New development steps of AFSI-ASCOT based synthetic neutron diagnostics and validation at JET are reported in this contribution. Synthetic neutron diagnostics are important not only in existing tokamaks, where they are used to interpret experimental data, but also in the design of future reactors including DEMO and beyond, where neutron detectors are one of the few diagnostics available. Thus, development and validation of realistic synthetic diagnostics is necessary for increasing confidence in existing models and future diagnostic designs. Recent development in AFSI includes physical corrections such as implementation of plasma rotation and reduction of the fast particle contribution in thermal reactant distribution. The rotation typically changes the beam-thermal reaction rates by 1–5%, while accounting for the fast particle density consistently reduces the neutron deficit (widely known inequality of the measured and calculated neutron rates) by up to 15% depending on the discharge. Further developments include implementation of angular dependence of DD differential fusion cross sections and accounting for finite Larmor radius effect, which is important for high-energy particles such as ICRH. Additionally, the role of data based analysis in synthetic diagnostics development with the help of JETPEAK database is discussed.