Agentless is a tool for automatically solving software development problems by breaking the task into three phases. First it localizes the fault to specific files, classes, functions, and edit locations using hierarchical analysis. Then it samples multiple candidate patches in diff format. Finally it validates patches by running regression tests and generated reproduction tests, re-ranking candidates until submitting a correct fix. The system achieved 40.7% solve rate on SWE-bench lite using Claude 3.5 Sonnet.