510 vs 501: Not Extended vs Not Implemented
510 and 501 can look similar in logs, but they tell clients, crawlers, and API consumers different things.
| Aspect | 510 | 501 |
|---|---|---|
| Meaning | Not Extended describes how the server processed the request and what the client should do next. | Not Implemented describes how the server processed the request and what the client should do next. |
| Typical use case | HTTP 510 Not Extended indicates a server errors response outcome. | HTTP 501 Not Implemented indicates a server errors response outcome. |
| Caching/client behavior | Check cache headers and downstream behavior for 510. | Check cache headers and downstream behavior for 501. |
| SEO implications | Search crawlers interpret 510 according to server-errors semantics. | Search crawlers interpret 501 according to server-errors semantics. |
| API/backend impact | API clients may branch logic specifically on 510. | API clients may branch logic specifically on 501. |
When to use one vs the other
Use 510 when the response should communicate not extended behavior; use 501 when not implemented is the accurate protocol signal.
A frequent mistake is swapping 510 and 501 for convenience; that causes client retry bugs, incorrect cache signals, and misleading monitoring data.
Decision summary: if user agents should receive the Not Extended signal, return 510; if they should receive Not Implemented, return 501.
FAQ
What is the biggest difference between 510 and 501?
510 communicates Not Extended, while 501 communicates Not Implemented. Choosing the right one keeps clients and intermediaries predictable.
Do 510 and 501 have SEO or caching impact?
Yes. Search engines and caches interpret status classes differently. Use each code according to its semantics to avoid accidental indexing, stale responses, or crawl inefficiency.
Can APIs safely return 510 instead of 501?
Only when it matches contract semantics. API clients often branch logic by exact code, so swapping them can break retries, auth handling, or user-facing errors.
Related guides: 510 Not Extended · 501 Not Implemented