Using a type I-F SRU as an example, Camara-Wilpert et al. demonstrate that SRUs inhibit CRISPR–Cas immunity via an RNA-based anti-CRISPR mechanism. The type I-F SRU produced a small non-coding RNA (Racr) that mimicked the type I-F CRISPR RNA but resulted in the formation of an aberrant Cas subcomplex that lacked Cas5f and Cas8f, which are essential for target recognition and cleavage. Moreover, the team identified Racr candidates for almost all CRISPR–Cas types, and nine candidates were shown to have anti-CRISPR function when expressed in bacteria. In addition, racr genes tend to cluster with acr genes within viral genomes, suggestive of their functional equivalence. Overall, this study not only found that phage-encoded RNAs can act as anti-CRISPRs but also provides future opportunities for precise regulation of CRISPR–Cas-based tools.