Hi Chris, as a heads up, Stan actually has had three built-in methods for a while now. There is a non-stiff Adams-Moulton solver introduced in 2018. It unfortunately was only just exposed in the Stan 2.23 documentation: https://mc-stan.org/docs/2_23/stan-users-guide/ode-solver-ch.... Certainly, the devs have been talking about adding more solvers for a while, including SDE and DDE solvers, and your DifferentialEquations.jl ecosystem is an excellent model; it is an area that we know Stan has been lacking in. I think Steve Gronder will be trying to work with you regarding benchmarking.