Should duplicate signups return 409 or 422?
Either is common. 409 emphasises the clash with existing data; 422 treats it as a validation error. Pick one and use it consistently across your API.
HTTP 409 CONFLICT
The request conflicts with the current state, such as an edit conflict or a duplicate. 409 belongs to the 4xx client error class and is defined in RFC 9110.
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About HTTP 409
What Does HTTP 409 Conflict Mean?
The request conflicts with the current state, such as an edit conflict or a duplicate. The code is registered with IANA and defined in RFC 9110.
Client error: the request has a problem, so it cannot be fulfilled as sent. Every code from 400 to 499 shares that general meaning, so a client that does not recognise 409 must treat it like 400.
When Do You See a 409 Response?
409 Conflict means the request clashes with the current state of the resource. Common cases are creating a user whose email already exists, editing a document someone else changed since you loaded it, uploading a file version older than the stored one, or a Git-style push that is not a fast-forward. The request itself is valid; it simply cannot be applied to the resource as it is now.
How Do You Fix a 409?
Servers should explain the conflict in the body clearly enough for the user or client to resolve it, for example which field is duplicated or what the current version is. Clients should fetch the latest state, merge or adjust, and resend. For lost-update protection, conditional requests with If-Match and an ETag, answered with 412 on mismatch, give a more precise and standard signal than a generic 409.
What Does a 409 Response Look Like?
HTTP/1.1 409 Conflict
Content-Type: application/problem+json
ETag: "v17"409 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.
To inspect a real one without sending it anywhere:
- Copy the response headers from your browser Network tab, or from
curl -i. - Paste them into the Header decoder to see every header explained.
- Check caching in the Cache lane.
Which Status Codes Are Related to 409?
- 412 Precondition Failed: An If-Match or If-Unmodified-Since condition was false. Used for optimistic locking.
- 422 Unprocessable Content: The syntax is fine but the content fails validation. Common in JSON APIs.
- 428 Precondition Required: The server requires a conditional request, to prevent lost updates.
Browse every code on the HTTP status codes page.
What Other Lanes Are in the Reference Section?
- HTTP status codes: Every IANA-registered status code, with a page per code.
- MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
- HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.
Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.
Frequently Asked Questions
How does 409 differ from 412?
412 is tied to an explicit precondition header such as If-Match that turned out false. 409 is a general conflict the server detected on its own.
Is HTTP 409 an error?
Yes. 409 is a client error response: the problem is in the request, so repeating it unchanged will usually fail again.
Is a 409 response cacheable by default?
No. Caches store a 409 only when Cache-Control or Expires explicitly allows it.
How Can You Verify That Nothing Leaves This Browser?
You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.
- Open your browser developer tools with
F12, orCmd+Option+Ion a Mac, and select the Network tab. - Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
- Click the clear icon in the Network tab so that the log is empty.
- Paste real headers into any lane and read the result.
- Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.
Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.
2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.