Bug 23336 introduces an API workflow for checkouts by adding two endpoints.. one for availability that creates a token for confirmations that can then be used by the checkouts endpoint to verify that availability has been checked. Whilst I think the availability endpoint is worthwhile in it's own right, I think we could also send back the token in the 409 pre-condition failed response of the checkouts endpoint so one could use that single endpoint for double duty in the right circumstances. I also think we could work on how the confirmation token is sent to the endpoint.. it's currently a simple query param but may lend itself to being part of the x-koha-overrides header or another header?