405 vs 501: Method Not Allowed vs Not Implemented
405 and 501 can look similar in logs, but they tell clients, crawlers, and API consumers different things.
| Aspect | 405 | 501 |
|---|---|---|
| Meaning | Method Not Allowed 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 405 Method Not Allowed indicates a client errors response outcome. | HTTP 501 Not Implemented indicates a server errors response outcome. |
| Caching/client behavior | Check cache headers and downstream behavior for 405. | Check cache headers and downstream behavior for 501. |
| SEO implications | Search crawlers interpret 405 according to client-errors semantics. | Search crawlers interpret 501 according to server-errors semantics. |
| API/backend impact | API clients may branch logic specifically on 405. | API clients may branch logic specifically on 501. |
When to use one vs the other
Use 405 when the response should communicate method not allowed behavior; use 501 when not implemented is the accurate protocol signal.
A frequent mistake is swapping 405 and 501 for convenience; that causes client retry bugs, incorrect cache signals, and misleading monitoring data.
Decision summary: if user agents should receive the Method Not Allowed signal, return 405; if they should receive Not Implemented, return 501.
FAQ
What is the biggest difference between 405 and 501?
405 communicates Method Not Allowed, while 501 communicates Not Implemented. Choosing the right one keeps clients and intermediaries predictable.
Do 405 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 405 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: 405 Method Not Allowed · 501 Not Implemented