This constraint is optional in a REST architecture style. According to Roy T. Fielding, REST allows client functionality to be extended by downloading and executing code in the form of applets or scripts. This simplifies clients by reducing the number of features required to be pre-implemented. In practice, not many RESTful APIs allow code to be downloaded and executed during runtime.

As long as a system adheres to the preceding constraints, it can be known as being RESTful. HTTP isn't mandatory to make a system RESTful, even though most RESTful APIs are built over HTTP, mostly because HTTP, as an open standard, doesn't bind the implementation of the API or the implementation of its clients to a specific technology. Developers who are familiar with HTTP can pick up a RESTful API easily.