Strasbourg, the BlueCo-owned French club that operates as a feeder side for Chelsea, have handed the 19-year-old a contract running until 2031.
The fee is fixed rather than the staged package initially reported, with no further add-ons attached, though Werder Bremen have secured a 10 percent sell-on clause that will pay out on any future transfer.
Coulibaly established himself as a first-team regular during his breakthrough Bundesliga season last term, form that briefly attracted interest from Chelsea and Paris Saint-Germain, though that speculation cooled following a difficult second half of the campaign.
The Germany under-21 international joined Werder on a free transfer from Hamburger SV in 2024, and his departure now hands the club a significant fee for a player who cost them nothing to sign.
What Werder do with that money remains to be seen, given the club's tight financial position, but two names are understood to be under consideration as they look to reinvest.
Cameron Puertas, who previously spent time on loan at Werder and is now at Al Qadsiah, sits at the top of the club's wish list, while Batista Mendy of Trabzonspor has also been discussed as an alternative option to strengthen the squad before the transfer window closes.
The confirmation brings clarity to a deal that had been reported in various forms over recent days, with earlier speculation over a bonus-heavy structure now replaced by the simpler fixed fee plus sell-on arrangement.
For Strasbourg, the move continues a pattern of recruitment focused on young, developing talent from the Bundesliga and elsewhere in Europe, part of the club's broader strategy of identifying prospects who can either progress within the Chelsea ownership group or be sold on for a profit further down the line.