400 vs 409: Bad Request vs Conflict
400 and 409 can look similar in logs, but they tell clients, crawlers, and API consumers different things.
| Aspect | 400 | 409 |
|---|---|---|
| Meaning | Bad Request describes how the server processed the request and what the client should do next. | Conflict describes how the server processed the request and what the client should do next. |
| Typical use case | HTTP 400 Bad Request indicates a client errors response outcome. | HTTP 409 Conflict indicates a client errors response outcome. |
| Caching/client behavior | Check cache headers and downstream behavior for 400. | Check cache headers and downstream behavior for 409. |
| SEO implications | Search crawlers interpret 400 according to client-errors semantics. | Search crawlers interpret 409 according to client-errors semantics. |
| API/backend impact | API clients may branch logic specifically on 400. | API clients may branch logic specifically on 409. |
When to use one vs the other
Use 400 when the response should communicate bad request behavior; use 409 when conflict is the accurate protocol signal.
A frequent mistake is swapping 400 and 409 for convenience; that causes client retry bugs, incorrect cache signals, and misleading monitoring data.
Decision summary: if user agents should receive the Bad Request signal, return 400; if they should receive Conflict, return 409.
FAQ
What is the biggest difference between 400 and 409?
400 communicates Bad Request, while 409 communicates Conflict. Choosing the right one keeps clients and intermediaries predictable.
Do 400 and 409 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 400 instead of 409?
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: 400 Bad Request · 409 Conflict