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.
Have a code, where you can really describe the error; try to use the correct HTTP status (your example doesn't); don't ever use status 200 for errors; and finally, have an "error" key set to something somewhere (I'd write the error code to it).
The message is optional.
So, the simplest version would be:
Status 200 for errors is common for non-REST HTTP APIs. An application error isn't an HTTP error, the request and response were both handled successfully.