The actress claims Cynthia Pett-Dante -- who happens to manage Brad Pitt -- was part of a seller group that convinced her to drop $3.2 million on a Malibu house in 2016, but failed to mention it was riddled with major problems.

According to the docs ... Fox was pregnant at the time and raising 2 toddlers, and led to believe she was buying a "paradise in bucolic Malibu where she could raise her young family.."

Instead, Fox claims the home is a "nightmarish living hell." For starters, she says it's on a slope that needs to be regraded, and a bunch of features are not up to code -- including animal pens, a gazebo, stone walls, and a storage building for livestock feed.

According to the docs ... there was also mold all over the house upon moving in, including in the master bedroom and bathroom and the kids' rooms.

Fox is suing for more than $5 million in damages

We reached out to Pett-Dante ... no word back so far.