A novel immune‐stimulating antibody‐drug conjugate, which links STING agonists to anti‐HER2 mAb through an optimized noncleavable Linker, remains highly stable at DAR = 5.7 and demonstrates superior efficacy over marketed HER2‐targeted ADCs in multiple tumor models with high, low, and drug‐resistant HER2 expression. This evidence concerns the gene ERBB2 and neoplasm.