With DARPA’s AI Cyber Challenge (AIxCC) semifinal starting today at DEF CON 2024, we want to introduce Buttercup, our AIxCC submission. Buttercup is a Cyber Reasoning System (CRS) that combines conventional cybersecurity techniques like fuzzing and static analysis with AI and machine learning to find and fix software vulnerabilities. Adaptive vulnerability discovery: Our system uses large language models (LLMs) to generate seed inputs for fuzzing, significantly reducing the time needed to discover vulnerabilities.