<p>Making a living in commodity production depends on knowledge of both <i>how</i> to produce and <i>what</i> is produced—a commodity’s qualities and value. Producers’ livelihoods and agency stem from the relationships between their knowledge systems and value chains. Bringing together agrarian studies with cultural-economic studies of value chains, this paper considers knowledge(s) as forces in the production of diverse kinds of value, complicating labels like expert, technical, lay or local. The paper presents the case of specialty coffee farming and processing in Vietnam, focusing on how fermentation during post-harvest processing creates qualities and value. The analysis shows coffee producers employ <i>working knowledge</i>: knowledge of production that <i>does</i> work by producing value. Though Vietnam is the world’s second-largest coffee producer, it has a reputation for poor quality. In the 2010s, a slim minority began producing specialty coffee—valued for exceptional flavors and aromas—in part by employing novel post-harvest fermentations. Through an ethnography of fermentation knowledges and practices, this paper shows that livelihoods depend on flows not only of coffee and its material qualities but also of information. Producers assemble and translate diverse knowledges and practices as their work depends on making a commodity legible for value chains. Their knowledge systems are thus not peripheral but central to global production, as they shape and are shaped by value chains and the production of quality.</p>