The Central African Republic has had an interesting semi-presidential history. Working backwards, the current constitution, dating back to 2005, is semi-presidential. This replaced an earlier semi-presidential constitution that dated from 1995. In between, in 2003 François Bozizé, the current president, seized power and suspended the 1995 constitution, replacing it over a series of weeks with a number of constitutional decrees. These decrees did not amount to a semi-presidential regime. Prior to the passage of the 1995 constitution, in 1991 there was an amendment to the 1986 constitution that, as far as I can tell, grafted a prime minister who was responsible to parliament onto the existing presidential system. (If anyone has a copy of the 1991 amendments, then please share them). Anyway, all of this means that, according to the current state of my research, the Central African Republic became semi-presidential in 1991.
Before that, though, there was also a short period of semi-presidentialism that is probably forgotten. In 1981 David Dacko, who had seized power from the self-styled ‘Bokassa I’ in 1979, proposed a new constitution that was approved in a referendum in February. This 1981 constitution was semi-presidential. (I have a hard copy of the text that I can share if required). In March 1981 David Dacko was elected president, winning just over 50% of the vote in an election that was at least somewhat free and fair.
However, the Central African Republic’s first experiment with semi-presidentialism ended soon after. On 1 September there was a coup and Dacko was deposed. The new military leader, André-Dieudonné Kolingba issued a constitutional act that ended Dacko’s earlier constitution.
So, for a few months in 1981 the Central African Republic operated under a semi-presidential constitution. More information, in French, including various texts, can be found at a really useful website here.