Viator

Viator is an agent. They do not run the tours, they book and farm out to the actual operators. You can find the actual operator on the Viator or TripAdvisor (TA owns Viator) listing.

Many of us prefer to book directly with the supplier. Others find no difficulty with Viator bookings.

I second that response - you can find a link to the actual tour providers and then I go to that tour operator's website for the actual booking. Also you can often find additional tours on that website that weren't mentioned on Viator/Trip Advisor. You can also look at the Rick Steves tour book for Paris/France for suggestions of day tour providers as well. Plus there are many things you do on your own, such as a tour of the Paris Opera, walking tours, museums using a guide book or audio guide as well as a day trip to Versailles or Rouen on the train. Have fun!

I will “third” this suggestion. Use Viator to scan tours available, and when you see one you like, find the name of the actual tour operator on the Viator listing, and go to the prover’s own website to book.

I accidentally booked something with Viator last year -- cancelled, found the actual tour operator and it was significantly cheaper. Plus when you book with the tour itself, you are in a better position to get satisfaction if something goes awry.

Look at “Paris Walks”. I did one for Montemarrte and I thought it was excellent. I have never used Viator, based on the negative feedback I’ve heard, I’d only use it at a last resort

Here is a quote from a Trip Report I wrote, that happened to be to Poland:

I was hesitant to book a car and driver in Wroclaw using Trip Advisor. But at least the name Viator was familiar to me. Viator was pretty transparent about the actual provider of the private excursion (Wratislavia Tour), so I tried to book it directly with the real provider. But their website only had “Church of Peace Świdnica and Some Dull Nazi Bunker” and I wanted “[BOTH] Churches of Peace, in Świdnica and Jawor”, the latter being only on Viator’s menu. When I confirmed (as advised to) with the provider, I said I would have preferred to book directly with them and give them all the money. They said, in effect, “Yeah, we know it’s not all on our own website.”

Our driver was on time (9AM at our hotel), cheerful, well-informed, and knew the area very well. Our huge 8-passenger minivan was very comfortable for 2. ..... [quoted material omitted here] Because of miserable highway traffic and rubbernecking of an accident in the other divided-highway direction, we did not get back to our hotel until 2 PM, an hour late. (No charge for that, of course.)
Because I made a rookie mistake in my Trip Advisor booking, I had to use the interface’s options to Change Pickup Point and Change Tour Date. I was far enough in advance that I was told There Will Be No Charge for This Change. That was a really good internet experience!

So I currently have nothing against Viator.

We only used them once, but the "Tours by Locals" website will find individual local guides that can do custom itineraries for you.

We used Viator once as well, and nothing against them. They are a US based company so can provide you another layer of oversight over the guide, and a financial transaction in the US. It's not unreasonable to charge extra for that.

The truth is that Viator does not schedule in "real time" So they take your reservation and your money and.... the night before your Eiffel Tower tour you get a "too bad, here's a refund email" . I know several people who have been stood up by them the night before. Their "oversight" of the guides appears to be "will you give us a cut for selling"

Unless there are no other options book direct!