Will my portal tell my customer whether they need a visa for the country they are flying to?

No, a portal does not check visa or passport rules for your customer on its own. The flight and hotel content that reaches your portal through an API is about availability, price and the booking itself, and it very rarely carries the entry rules for the destination. Unless something is added for this, a customer can book and pay for a trip they are not allowed to take, and the first anyone hears about it is at the airport check in desk.

What a portal can do well is put the warning in front of the traveller at the right moment. You can show a clear notice on the passenger details page and repeat it on the confirmation, saying that visas, entry rules and passport validity are the traveller's own responsibility, and pointing them to the embassy or the official government site of the country they are visiting. That wording is yours to write, so you can make it as firm and as specific as your business needs.

Some portals go further and connect to a separate travel document service that returns the likely rules for a given nationality and destination. Whether you can add that depends on your plan and on whether such a service covers the markets you sell in, and it is normally a service you pay for on its own, so ask before you assume it is included. Even when it is switched on, treat the result as guidance rather than a guarantee, because these rules change at short notice and only the embassy or consulate can give a final answer.

The sensible next step is to decide what your own agency is willing to promise about travel documents, write that into the notice your customers see before they pay, and then ask what document checking options your plan supports.