Platform ecosystems shift the locus of innovation from internal departments to third‐party developers, leading to knowledge boundaries between the platform owner and these external developers. Whereas traditional knowledge management approaches focus on intra‐organizational knowledge sharing, platform owners have to adopt an inter‐organizational approach to integrate knowledge across firm boundaries. Platform owners thus rely on knowledge boundary resources such as technical documentation, information portals, training materials, sample code, blogs, and online communities to overcome knowledge boundaries in their ecosystems. Existing research has identified different types of knowledge boundaries and knowledge boundary resources, but further research is needed to clarify how they influence third‐party application development. Against this background, we study how third‐party developers integrate knowledge boundary resources into their development practices. To derive an in‐depth understanding of the phenomenon, we used cognitive fit theory and conducted a single case study of SAP's Business Technology Platform. Based on 23 interviews, we developed a process model and distilled two primary purposes why third‐party developers use knowledge boundary resources: learning and problem‐solving. Based on these purposes, we explain the emergence of a knowledge boundary resource‐purpose fit that shapes the selection, evaluation, and usage of knowledge boundary resources. We then draw upon various knowledge boundary resource characteristics to condense four cases with a particularly favorable knowledge boundary resource‐purpose fit. Based on our results, we derive five propositions to help platform owners design effective knowledge boundary resources.