The 30-year-old is to sign a new contract until 2007 and is also poised to play for Les Bleus in their November Test matches for the first time since April 2003.
"I met with my president and he offered me the chance to extend my contract by two years," said Castaignede in L'Equipe.
His current contract runs out in June 2006.
"The only club that could make me leave Saracens is Stade Toulousain, however, I think I am going to stay with Saracens but not for two years.
"I just want one more year, until the 2007 World Cup, and then I will retire."
Castaignede is expected to make his international return with France with coach Bernard Laporte saying at the start of the week he was counting on him for the November Test matches.
"The World Cup is the main objective of my career," added Castaignede, who underwent fitness tests in Marcoussis this summer.
"It will also be played in France and the French team has what it takes to win it.
"We have a lot of good players and I believe this is the best team I've seen since I became an international."
Laporte is unsure whether the versatile Castaignede will play at centre or fly-half, but the Saracens player is ready to accept any challenge.
"I don't mind, I have matured with age," he said. "If I join the French team, I will do as I'm told.
"There is no ideal fly-half. For certain matches (Jonny) Wilkinson is the best but for some others, I would prefer (Frederic) Michalak.
"Everything depends on the tactics that are put into place."
France kick start their Autumn tour on November 5 against Australia and will then take on Canada, Tonga and South Africa.
Laporte will announce his squad on October 25.