Hello, test file: \def\msg#{\immediate\write16} \msg{\meaning\Xpdfoutput} \begingroup \csname Xpdfoutput\endcsname \endgroup \msg{\meaning\Xpdfoutput} \directlua{tex.enableprimitives('X', {'pdfoutput'})} \msg{\meaning\Xpdfoutput} \csname @@end\endcsname\end Result: 3 x "undefined" After reading the documentation I expected: 2 x "undefined", 1 x "\pdfoutput" | Then it will check for the existence of the constructed csname. If the | csname is currently undefined (note: that is not the same as \relax), it | will globally define the csname to have the meaning: run code belonging to | the primitive `name'. If for some reason the csname is already defined, it | does nothing and tries the next pair. Either there is a bug in the implementation or in the description ("is already defined" --> "is or was already defined"). Workaround: \directlua{tex.enableprimitives('Workaround', {'pdfoutput'})} \global\let\Xpdfoutput\Workaroundpdfoutput Yours sincerely Heiko