We establish a Harder-Narasimhan formalism for modifications of G-bundles on the Fargues-Fontaine curve. The semi-stable stratum of the associated stratification of the $B_{dR}^+-Grassmannian$ coincides with the weakly admissible locus. When restricted to minuscule affine Schubert cells, it corresponds to the Harder-Narasimhan stratification of Dat, Orlik and Rapoport. I will also explain the relation to the Newton stratification as well as some geometric properties of the strata. This is joint work with K.H. Nguyen.