[galaxyLogic]

If you only use "simple types" then you often get implicit assumptions about the "values". For instance note that "web-services" basically means you have a very simple interface defined by the HTTP-protocol.

But what is actually inside the HTTP-payloads can then have many constraints on them which are not declared anywhere. For instance your code might assume the payload is JSON with several required fields in it.

[leidenfrost]

For that you have the "closed for modification but open for extension" principle.

If the server is new and fresh, yeah it's ok to assume the payload of a request will be a JSON object with some required fields, but leave the parameter there in case someone decides they will start sending XML payloads to it