I think I may have stumbled on an incompleteness in this method. Here's the test case: 1. Define a system. S1, with a version loaded from a version file; 2. Load S1 into session. 3. Hack hack hack. 4. Bump S1's version number in the version.lisp-expr file 5. Load S2 that depends on new version of S1. As far as I can tell, ASDF is simply checking the .asd file, not S1-version.lisp-expr, to see if it's stale. So my load in step 5 is failing. This is, I believe, because S1-version.lisp-expr is not in the set of INPUT-FILES for S1's DEFINE-OP. It's a little unsettling to change this on the fly, but I think the right thing is probably to push the version.lisp-expr file onto a set of ancillary dependencies when it's read. If this sounds right, I'll stick this in as a ticket, and try to find where we could inject a fix. Best, r