HTTP 409 Conflict

HTTP 409 Conflict — The request conflicts with the current state of the resource.

Class

4xx Client Error

Defined in

RFC 9110 §15.5.10

Official

Yes (IANA registry)

Cacheable by default

No (unless Cache-Control/Expires say so)

Safe to retry

Not without changing the request

What does HTTP 409 mean?

409 Conflict means the request couldn't be completed because it conflicts with the current state of the target resource. The client can usually resolve the conflict and resubmit, so the response body should explain what conflicted.

When is 409 used?

• Creating something that must be unique and already exists (a username or email already registered).
• Concurrent edits: saving changes based on an outdated version of a document.
• A state transition that isn't allowed (cancelling an order that has already shipped).
• Git hosting and package registries rejecting a push or publish of an existing version.

Common causes of a 409

• Someone (or another tab/device) changed the resource since you loaded it.
• A duplicate submission — e.g. double-clicking a submit button.
• A unique constraint in the database.

How to fix 409 Conflict

If you're visiting the site

1. Refresh the page to load the latest version, then make your change again.
2. If signing up, the email or username may already be in use — try logging in or resetting your password.

If you run the site or API (developers)

1. Return a body that names the conflict, e.g. {"error":"email_taken"} or the current version.
2. For lost-update protection use ETags with If-Match, answering 412 when they don't match (409 is fine for domain conflicts).
3. Map database unique-constraint violations to 409 instead of letting them bubble up as 500.
4. Use idempotency keys so retried requests don't produce spurious conflicts.

Returning or handling 409: server examples

app.post("/api/users", async (req, res) => {
  try {
    res.status(201).json(await db.users.create(req.body));
  } catch (e) {
    if (e.code === "23505") // Postgres unique_violation
      return res.status(409).json({ error: "email_taken" });
    throw e;
  }
});

// app/api/example/route.ts (Next.js App Router)
export async function PATCH(req: Request) {
  const { version, ...changes } = await req.json();
  const doc = await getDoc();
  if (doc.version !== version)
    return Response.json({ error: "conflict", current: doc }, { status: 409 });
  return Response.json(await saveDoc(changes));
}

Example 409 response

HTTP/1.1 409 Conflict
Content-Type: application/json

{"error":"email_taken","message":"An account with this email already exists."}

Related status codes

• 412 Precondition FailedA condition in the request headers (If-Match, If-Unmodified-Since…) evaluated to false.
• 422 Unprocessable ContentThe request is well-formed, but its contents fail validation.
• 400 Bad RequestThe server can't process the request because it's malformed or invalid.

Related tools

• HTTP status codes — full list
• HTTP headers reference
• cURL converter
• Mock API — fake REST endpoints with custom status codes
• Webhook tester — inspect incoming requests