Literacy has become inextricably bound with machine processes, especially in the age of ubiquitous, consequential artificial intelligence (AI). Despite a relatively long history of AI involvement in our everyday reading and writing practices, the public availability of generative AI tools has set off a wave of heated debate--and concern--about AI's role in our lives. We argue that critical literacy theory and tools can serve as a foundation, when combined with posthumanist ideas and some technical knowledge, for understanding, teaching, and participating in our AI-infused world. In this paper, we outline our theory of critical posthumanist literacy, which draws on posthumanist scholarship to re-imagine critical literacy with respect to concepts of ontology, agency, ethics and justice, and pedagogy. For each concept, we build on humanist, critical perspectives to show how posthumanist scholarship can help theorize for literacy in the age of AI, especially as AI presents both lingering and new challenges to conceptions of human text production and consumption. Posthumanism provides us with alternative modes of thinking about the nature of "things" (and ourselves); with an understanding of agency as not a human possession but an accomplishment among/within many human and non-human actors; with an expanded ethics that accounts more deeply for non-humans; and with pedagogy that embraces ambiguity, movement, and speculation. Using these ideas to expand critical literacy practices, we offer concepts and questions for guiding literacy practice and research, with the understanding that these are no longer separable from complex computational systems.