<span class="paragraphSection"><div class="boxTitle">Abstract</div>CRISPR–Cas12a has transformative potential in molecular diagnostics owing to its robust signal amplification, but its sustained activity state severely limits temporal programmability and precise nuclease control in complex detection workflows. Here, we demonstrate that the conserved crRNA scaffold secondary structure itself can be repurposed as a reversible and programmable conformational switch to regulate Cas12a activity. By introducing short complementary DNA blockers of tunable length, we achieved length-dependent disruption and remodeling of scaffold secondary structure, shifting LbCas12a into an inactive conformation. Scaffold structure was subsequently reinstated through either single or cooperative strand displacement activation, enabling time-resolved and on-demand restoration of Cas12a activity. The conserved scaffold ensures intrinsic assay universality, while its programmable rewiring markedly improves SNVs discrimination and enables compatibility with one-pot isothermal amplification assays, delivering analytical sensitivity comparable to conventional two-step assays. This regulatory framework was further demonstrated in the detection of <span style="font-style: italic;">Klebsiella pneumoniae</span> and <span style="font-style: italic;">Mycobacterium tuberculosis</span>. By validating the crRNA scaffold as a practical and programmable switch for Cas12a activity control, this work establishes a universal and reversible framework for scaffold rewiring to modulate CRISPR nucleases and offers mechanistic insight to guide future assay engineering.</span>