The 2023 Voyager agent kept a growing “skill library” in which every capability it developed in Minecraft was stored as a piece of executable code, so a mastered skill could be retrieved and composed into harder behaviors rather than relearned from scratch. Driven by GPT-4 through plain API calls, with no fine-tuning, this code-as-memory design helped Voyager collect 3.3 times more unique items than prior state-of-the-art methods, travel 2.3 times farther, and unlock key tech-tree milestones up to 15.3 times faster.