The Boston Red Sox trade infielder prospect Marcelo Mayer to the San Francisco Giants in a deal that brings a left-handed relief pitcher to Boston. Multiple outlets describe Mayer as a former top prospect whose status has changed since earlier expectations about his future with the organization. The trade occurs ahead of the 2026 MLB trade deadline. The agreement reflects a shift in Boston’s roster planning, with the team exchanging a high-profile minor-league talent for bullpen help. For San Francisco, the move adds a prospect with a notable reputation from Boston’s system. The reports characterize the core of the transaction as Mayer going to the Giants and Boston receiving a lefty reliever in return, though details such as additional players or specific compensation are not provided in the excerpts. Both sources frame the transaction as a reassessment of Mayer’s path with Boston and an effort by each team to address its near-term needs—Boston’s bullpen on one side, and prospect acquisition on the other.