Will a flight booking API show the budget airlines as well as the big carriers?

Yes, a flight booking API can usually show low cost airlines alongside the regular scheduled airlines, but the two do not always arrive from the same place. Full service airlines are mostly reached through the global distribution systems that agents have used for years, while many budget carriers sit outside those systems and have to be connected one at a time, either directly or through an aggregator that has already done that work.

What this means in practice is that your search results may be built from more than one source at once. Your customer still sees a single list, sorted by price or by departure time, and does not need to know where each fare came from. Whether a particular budget airline appears at all depends on your supplier and on the plan you are on, so it is worth asking for the list of carriers included before you commit to anything.

There are also real differences in how these fares behave after the search. Budget carriers often charge separately for baggage, seats and meals, they may not allow the same changes and refunds as a normal ticket, and some will not let a booking be cancelled once it has been issued. A good API passes those conditions through so they can be shown on your results page, but how much detail comes back varies from airline to airline.

The sensible next step is to ask your supplier which low cost carriers are live on your account today, and then run a few test searches on the routes your customers actually fly, so you can see for yourself what comes back before you go live.