This article calls for a re-examination, from a Relational Contract Theory perspective, of the ‘copyleft’ agreement, which mandates the source code of publicly released software to be perpetually shared with the public. It deals with standard-form copyleft licences such as the GNU General Public Licence, whose legal enforceability has often been a subject of debate. From the viewpoint of classical contract law, these licences would lack affirmatively expressed consent from the licensees and would thus be unlikely to be legally binding. There are two alternatives to this classical contractual view. The first is the neoclassical contract approach as represented in Easterbrook’s ruling in ProCD v Zeidenberg, which attempts to re-engineer the legitimisation of standard-form information product licensing schemes with the goal of maximising individual utility gains. The second alternative is a (Ian) Macneilian relational approach, which is endorsed by this article but has remained conspicuously absent from the existing legal literature.