Super interesting. In SMLib (via smlibpy) fillets are both more robust than OCCT, and apparently somewhat non-deterministic with some new and different failure modes. In the below compressor wheel, the script is much less convoluted in smlibpy than in build123d with regard to the root fillet (cause of all problems). Three issues though, and they are a bit weird: 1) Fillet chains crossing the seam line of the core fail. Seam needs to be placed where it doesn't intersect any of the blade root edges 2) Fillets which cover the seam line of the core build only in G1 mode (standard rolled fillet) 3) There is a non-deterministic fillet failure on the blade opposite of the seam where it fails on 1 out of 5 builds. My script had to build-in a self-healing routine. Verdict: Better-ish than OCCT, but odd edge cases, which in a complex design, aren't really edge cases.