clos-diff is a GPL licensed library which will be able to create and
apply diffs of CLOS objects in a compact representation that will not
use too much bandwidth or disk space. The library also preforms the
diffs recursively, so that (theoretically) infinite layers of objects
can be diffed. The only dependency is closer-mop.

The API is simple, and documented below:

function: clos-diff:diff (old new &key (test #'equalp))
  This function will create a diff that represents the modification
  that need to be applied to object old to create the object new. The
  test parameter will control the test used for equality. As a special
  case is the old parameter is nil, a diff from an object with no
  bound slots will be created.

function: clos-diff:apply-diff (diff &optional object)
  This function will apply the diff given in the parameter diff to the
  object contained in object. If object is nil, a fresh object of the
  appropriate type (with all slots unbound) will be created, and the
  diff will be applied to that (See the documentation for diff for
  more information).

This project is hosted on github. Submit bug reports there. Also, you
may contact the author at krzysdrewniak AT gmail DOT com .