philosophy

Marxism is a rejection of metaphysics, so the determinism debate is mostly irrelevant. Marxism is a concrete analysis of concrete observable conditions. Although as @FunkyStuff@hexbear.net said, there is a degree of 'predetermination' of one's worldview by their class stand.

Marxism doesn't really encompass all the different aspects of ontology, at least not in any monolithic way that could provide an answer to this specific question. Some Marxists believe in determinism down to the level of individual people, while others believe that free will at the individual level exists but material conditions create soft determinism at the social level, where the material circumstances for large groups override any personal decision making. At the end, political economy doesn't really depend on which of the two is true, they're functionally the same.

It's a little like asking if Marxists are atheists or if they just think God doesn't do anything. Those possibilities are functionally the same thing and the ideology is principally concerned with the project of capturing political power, not answering the fundamental questions of the universe.