It is always the person who travels who has ultimate responsibility. This is also in many (most / all?) Travel agency contracts I've ever seen, and it's also a situation we've seen here at Travel Stack Exchange a lot.
A good travel agency will advise you if you need a visa, but most travel agencies only know the standard for the country in which they are located. These are the people in this country and often the most common foreign nationalities.
Very good agencies also check this on a Visa website, but they do so only as a service and (usually) take no responsibility.
And most travel agents will realize that they are not visa specialists and will tell you that you need to check (and often how) that you are not a national of the country where you are booking.
At Travel Stack Exchange we usually refer to one of the database pages where the airlines also check whether you need a visa. Many travel agencies in many countries will also use one of them (often the portal used by the national airline), but other websites that are not kept up to date.
So it's best to check again, even if the travel agency has checked.
Even if you've done everything right, visa requirements can change overnight.
Once my brother, his girlfriend and I wanted to check in France to take a ferry to the UK to find out that the girlfriend needed a visa. This started at midnight that day. And announced less than 24 hours before the start of the new rule (I think less than 8 hours before the start, but we still missed this information because it wasn't in their home country.)