Here’s the text of a July letter from Foreign Minister Zarif to the UNSG. This paragraph stands out:

The US and E3 illusion that there can ever be a renegotiation of the timetable enshrined in the JCPOA and Resolution 2231 represents utter bad faith. The timetable for termination of voluntary restrictions accepted by Iran in the JCPOA—maliciously called “JCPOA sunset clauses,” in order to evoke fear— were the subject of the longest and most difficult negotiating process—which began from the very first day of the Muscat discussions in August 2012, and continued until the evening of July 13, 2015. Agreement on the current timetable required great flexibility and compromise on the part of the Islamic Republic of Iran, and huge sacrifices by Iran on other significant issues.

The letter includes a useful annex containing a slew of public Iranian documents concerning the JCPOA. Nice to have them all in one place.