What a response said about your remaining request allowance.

Duffel reports the allowance in response headers. This struct holds whichever of them arrived; a field is nil when its header was absent or unreadable, and the whole struct is nil when none of them were sent.

You get one on a Duffel.Error and on every [:duffel, :request, :stop] telemetry event, so you can slow down before Duffel starts refusing requests rather than after:

:telemetry.attach(
  "duffel-rate-limit",
  [:duffel, :request, :stop],
  fn _event, _measurements, %{rate_limit: rate_limit}, _config ->
    if rate_limit && rate_limit.remaining && rate_limit.remaining < 10 do
      Logger.warning("duffel allowance nearly spent: #{rate_limit.remaining}")
    end
  end,
  nil
)

Requests that hit the limit are retried automatically, honouring retry-after, so reach for this when you want to shape your own traffic or to decide what to do once the retries are spent.