A Study of Radiation Therapy and Cemiplimab With or Without Fianlimab In People With Bladder Cancer

Inclusion Criteria: * Age ≥ 18 years at the time of informed consent * ECOG 0-1 * Histologically confirmed diagnosis of urothelial carcinoma. ° Variant histology is acceptable if there is a predominant urothelial component. Any neuroendocrine / small cell components are excluded (Investigators are encouraged to discuss with study team and PI) * Cystoscopically and radiographically confirmed cT2-4a cN0 cM0 disease per American Joint Committee on Cancer Staging Manual, 8th edition. * Patients with cT4a disease invading into the prostatic stroma with no cystoscopic confirmation of muscle invasion are eligible. * Clinically node-negative pelvis (cN0) on CT or MRI within 56 days: no pelvic lymph node ≥15 mm short-axis. Pelvic nodes 10-14 mm are permitted if not suspicious by GU radiology morphology; if deemed suspicious, biopsy or repeat imaging in 4-6 weeks must confirm cN0 prior to enrollment. PET may inform adjudication but is not required. * Patients declines cisplatin-based therapy or is ineligible for cisplatin-based therapy based on any of the following criteria: * Estimated or calculated creatinine clearance ≥ 30ml/min but \< 60 ml/min * Grade 2 or above audiometric hearing loss (per CTCAE v5.0) * Grade 2 or above peripheral neuropathy (per CTCAE v5.0) * Availability of tumor specimen block or 20 unstained slides from diagnosis of muscle-invasive disease. Patients with fewer than 20 slides available may be enrolled after discussion with the Principal Invest…