Lateral flow immunoassays (LFAs) are widely used in point-of-care testing (POCT) for detecting small molecules. However, their application is often hindered by the complex synthesis of traditional chemically synthesized antigens. Nanobody-based coating antigen mimics have shown excellent analytical performance in various immunoassay platforms, but their application in LFAs still faces challenges. Here, we demonstrate the use of nanobody fusion proteins as antigen mimics in the construction of LFAs for detecting small molecules. Anti-idiotypic nanobody (B26) specific to Fumonisin B