The 25-year-old made 38 appearances in all competitions for the Ligue 1 side last season, scoring seven times as they finished 10th in the table.
His performances earned him a place in Didier Deschamps' France side, and attention from around Europe.
But Cardiff now appear to be in pole position for his signature after Mackay admitted that the two clubs had agreed a fee, although personal terms are still to be discussed.
"Ourselves and Toulouse have come to an agreement," Mackay told Sky Sports.
"We've got to then have the conversation with Etienne and his advisors, they've been very professional in terms of what they're talking about as well.
"We've been in constant dialogue with them in the last week and Toulouse have been excellent to deal with.
"It's pretty delicate at the moment. We're in discussions and negotiations but it's something that's ongoing."
Capoue would be the third new signing for Cardiff, joining Andreas Cornelius and John Brayford, as the club prepare for their first top flight campaign in 51 years.
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