Strikingly, SARS-CoV and MERS-CoV-infected patients are insensitive to ribavirin, a FDA-approved broad-spectrum antiviral drug. Here, we report an unprecedented RNA correction machinery that was developed by the SARS-CoV. We show that the viral 3’-5’ exonuclease is able to excise either a misincorporated ribonucleotide or ribavirin 5’-monophosphate and to resume RNA synthesis. This proofreading pathway has allowed coronavirus RNA genome size expansion (~ 32 kb in size). This work paves the way for the design of future antiviral strategies using nucleoside analogues.