resources by sending representations (JSON bodies) — the server's internal implementation is hidden. Self-descriptive messages: each request/response includes enough metadata (Content-Type, Authorization, status code) to describe how to process it. HATEOAS: responses include links to related actions the client can take next. The uniform interface simplifies the overall system architecture by decoupling client from server and enabling each to evolve independently.