I'm using ASDF to build an open source implementation of Google's protocol buffer data serialization library. The code is here: http://github.com/brown/protobuf I've defined a new component type that allows developers to include protobuf message definition files in their Lisp ASDF files: (defclass protobuf-source-file (cl-source-file) ((relative-proto-pathname ... ) (search-path ... ) (:documentation "A protocol buffer definition file.")) When used like this in a system definition: :components ((:protobuf-source-file "foo")) executing the ASDF:LOAD-OP operation causes the file called foo.proto to be translated into foo.lisp by a C++ program, and then compiled into foo.fasl and loaded. Note that foo.lisp is created in the same directory as the system definition, but I would prefer it to be created in the same directory as foo.fasl, which for me is in a subdirectory of ~/.cache/common-lisp. What's the easiest way to accomplish this? I initially tried overriding INPUT-FILES when applied to CL-SOURCE-FILE instances, but ASDF::PERFORM is defined like this: (defmethod perform ((operation compile-op) (c cl-source-file)) (let ((source-file (component-pathname c)) ... ) ... ) Since it calls COMPONENT-PATHNAME to find Lisp source, it will always look in the system definition directory. bob