Should a 409 body say what conflicted?

Yes, and specifically enough for the client to resolve it — "seat 14A is taken" beats "conflict".

Is a duplicate POST a 409?

409 if the duplicate is genuinely unacceptable. If the caller sent an Idempotency-Key, replaying the original response is friendlier.

HTTP 409 Conflict

The request is valid but clashes with the current state — a duplicate, a seat already taken, a state transition that is not allowed from here.

Defined in RFC 9110 §15.5.10 — 409 Conflict · MDN reference

Where you meet HTTP 409 in production

Double-booked seats, duplicate usernames, deleting a record that something else still points at, cancelling an order that already shipped, and any state machine asked to make a transition it does not have.

Why you would test it

A 409 is a state problem, not a transport problem, so blind retries make it worse. Test that your retry policy distinguishes it from a 503.

What your client should do about a 409

A 409 is about state, so a blind retry usually fails identically — the seat is still taken. Re-read the current state, decide whether the operation still makes sense, and only then try again. This is the status where an automatic retry policy does the most damage for the least benefit.

409 versus the codes it gets confused with

409 vs 412 Precondition Failed — 412 is a conflict the client explicitly asked to be checked, by sending a validator. 409 is a conflict the server detected on its own.
409 vs 422 Unprocessable Content — 422 rejects the content of the request. 409 accepts the content and rejects it against the current state of the resource.

Endpoints that return 409

46 endpoints in this playground answer with 409. Every one is free, needs no signup, and can be called from the browser or with curl.

DELETE https://funapi.dev/api/pizza/v1/orders/{id} — Order already delivered — cannot cancel (Pizza Orders API) Open & run this endpoint
PUT https://funapi.dev/api/coffee/v1/orders/{id}/ready — Order is already ready (Coffee Shop API) Open & run this endpoint
DELETE https://funapi.dev/api/football/v1/teams/{id} — Team has played matches (Football API) Open & run this endpoint
PATCH https://funapi.dev/api/holmes/v1/cases/{id} — Case is already in that status (Sherlock Holmes API) Open & run this endpoint
POST https://funapi.dev/api/webhooks/v1/deliveries/{id}/replay — Max attempts (3) already reached (Webhooks API) Open & run this endpoint
POST https://funapi.dev/api/dinopark/v1/dinosaurs — Enclosure is at capacity (Dino Park API) Open & run this endpoint
POST https://funapi.dev/api/marvel/v1/missions/{id}/complete — Mission is already complete (Marvel API) Open & run this endpoint
POST https://funapi.dev/api/got/v1/nobles/{id}/kill — Noble is already dead (Game of Thrones API) Open & run this endpoint
POST https://funapi.dev/api/got/v1/battles — A house cannot fight itself (Game of Thrones API) Open & run this endpoint
POST https://funapi.dev/api/bank/v1/transfers — An account is frozen (Bank API) Open & run this endpoint
POST https://funapi.dev/api/bank/v1/accounts/{id}/freeze — Already frozen (Bank API) Open & run this endpoint
POST https://funapi.dev/api/airline/v1/bookings — Seat taken, flight cancelled, or flight full (overbooked) (Airline API) Open & run this endpoint
PUT https://funapi.dev/api/airline/v1/bookings/{id}/seat — Seat taken or booking checked-in (Airline API) Open & run this endpoint
POST https://funapi.dev/api/airline/v1/bookings/{id}/checkin — Already checked-in, or the flight is cancelled (Airline API) Open & run this endpoint
DELETE https://funapi.dev/api/airline/v1/bookings/{id} — Cannot cancel after check-in (Airline API) Open & run this endpoint
PATCH https://funapi.dev/api/protocol/v1/documents/{id} — A "test" operation failed (Protocol Lab API) Open & run this endpoint
POST https://funapi.dev/api/shop/v1/carts/{id}/items — Not enough stock (product 3 is always out) (Shop API) Open & run this endpoint
POST https://funapi.dev/api/shop/v1/carts/{id}/checkout — Cart already checked out, or stock ran out since adding (Shop API) Open & run this endpoint
POST https://funapi.dev/api/cargo/v1/shipments/{id}/advance — Shipment is in a terminal state (Cargo Tracking API) Open & run this endpoint
POST https://funapi.dev/api/cargo/v1/shipments/{id}/attempt-delivery — Not out-for-delivery (Cargo Tracking API) Open & run this endpoint
PUT https://funapi.dev/api/cargo/v1/shipments/{id}/address — Already delivered (Cargo Tracking API) Open & run this endpoint
POST https://funapi.dev/api/space/v1/launches/{id}/scrub — Already launched or already scrubbed (Space Agency API) Open & run this endpoint
POST https://funapi.dev/api/space/v1/launches/{id}/launch — Not scheduled, or an auto-abort at T-0 (~25% chance) (Space Agency API) Open & run this endpoint
GET https://funapi.dev/api/space/v1/launches/{id}/countdown — Launch is not scheduled (Space Agency API) Open & run this endpoint
POST https://funapi.dev/api/starfleet/v1/ships — A ship with that name already exists (Starfleet API) Open & run this endpoint

…and 21 more endpoints across the catalog.

Questions about HTTP 409

Should a 409 body say what conflicted?: Yes, and specifically enough for the client to resolve it — "seat 14A is taken" beats "conflict".
Is a duplicate POST a 409?: 409 if the duplicate is genuinely unacceptable. If the caller sent an Idempotency-Key, replaying the original response is friendlier.

Other client error codes

All HTTP status codes · All 39 mock REST APIs · Getting started guide

Last updated 6 September 2026