CRISPR-Cas-based diagnostics have the potential to elevate nucleic acid detection. CRISPR-Cas systems can be combined with a pre-amplification step in a one-pot reaction to simplify the workflow and reduce carryover contamination. Here, we report an engineered Cas12b with improved thermostability that falls within the optimal temperature range (60°C–65°C) of reverse transcription-loop-mediated isothermal amplification (RT-LAMP). Using de novo structural analyses, we introduce mutations to wild-type BrCas12b to tighten its hydrophobic cores, thereby enhancing thermostability. The one-pot detection assay utilizing the engineered BrCas12b, called SPLENDID (single-pot LAMP-mediated engineered BrCas12b for nucleic acid detection of infectious diseases), exhibits robust trans-cleavage activity up to 67°C in a one-pot setting. We validate SPLENDID clinically in 80 serum samples for hepatitis C virus (HCV) and 66 saliva samples for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with high specificity and accuracy. We obtain results in as little as 20 min, and with the extraction process, the entire assay can be performed within an hour.