If I book a flight through the API and the airline cancels the flight, will I find out automatically?

No, the flight booking API will not automatically notify you if an airline cancels a flight after you have booked it. You need to set up your own process to monitor for cancellations, either by checking with the airline directly or by asking your customers to contact you if they hear about a change.

What the API can do is tell you about schedule changes when you ask it to look up a booking. Some travel management systems include tools that watch for changes in the background and alert you, but this depends on the software you are using and how it connects to the airlines. The airline itself will contact your customer about a cancellation, but there may be a delay between when the airline makes the decision and when that information reaches you through the booking system.

Because airlines can cancel or reschedule flights at any time, many travel agencies build customer service processes that include checking key bookings regularly in the days before travel, especially for flights that are further out. You should also make sure your customers know how to reach you quickly if they hear about a change, so you can help them rebook or handle refunds right away.

Check with your booking system provider about what monitoring tools they offer, and ask whether airlines push cancellation notices to them automatically or whether you need to pull that information yourself.