Clients and servers can negotiate representation formats using headers such as Accept and Content-Type.

A client might send Accept: application/json, indicating that it wants JSON. A request containing JSON normally sends Content-Type: application/json.

Media types can also be vendor-specific or versioned, for example application/vnd.example.user+json. Content negotiation is different from URI versioning because the representation format is negotiated through HTTP metadata rather than encoded in the path.

A robust API validates unsupported media types and can respond with 415 Unsupported Media Type when the request body format is not supported.