"I wanted to return to Agen in France because it's my club, the club of my heart with who I want to pull off one last challenge," Caucaunibuca told a media conference in France.
"I do not fear competition. The team plays very well and I want to show that a 34-year-old can still do beautiful things. I'll try to bring my experience."
Agen operated in the French second division and were desperate to return to the Top 14 League. They firmly believed Caucaunibuca could help their cause and stressed he had passed all of their medical and fitness tests.
He has been plagued by knee problems in recent years.
Caucaunibuca will have to compete with two other Fijian players, Viliame Waqaseduadua and Laitia Tagotago, and Canadian international Taylor Paris for a wing position at Agen.