Foreign circRNAs are potent adjuvants to induce antigen-specific T cell activation, antibody production, and anti-tumor immunity in vivo, and m<sup>6</sup>A modification abrogates immune gene activation and adjuvant activity. m<sup>6</sup>A reader YTHDF2 sequesters m<sup>6</sup>A-circRNA and is essential for suppression of innate immunity.