"I've been working with the Cubs on contract negotiations for a period of time," Castro's agent Paul Kinzer, of the Wasserman Group, told ESPNChicago.com's Bruce Levine on Thursday. "Starlin would like to be a Chicago Cub for many years to come."
Comcast SportsNet Chicago first reported Thursday that the Cubs and Castro are working on a deal that could be as long as six years and is expected to be completed by the end of the season.
Kinzer told Levine that talk of a six-year deal is "premature at this time."
The 22-year-old Castro is in his third year and will be arbitration eligible for the first time in 2012 because of his status as a Super 2 player. He is making $576,000 this season. A long-term deal would take him into his free agent years, according to a major league source.
Castro is a two-time All-Star in his two and a half seasons. He was the youngest player in history to lead the National League in hits when he finished with 207 at age 21 years, 188 days at the end of the 2011 season.
A career .296 hitter, Castro has struggled at the plate in the second half, batting .236 after the All-Star break, dropping his average to .276 to go along with 12 home runs and 58 RBIs. Castro has been plagued by mental lapses in the field during his young career, most recently on Friday when he got thrown out on the bases after losing track of the ball.
A USA Today report in May cited two Cubs officials that claimed "everyone but Jeff Samardzija" can be acquired by trade. But Cubs president of baseball operations Theo Epstein quickly shot down the talk.
"Starlin Castro is the type of player we're looking to build around," Epstein said on May 31. "There has been no trade consideration with him, whatsoever."
Castro, 23-year-old first baseman Anthony Rizzo and Samardzija appear to be the building blocks for Epstein and general manager Jed Hoyer.