FTR, i have managed to solve it with this: (defclass generate-lisp-op (selfward-operation) ((selfward-operation :initform '()))) ; we will specify it in our own COMPONENT-DEPENDS-ON (defmethod component-depends-on ((op generate-lisp-op) (c c2ffi-file)) `((load-op ,(find-system "cffi/c2ffi-generator")) ;; Regenerating the spec file is a lot of headache, so we ignore ;; the file modification times, and only communicate the ;; dependency to ASDF if the spec file is missing. ,@(let ((spec-file (input-file op c))) ; TODO is it legal to call ASDF:INPUT-FILES here? (when (or (not (probe-file spec-file)) (zerop (with-input-from-file (s spec-file) (file-length s)))) `((generate-spec-op ,c)))) ,@(call-next-method))) the commit: https://github.com/cffi/cffi/commit/46975c644aeb0a832cd170e953cc8ec3cbdc78df - attila PS: sorry for the double send, i have cleaned up my mail settings.