Sundials does have forward sensitivities, though the implementation does not SIMD the calculations of the primal with the dual parts like direct AD of the solver does. This makes it a bit worse on forward mode than direct differentiation of a solver (with an optimizing compiler) would do, which was something that took us a few years to really pin down (we had to implement our own to really benchmark it, and got the same phenomena demonstrated in https://arxiv.org/abs/1812.01892). The nice thing about the direct forward mode is that it is easier to opt the extra equations out of the linear solver, something that AD systems won't do by default (but it can be done)
