What is the (current) lower mass limit of the lensing object(s) for which astronomers have been able to determine that the apparent position of an object behind it has been deflected or otherwise lensed gravitationally?
During the solar eclipse of May 1919 gravitational lensing around the Sun (about 1000 Jupiter masses) was observed for the first time. But whether the lensing is visible depends not only on mass of the body in question, but also on density and surface gravity. Since red and brown dwarfs are much denser than the Sun they also have a higher surface gravity than the Sun's 28g despite being much less massive.
A sensitive enough instrument might also detect gravitational lensing around Jupiter (318 Earth masses and 2.528g on average). Therefore I guess any gas giant and brown, red, orange and yellow dwarf will provide for that effect. The lower limit may be one Jupiter mass and 2.5g. The body's density is more important for that effect. Jupiter's density is 1.326 g/cc (0.048 lb/in³).
I should add that black holes always produce visible gravitational lensing, as singularities, they have infinite density. If the hypothetical so-called primordial black holes at 5 Earth masses exist, the lens would be clear.