AI has made rapid progress on software engineering benchmarks in the past few years. However, most such benchmarks tend to focus on shorter tasks like fixing bugs or implementing individual features. MirrorCode is our benchmark, co-developed with METR, to test AI models on long-horizon coding tasks. In a MirrorCode task, AI models are tasked with reimplementing an entire program end-to-end, without access to the original source code. AI-generated solutions must match the original program’s outpu...