I’ve been thinking about using single pass for pullback calls with a single function argument or making this configurable (maybe something like fwddiff(f, SinglePass()) or so). Using thunks (current ForwardDiffPullbacks behavior) should be helpful esp. for multi-arg function when we finally bring Zygote PR 966 home (help very welcome). Also, some functions (lot’s of distributions-related stuff) don’t support dual numbers for all their arguments.