Status Codes: Five Classes, Five Meanings

The first digit says what happened and whose problem it is before the client reads anything else.

An HTTP status code is a three-digit number at the start of every response, and its first digit puts it in one of five classes: 1xx means keep going, 2xx means it worked, 3xx means look elsewhere, 4xx means the request was wrong, and 5xx means the server failed on a request that may have been fine. A client should branch on the class first, because the class alone tells it whose problem the response is and whether trying again could help.

The five classes, what each says, who is at fault, and what the client does next
Class	Says	At fault	Client's next step
1xx Informational	Keep going; the final answer is still coming	Nobody	Wait for the final response
2xx Success	The request worked	Nobody	Use the response
3xx Redirection	The answer is somewhere else	Nobody	Follow the Location header
4xx Client error	The request was wrong	The client	Fix the request; sending it unchanged will fail again
5xx Server error	The server failed	The server	Retry later if the method is safe to repeat

Why the class matters more than the code. HTTP defines dozens of codes, and servers may invent new ones. A client that meets a code it does not know must treat it like the first code of its class, x00. So a client that has never heard of 418 still knows it is a client error, and one that meets 599 knows the server failed.

The two classes that matter most. The line between 4xx and 5xx decides retries. A 4xx says the request itself is the problem: a missing field, an unknown resource, a caller with no permission. Repeating it unchanged gets the same answer and only adds load. A 5xx says the server could not do something it should have, so the same request may succeed a moment later.

1xx in APIs. Codes such as 100 Continue and 101 Switching Protocols are sent before the final response, and HTTP libraries handle them without the application seeing them. An API designer rarely sends one on purpose.

The reason phrase is decoration. The words after the number, such as "Not Found", are for people reading a log. Software reads the number, and HTTP/2 does not send the phrase at all.

So choose a code by asking two questions: did it work, and if not, who must change something for it to work?

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