When al-Mahdī died suddenly, and unexpectedly, on a hunting expedition on 4 August 785, Hārūn al-Rashīd was with the caliph at Māsabadhān (the foothills of al-Jibāl, some 250 kilometres east of Baghdad). Al-Hadī, the senior successor, was campaigning in Jurjān, east of the Caspian Sea (a further 750 kilometres north-east of the capital). Hārūn al-Rashīd duly had Mūsā recognised as caliph, but Mūsā was to reign for less than fourteen months; he died in September 786. As a result, accounts of the events of his very brief caliphate (and of the succession arrangements under al-Mahdī) were shaped during the caliphate of Hārūn al-Rashīd, who reigned for the next twenty-three years. What may have amounted to a coup, orchestrated by Hārūn's supporters at the imperial court and in the provincial armies, appears to have been partially suppressed in the extant accounts.

According to al-Ṭabarī, the mawlā, al-Rabīc b. Yūnus, took the opportunity of proclaiming al-Hādī's accession from Māsabadhān also to proclaim Hārūn al-Rashīd as the walī al- cahd. The letters to the provinces requested the pledge of allegiance to al-Hādī as caliph and Hārūn al-Rashīd as walī al- cahd. As we have seen, this may have been the first occasion on which Hārūn was formally proclaimed as such; if so, it amounted to an arrogation of the rights of al-Hādī to decide the succession, and thus set the scene for what appears to have amounted to near civil war.