If someone said something ("It doesn't work well") and you want to express your agreement, here are the most common ways: - I agree (completely). - I agree with you. - I agree that it doesn't work well. - I agree with you that it doesn't work well. Here are less common phrases: - I agree on that (point). - We agree on that (point). - I agree with you on that (point). - We are in agreement on that (point). - I have the same opinion about that (as you). - We are of the same opinion about that. Some people say things like "I agree with the fact ...", but I don't think it's a correct way.

"In confirmation" is not a common way to say it. "I agree", "I am in complete agreement" would both work well in a formal setting.