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.

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.

Endpoints that return 409

35 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)
PUT https://funapi.dev/api/coffee/v1/orders/{id}/ready — Order is already ready (Coffee Shop API)
DELETE https://funapi.dev/api/football/v1/teams/{id} — Team has played matches (Football API)
PATCH https://funapi.dev/api/holmes/v1/cases/{id} — Case is already in that status (Sherlock Holmes API)
POST https://funapi.dev/api/webhooks/v1/deliveries/{id}/replay — Max attempts (3) already reached (Webhooks API)
POST https://funapi.dev/api/dinopark/v1/dinosaurs — Enclosure is at capacity (Dino Park API)
POST https://funapi.dev/api/marvel/v1/missions/{id}/complete — Mission is already complete (Marvel API)
POST https://funapi.dev/api/got/v1/nobles/{id}/kill — Noble is already dead (Game of Thrones API)
POST https://funapi.dev/api/got/v1/battles — A house cannot fight itself (Game of Thrones API)
POST https://funapi.dev/api/bank/v1/transfers — An account is frozen (Bank API)
POST https://funapi.dev/api/bank/v1/accounts/{id}/freeze — Already frozen (Bank API)
POST https://funapi.dev/api/airline/v1/bookings — Seat taken, flight cancelled, or flight full (overbooked) (Airline API)
PUT https://funapi.dev/api/airline/v1/bookings/{id}/seat — Seat taken or booking checked-in (Airline API)
POST https://funapi.dev/api/airline/v1/bookings/{id}/checkin — Already checked-in, or the flight is cancelled (Airline API)
DELETE https://funapi.dev/api/airline/v1/bookings/{id} — Cannot cancel after check-in (Airline API)
PATCH https://funapi.dev/api/protocol/v1/documents/{id} — A "test" operation failed (Protocol Lab API)
POST https://funapi.dev/api/shop/v1/carts/{id}/items — Not enough stock (product 3 is always out) (Shop API)
POST https://funapi.dev/api/shop/v1/carts/{id}/checkout — Cart already checked out, or stock ran out since adding (Shop API)
POST https://funapi.dev/api/cargo/v1/shipments/{id}/advance — Shipment is in a terminal state (Cargo Tracking API)
POST https://funapi.dev/api/cargo/v1/shipments/{id}/attempt-delivery — Not out-for-delivery (Cargo Tracking API)
PUT https://funapi.dev/api/cargo/v1/shipments/{id}/address — Already delivered (Cargo Tracking API)
POST https://funapi.dev/api/space/v1/launches/{id}/scrub — Already launched or already scrubbed (Space Agency API)
POST https://funapi.dev/api/space/v1/launches/{id}/launch — Not scheduled, or an auto-abort at T-0 (~25% chance) (Space Agency API)
GET https://funapi.dev/api/space/v1/launches/{id}/countdown — Launch is not scheduled (Space Agency API)
POST https://funapi.dev/api/starfleet/v1/ships — A ship with that name already exists (Starfleet API)

…and 10 more endpoints across the catalog.

Other client error codes

All HTTP status codes · All 37 mock REST APIs · Getting started guide