How Codex Happened (every.to)

OpenAI's Codex has evolved into a pivotal tool for programming, stemming from its earlier, less successful iterations. Following the launch of GPT-5 in 2025, the Codex team reimagined its approach to coding by leveraging new reasoning models that allow the AI to plan and refine its responses before presenting them. This significant shift in functionality was particularly useful for programming tasks, where models could test and revise code autonomously, demonstrating a leap in AI capability. By mid-2026, Codex had become integral to OpenAI's flagship ChatGPT app, achieving over 40 million weekly active users and merging programming assistance with broader knowledge work applications. The impact of this development on the AI/ML community is profound, highlighting a transition toward more autonomous AI systems in software development. The introduction of the "Thinking" interface, which symbolizes internal reasoning processes, marked a departure from simply reactive models to more strategic ones capable of complex problem-solving. As Codex and similar reasoning models gain traction, they are set to redefine workflows in coding and extend beyond traditional programming tasks, potentially democratizing software development for users with varying expertise. This evolution represents a significant milestone in the quest for sophisticated AI-driven solutions in knowledge work.