Updated project metadata. Cytokines of the interleukin 12 (IL-12) family are assembled combinatorially from shared alpha and beta subunits. A common theme is that human IL-12 family alpha subunits remain incompletely structured in isolation until they pair with a designate beta subunit. Accordingly, chaperones need to support and control specific assembly processes. Here, we site-specifically introduce photo-crosslinking amino acids into the IL-12 and IL-23 alpha subunits (IL-12alpha and IL-23alpha). This allows to stabilize transient chaperone:client complexes for mass spectrometry and reveals a large set of ER chaperones to interact with IL-12alpha and IL-23alpha. Among those, we focus on protein disulfide isomerase (PDI) family members and reveal IL-12 family subunits to be clients of several ill-characterized PDIs. We find that different PDIs show selectivity for different cysteines in IL-12alpha and IL-23alpha. Despite this, PDI binding generally stabilizes unassembled IL-12alpha and IL-23alpha against degradation. In contrast, alpha:beta assembly appears robust and only multiple simultaneous PDI depletions reduce IL-12 secretion. Our comprehensive analysis of the IL-12/IL-23 chaperone machinery reveals a hitherto uncharacterized role for several PDIs in this process. This extends our understanding of how cells accomplish the task of specific protein assembly reactions for signaling processes. Furthermore, our findings show that cytokine secretion can be modulated by targeting specific ER chaperones.