Which international border follows the Hindu Kush watershed?

The international border that follows the main watershed of the Hindu Kush is the boundary between Pakistan and Afghanistan.1

Specifically, this border follows the range's main watershed throughout its eastern region, stretching from Lupsuk Peak—located just north of the Karambar Pass—to the Dorāh Pass, which is situated just south of Mount Tirich Mir. This boundary line, known as the Durand Line, was established through 19th-century negotiations between the British and the ruler of Afghanistan. Near the Dorāh Pass, the border departs from the main watershed to follow minor spurs before crossing the Kābul River and continuing along the crest of the Spin Ghar Mountains.1