id: BOOK-012
title: The last seat cannot be oversold to two agents at once
layer: binding
area: Booking and cancelling
risk: high
owner: qa

preconditions:
  - signed in as agent@skyline.test / skyline
  - a flight with exactly 1 seat remaining

steps:
  given: a flight showing "1 left"
  when: two bookings for that flight are confirmed at the same moment
  then: one succeeds (201 with a PNR), the other is refused (409), and seats reach 0

evidence:
  - the POST /api/bookings request and response for BOTH attempts
  - the seat count before and after (recorded data)
  - screenshot of the 409 "just sold" message on the losing attempt

data:
  route: JFK -> LHR
  passengers: 1

must_not:
  - the flight must never show negative seats
  - both bookings must never both succeed