NFL Network's Mike Garafolo reported the news on Twitter on Wednesday.
San Francisco acquired Tomlinson for a 2019 fifth-round draft pick in an August 2017 trade with the Detroit Lions, who selected him in the first round of the 2015 draft.
Although the 26-year-old Jamaica native struggled to reach those initial expectations in Detroit, he showed signs of progress in his first season with the 49ers.
Tomlinson made 15 starts at left guard, and Pro Football Focus graded him as the league's No. 32 overall guard, including players on both the left and right sides.
While the Niners sought more consistency from him—he struggled to protect the quarterback at times—he was often the bright spot for an offensive line that rated 11th in run blocking and 18th in pass protection, per Football Outsiders.
Ultimately, even though Tomlinson made positive strides after arriving in San Francisco, it wasn't enough for the front office to increase its financial commitment. His cap hit would have jumped from $1.6 million in 2018 to $8.6 million in 2019 if the option were picked up, per Spotrac.
So, while the 49ers opted against going that route, they may still look to eventually re-sign the lineman to a more team-friendly deal.