Tollen's reagent is (AgNO3+NH4OH) and it enables basic medium. Fructose is a keto-hexose, still it can reduce Tollen's reagent. Because in basic medium, it undergoes epimerisation (Lobry de-Bruyn Van Ekenstein rerrangement) to give an aldehyde (D-glucose/D-mannose).
Throughout the equilibrium, keto-enol tautomeism takes place.