The agreement doesn’t guarantee that Tehran will never produce nuclear weapons—because no agreement could do so.
A week ago I volunteered my way into an Atlantic debate on the merits of the Iran nuclear agreement. The long version of the post is here; the summary is that the administration has both specific facts and longer-term historic patterns on its side in recommending the deal.
On the factual front, I argued that opponents had not then (and have not now) met President Obama’s challenge to propose a better real-world alternative to the negotiated terms. Better means one that would make it less attractive for Iran to pursue a bomb, over a longer period of time. Real world means not the standard “Obama should have been tougher” carping but a specific demand that the other countries on “our” side, notably including Russia and China, would have joined in insisting on, and that the Iranians would have accepted.