The biased agonism of glucagon-like peptide-1 receptor (GLP-1R) plays a key role in the efficacy and side effects of drugs used to treat type II diabetes mellitus and obesity. Despite its therapeutic potential, the mechanisms underlying GLP-1R biased agonism remain poorly understood. In this study, we investigate the role of the extracellular domain (ECD) in GLP-1R signaling bias through saturation mutagenesis at seven key sites. We examined 126 mutations and identified several that selectively abolished β-arrestin recruitment while retaining cAMP production. Additionally, we employed a large language model (LLM) to interpret the functional impacts of these mutations, uncovering correlations between sequence features and signaling outcome. These findings provide new insight into the "two-domain" model of class B1 G protein-coupled receptors (GPCRs), highlighting the ECD's role in biased agonism and offering novel information for designing more effective and selective GLP-1R agonists.