All of the 1993–1995 comics are based on short stories by Sapkowski, (except the last one, Zdrada/The Betrayal). List from Wikia:
- Droga bez powrotu (A Road with No Return, based on the short story Droga, z której się nie wraca)
- Geralt (based on the short story The Witcher)
- Mniejsze zło (based on the short story The Lesser Evil)
- Ostatnie życzenie (based on the short story The Last Wish)
- Granica możliwości (based on a short story The Bounds of Reason)
- Zdrada (The Betrayal, not based on a short story, but on an outline by Andrzej Sapkowski)
This is the reason Sapkowski is credited as co-writer – he did write the source material for all of them. As far as I know they don't condradict anything established in the novels.
Regarding canonicity, Sapkowski does not seem to consider anything canon beyond the books. Excerpts from an interview about the Witcher video games:
"Adaptations - although they can in a way relate to the story told in the books - can never aspire to the role of a follow-up. They can never add prologues nor prequels, let alone epilogues and sequels.
"If we level the field between books and their adaptations in other media, only the former can be the ones telling a story," he stated. "There can never be a different relation between a book and its adaptation, other than the one that without the book the adaptation would not exist at all.
"I realise that current times accustom us - which I find terrible - to the strange convergence of media and the freedom of mixing them. To me as a writer, the idea to write 'adjuvant content' and create something 'complementary' to a game or a comic is an absolute pinnacle of idiocy."
"I will definitely skip any 'alternative ideas'," Sapkowski promised. "It'll come easily to me anyway, as I don't know any of them. And even if I knew, it would be funny and silly were I to write based on the game's suggestions. I suppose I have made myself clear when I said that I will never accept any ideas and concepts of 'complementarity plots' and 'building coherent stories'. A story can only be contained in a book."
And from another interview in Polish:
All adaptations - movies, comics or games, including "The Witcher" - are only adaptations, they are completely separate feature films, the work of their creators . I have no references to Geralt's witchcraft, and they can not be treated as any canon, because they are the only ones I have printed in printed form. Therefore, if the story of the witcher is actually taken up - whether in the form of a prequel, a sequel or a spin-off - will not be the least adaptable to any adaptation, the elements of these adaptations will be completely overlooked - in the actual canon of the witcher's tales.