HTTP 510 Not Extended (RFC 2774) indicates that further extensions to the request are required for the server to fulfill it. The server is saying 'I understand your request but I need additional protocol extensions to handle it.' This is extremely rare — RFC 2774 (HTTP Extension Framework) was experimental and never widely adopted. You're unlikely to encounter 510 in production systems.
Response includes the status code, standard headers (including Content-Type), and a small diagnostic JSON body describing the request and returned status.
Simulator URL (copy in the app after load — not a normal link):
https://httpstatus.com/api/status/510
Example request:
curl -i "https://httpstatus.com/api/status/510"Further extensions to the request are required for the server to fulfill it.
On this code, Inspector focuses on semantics, headers, and correctness warnings that commonly affect clients and caches.
HTTP 510 Not Extended represents a specific server-side condition that requires different handling than other 5xx errors. Understanding the precise cause helps operations teams diagnose and resolve issues faster. Monitoring systems should distinguish 510 from other 5xx codes for accurate alerting and diagnosis.
// Handle 510 Not Extended
process.on('unhandledRejection', (reason) => {
console.error('Unhandled rejection:', reason);
});
app.use((err, req, res, next) => {
console.error(`${req.method} ${req.url}:`, err.stack);
res.status(err.status || 500).json({
error: process.env.NODE_ENV === 'production'
? 'Internal Server Error'
: err.message,
requestId: req.id
});
});from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
import logging
logger = logging.getLogger(__name__)
@app.exception_handler(Exception)
async def server_error_handler(request: Request, exc: Exception):
logger.error(f'{request.method} {request.url}: {exc}',
exc_info=True)
return JSONResponse(
status_code=510,
content={'error': 'Not Extended', 'request_id': request.state.id}
)@ControllerAdvice
public class GlobalErrorHandler {
private static final Logger log = LoggerFactory.getLogger(
GlobalErrorHandler.class);
@ExceptionHandler(Exception.class)
public ResponseEntity<ErrorResponse> handleException(
Exception ex, HttpServletRequest req) {
log.error("{} {}: {}", req.getMethod(),
req.getRequestURI(), ex.getMessage(), ex);
return ResponseEntity.status(510)
.body(new ErrorResponse("Not Extended",
"An unexpected error occurred"));
}
}func errorMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
defer func() {
if err := recover(); err != nil {
log.Printf("%s %s: %v\n%s",
r.Method, r.URL, err, debug.Stack())
w.WriteHeader(510)
json.NewEncoder(w).Encode(map[string]string{
"error": "Not Extended",
})
}
}()
next.ServeHTTP(w, r)
})
}