In either case, the actual information on whether it is painless is immaterial to the choice. The only thing that can factor into an agent's decision is the information that he learns about the suicide agent, and the agency has a lot of control over that information.

It would be pretty easy to tell a plausible lie, though, especially if you can think of a plausible less-painful poison to pretend you used. Tell them it's something moderately painful, so you can still tell them, "look, it's still better than torture". Just don't remind them about the edge-case scenarios where it really has to be fast and reliable, and they probably won't question it too hard...