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Complete reference with usage guidance
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HTTP status codes are grouped into five categories: 1xx (Informational) for request acknowledgment, 2xx (Success) for successful operations, 3xx (Redirection) when the client must take further action, 4xx (Client Error) for problems with the request, and 5xx (Server Error) when the server fails to fulfill a valid request.
A 401 Unauthorized response means the client has not provided valid authentication credentials - the user needs to log in or provide a valid token. A 403 Forbidden response means the client is authenticated but does not have permission to access the requested resource. Re-authenticating will not help with a 403 because it is an authorization issue, not an authentication issue.
A 503 Service Unavailable status code means the server is temporarily unable to handle the request. Common causes include server maintenance, temporary overload, or downstream dependencies being unavailable (such as a database being down). Servers should include a Retry-After header to tell clients when to try again.
Use 200 OK for successful GET, PUT, or PATCH requests that return data. Use 201 Created after a successful POST that creates a new resource - include a Location header pointing to the new resource. Use 204 No Content for successful requests that do not need a response body, such as DELETE operations or updates where the client does not need the result.
The most common HTTP error code is 404 Not Found, which indicates the server cannot find the requested resource. It is returned when a URL does not match any existing endpoint or resource. Other frequently encountered error codes include 500 Internal Server Error (generic server failure), 403 Forbidden (access denied), and 400 Bad Request (malformed client request).