Bring a large pot of salted water to a boil.
Add the pasta and cook for 2 minutes less than the package directions.
Scoop out 1 cup of water then drain the pasta.
Add the drained pasta, ½ cup of the water, and cannellini beans to the empty pot and bring to a simmer. Cook for 2 minutes, stirring frequently.
Take the pasta off the heat and add the pesto, stirring to make a creamy sauce. Add more water if needed. Serve.
