I prefer simplicity and using the first example but I'd be happy to hear other options. Here's a few examples:
HTTP/1.1 403 POST /endpoint
{ "message": "Unauthorized access" }
HTTP/1.1 403 POST /endpoint
Unauthorized access (no json)
HTTP/1.1 403 POST /endpoint
{ "error": "Unauthorized access" }
HTTP/1.1 403 POST /endpoint
{
"code": "UNAUTHORIZED",
"message": "Unauthorized access",
}
HTTP/1.1 200 (🤡) POST /endpoint
{
"error": true,
"message": "Unauthorized access",
}
HTTP/1.1 403 POST /endpoint
{
"status": 403,
"code": "UNAUTHORIZED",
"message": "Unauthorized access",
}
Or your own example.
You should consider if you really want to integrate your application super tightly with the HTTP protocol.
Will it always be used exclusively over a REST-ful HTTP API that you control, and it has exactly one hop to the client, or passes through hops that can be trusted to never alter the HTTP metadata significantly? In that case you can afford to make HTTP codes semantically relevant for your app.
But maybe you need to pass data through multiple different types of layers and different mechanisms (socket protocols, pub-sub, file storage etc.) In that case you want all your semantics to be independent from any form of transport.