This phase II clinical trial evaluates the efficacy and safety of three neoadjuvant regimens in patients with locally advanced microsatellite instability-high/mismatch repair-deficient (MSI-H/dMMR) colorectal cancer (CRC): 1) Regimen A: Dual immune checkpoint blockade with nivolumab plus ipilimumab. 2) Regimen B: Nivolumab plus radiotherapy. 3) Regimen C: Nivolumab monotherapy. The primary objectives are to determine whether: 1) Dual immune checkpoint blockade (Regimen A) is superior to nivolumab monotherapy (Regimen C); and 2) Immunotherapy plus radiotherapy (Regimen B) is superior to nivolumab monotherapy (Regimen C). Methods: Participants will be randomized in a 1:1:1 ratio to one of the three arms. For patients with resectable tumors, surgical resection will be performed. In patients with low rectal cancer and poor prospects for sphincter preservation, a watch-and-wait (WW) strategy is an option if a clinical complete response (CR) is achieved following neoadjuvant therapy.