Do I have to enter my customer's passport details when I book a flight through your flight booking API?

Not always, it depends on the airline and the route your customer is flying. For many domestic flights, the name, date of birth and contact details are enough to issue the ticket, and nothing else is asked for. For international flights, most airlines want passport number, nationality, issuing country and expiry date, because border authorities require that information to be passed on before departure. Some airlines insist on it at the moment of booking, others accept the ticket without it and collect the details at online check in or at the airport counter.

The good news is that you do not have to guess. When you price a flight, the response tells you which passenger fields are mandatory for that particular fare and airline, so your booking screen can show the passport boxes only when they are actually needed. If you send a booking without a field the airline treats as compulsory, the request will come back with an error naming the missing information rather than failing silently, so you can go back to your customer and ask for it.

Even where it is optional, it is usually worth collecting passport details and storing them against the customer record. It saves you chasing people later, and it makes repeat bookings much quicker because the details are already there to reuse.

Before you go live, run a few test bookings on the kind of routes you actually sell, domestic and international, and note which fields come back as required so your booking form asks for the right things first time.