I see your dilemma with this one... if we add a few extra words for context... Person A : We don't need to work on this, do we? Person B: I don't think so. = Person B agreeing with person A's supposition. Person A : We don't need to work on Sundays / We don't need to work on the next project Person B: I don't think so. / I dont think we do. = Person B agreeing with person A's statement Person A : We don't need to work on our relationship Person B: I don't think so either = Person B agreeing with person A's statement This kind of thing comes up in casual chit-chat, but usually is avoided by not having negatives in both statements, or throwing "either" in there: Person A : We don't need to work on blah blah... Person B: I agree / I thought the same! / That's also what I thought!

Person B is telling Person A that in fact they DO need to work on blah blah

I agree with Ammo D. I disagree with Amber and Hilary... Person B is agreeing with person A. Person B doesn't think they need to work on blah blah. Person A doesn't think they need to work on blah blah either. So, they are of the same opinion: in other words, they agree. As Ammo D suggests, this is a bit of a dilemma for learners, because in many languages people will reply in the affirmative - even if the proposition by the other person is in the negative. e.g. If I said to a Chinese learner of English: "You're not British, are you?" many learners would quickly reply: "Yes". But the correct answer would be "No", meaning: "No, I'm not British." So now, the Chinese learner is agreeing with my supposition that they are not British.

Person B is disagreeing with person A. Person B doesn't think they need to work on something.