As readers probably know, the State Dept’s 2019 CWC compliance report states that Iran transferred “CW to Libya during the 1978-1987 Libya-Chad war.”

Here’s some more detail:

The United States assesses that in 1987 Iran transferred CW munitions to Libya during the 1978-1987 Libya-Chad war. Following the collapse of the Gaddafi regime, the Libyan Transitional National Council located sulfur mustard-filled 130mm artillery shells and aerial bombs, which are assessed to have originated from Iran in the late 1980s. In 2011, Libya declared to the Organization for the Prohibition of Chemical Weapons (OPCW) that it discovered 517 artillery shells and 8 aerial bombs comprising 1.3 Metric Tons of sulfur mustard but did not address the provenance of the items. 

Well, the CIA was on this a while back. The 1988 report (which I cited here) mentions the transfer at least a couple of times. The report first observes that Iran’s CW production capability and stockpile were “sufficient to permit the shipment of chemical weapons to Libya in 1987.” It later mentions that “Iranian-produced chemical weapons have been transferred to Libya.”