What’s the status code number for an invalid HTTP request (server can’t understand it)?

Final Answer

The status code number for an invalid HTTP request, where the server cannot understand the request due to malformed syntax, is 400.

Explanation

A status code of 400 Bad Request indicates that the server couldn’t understand the request due to invalid syntax. When a client sends a request to a server, it expects a proper syntax that adheres to the HTTP protocol specifications. If the syntax is incorrect, or if the request is not formulated properly (such as a missing header or an invalid URL), the server responds with a 400 status code to inform the client that there was an error with the request's format. This serves as a prompt for clients to verify their request construction and try again, ensuring adherence to the expected protocols in future communications.