OpenAI used its OpenDev stage on October 6, 2025 to push ChatGPT from a capable assistant into a platform you can build on. The headliners were apps that run directly inside ChatGPT via a new Apps SDK, AgentKit for building and shipping agentic workflows, general availability of Codex with a fresh SDK and Slack integration, and a strategic AMD partnership aimed at scaling compute to six gigawatts. This post breaks down what shipped, why it matters, and how developers can start building today.